Tesi di Dottorato GEOMECHANICAL MODELLING OF …...tesi di dottorato geomechanical modelling of...

UNIVERSITÀ DEGLI STUDIDI SALERNO

Dipartimento diIngegneria Civile

CORSO DIDOTTORATO DI RICERCA IN

Ingegneria Civile perl’Ambiente ed il Territorio

Correlatori_

Prof. ing. Manuel PASTORProf. Ing. Giuseppe SORBINO

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Relatore_

Prof. ing. Leonardo CASCINI

UNIVERSITÀ DEGLI STUDI DI SALERNOVia Ponte don Melillo84084 Fisciano (SA)Tel. 089 96 4029 Fax 96 4343www.unisa.it

Ciclo IV Nuova Serie - Coordinatore: prof. Ing. Rodolfo M.A. NAPOLI

GEOMECHANICAL MODELLING OF TRIGGERING MECHANISMS

FOR FLOW-LIKE MASS MOVEMENTS IN PYROCLASTIC SOILS

Sabatino Cuomo

_

Tesi di Dottorato

i

UNIONE EUROPEA

FONDO SOCIALE EUROPEO PROGRAMMA OPERATIVO NAZIONALE 2000/2006

“Ricerca Scientifica, Sviluppo Tecnologico, Alta Formazione”

Regioni dell’Obiettivo 1 – Misura III.4 - “Formazione superiore ed universitaria”

DOTTORATO DI RICERCA IN INGEGNERIA CIVILE PER L’AMBIENTE ED IL TERRITORIO

Università degli Studi di Salerno

IV Ciclo Nuova Serie (2002-2005)

U

NIVERSITY OF SALERN

O

TESI DI DOTTORATO

GEOMECHANICAL MODELLING OF TRIGGERING

MECHANISMS FOR FLOW-LIKE MASS MOVEMENTS

IN PYROCLASTIC SOILS

(MODELLAZIONE GEOMECCANICA DEI MECCANISMI DI INNESCO

DI COLATE RAPIDE IN TERRENI PIROCLASTICI)

SABATINO CUOMO

Coordinatore del Dottorato Relatore

PROF. ING. RODOLFO M.A. NAPOLI PROF. ING. LEONARDO CASCINI

Correlatori

PROF. ING. MANUEL PASTOR

PROF. ING. GIUSEPPE SORBINO

On the front cover: Vesuvius volcano (Italy) GEOMECHANICAL MODELLING OF TRIGGERING MECHANISMS FOR FLOW-LIKE MASS MOVEMENTS IN PYROCLASTIC SOILS ____________________________________________________________________ Copyright © 2005 Università degli Studi di Salerno – via Ponte don Melillo, 1 – 84084 Fisciano (SA), Italy – web: www.unisa.it Proprietà letteraria, tutti i diritti riservati. La struttura ed il contenuto del presente volume non possono essere riprodotti, neppure parzialmente, salvo espressa autorizzazione. Non ne è altresì consentita la memorizzazione su qualsiasi supporto (magnetico, magnetico-ottico, ottico, cartaceo, etc.). Benché l’autore abbia curato con la massima attenzione la preparazione del presente volume, Egli declina ogni responsabilità per possibili errori ed omissioni, nonché per eventuali danni dall’uso delle informazione ivi contenute.

Finito di stampare: Ottobre 2005

iii

To my sister Giovanna

1

INDEX

INDEX ........................................................................................................................... 1 INDEX OF FIGURES AND TABLES ................................................................... 3 ABSTRACT ................................................................................................................. 14

SOMMARIO ............................................................................................................... 16 ACKNOWLEDGEMENTS .................................................................................... 18

ABOUT THE AUTHOR .......................................................................................... 19 1 INTRODUCTION............................................................................................... 20

2 PYROCLASTIC SOILS ....................................................................................... 23 2.1 General features ...................................................................................... 24

2.1.1 Origin ......................................................................................... 24 2.1.2 Soil taxonomy ........................................................................... 31

2.2 Geotechnical characteristics .................................................................. 32

2.2.1 Micro scale ................................................................................ 32 2.2.2 REV scale .................................................................................. 35

2.2.3 Macro scale ............................................................................... 43 2.3 Worldwide diffusion ............................................................................... 45

3 FLOW-LIKE MASS MOVEMENTS IN PYROCLASTIC SOILS ............ 51 3.1 General features ...................................................................................... 53 3.2 Case histories ........................................................................................... 57

3.3 Remarks on landslides classification .................................................... 66 4 MODELLING OF RAINFALL-INDUCED TRIGGERING

MECHANISMS .......................................................................................................... 71 4.1 Rainfall-induced triggering mechanisms ............................................. 72

4.2 An overview of the available models ................................................... 74 4.3 Geomechanical modelling ..................................................................... 84

4.3.1 Pore water pressure modelling .............................................. 84 4.3.2 Remarks on failure and instability in geomaterials ............. 85 4.3.3 Modelling of the triggering stage ........................................... 87

5 THE CASE STUDY............................................................................................. 91 5.1 Southern Italy .......................................................................................... 92

5.1.1 Pyroclastic soils in the Campania region.............................. 92 5.1.2 Environmental settings ........................................................... 99

5.1.3 Flow-like mass movements in the Campania region ....... 101

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5.2 The study area “Pizzo d’Alvano massif” .......................................... 104 5.2.1 The May 1998 event ............................................................. 104 5.2.2 The scientific debate ............................................................. 109

5.2.3 Remarks on used approaches .............................................. 123 6 LANDSLIDES CHARACTERIZATION IN THE STUDY AREA ...... 127

6.1 Geological aspects ................................................................................ 128 6.1.1 General features..................................................................... 128

6.1.2 Hillslope models .................................................................... 136 6.2 Triggering mechanisms ........................................................................ 139

6.3 Geotechnical data set ........................................................................... 151 6.3.1 In-situ conditions .................................................................. 151 6.3.2 Soil mechanical properties ................................................... 156

7 GEOMECHANICAL MODELLING IN THE STUDY AREA ............ 163 7.1 Insights towards modelling ................................................................. 164

7.2 M1: A bedrock outlet induced triggering mechanism .................... 167 7.2.1 Site-scale analyses by limit equilibrium method ............... 168

7.2.2 Massif-scale analyses by limit equilibrium method .......... 171 7.2.3 Massif-scale analyses by finite element method ............... 177 7.2.4 Remarks on modelling .......................................................... 179

7.3 M2: Triggering mechanisms for triangular shaped landslides ....... 184 7.3.1 Upper outlets induced phenomena .................................... 186

7.3.2 Impact-induced phenomena ................................................ 190 7.3.3 Remarks on modelling .......................................................... 197

7.4 M3: A track induced triggering mechanism ..................................... 200 7.4.1 Insights towards massif-scale analyses ............................... 202

7.4.2 Massif-scale analyses ............................................................. 204 7.4.3 Remarks on modelling .......................................................... 208

8 CONCLUDING REMARKS .......................................................................... 211

8.1 The lesson from the case study .......................................................... 211 8.2 Future developments ........................................................................... 217

REFERENCES ......................................................................................................... 219 APPENDIX A Landslide classifications ............................................................... 237

APPENDIX B Volcanism in the Campania region ............................................ 247 APPENDIX C May 1998 flow-like mass movements........................................ 257 APPENDIX D Used geomechanical methods .................................................... 265

3

INDEX OF FIGURES AND TABLES

Figure 2.1 Scheme of a volcanic eruption (Myers et al., 2004)............................. 25 Figure 2.2 Explosivity index (Kobayashi & Okuno, 2002) ................................... 25 Figure 2.3 Some examples of explosive eruptions: a) 1996 Popocatepelt

(Mexico) (Granados et al., 2000), b) 1990 Radoubt (Alaska)

(Nael et al., 1997) ..................................................................................... 26

Table 2.4 Examples of explosive eruptions (Sigurdson et al., 2000) ................... 26 Figure 2.5 Examples of hysopach from (a) Fuego volcano (Guatemala),

(b) Spurr volcano (Alaska), (c) Nebraska (United States) (Riley

et al., 2003) ................................................................................................ 27

Figure 2.6 Ash transported by wind (Neal et al., 1997) ......................................... 28 Figure 2.7 Volcanic ash fall from some ancient and modern eruptions in

the western United States (Kenedi et al., 2000) .................................. 28

Figure 2.8 Pyroclastic ejecta: a) ash, b) lapilli, c) blocks and bombs d)

pumice ....................................................................................................... 29

Figure 2.9 A scheme for soil formation process .................................................... 30 Figure 2.10 Early stages of soil formation process: five months after the

1991 Pinatubo volcano eruption (Newhall et al., 1991) .................... 31 Figure 2.11 Products of volcanic eruptions: (a, b) Volcanic clasts from

1992 Spurr eruption (Alaska), (c) Angular glass bubble-wall

shards from 1974 Fuego eruption (Guatemala), (d) Bubble-wall

shards from the Ash Hollow deposit in Nebraska (United

States) (Riley et al., 2003)........................................................................ 33 Figure 2.12 SEM photographs showing (a) the surface of a flow pumice

and (b) its internal voids (Esposito & Guadagno, 1998). ................. 34 Figure 2.13 Bulk density versus humus (Nanzjo, 2002) ........................................ 35

Figure 2.14 Dry unit weight (a) and void ratio (b) for some pyroclastic soils

(Bilotta et al., 2005) ................................................................................. 36 Figure 2.15 Variability of some features for pyroclastic soils: a) grain size

distributions (Bilotta et al., 2005) b) porosity (Picarelli et al.,

2001), c) friction angle and d) cohesion (Crosta & Dal Negro,

2003) .......................................................................................................... 36 Figure 2.16 In situ negative pore pressures for some pyroclastic covers of

Southern Italy: a) at massif scale (Cascini & Sorbino, 2003);

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b,c) at site scale (respectively Scotto et al., 2005; Damiano,

2004) ........................................................................................................... 38 Figure 2.17 Direct shear tests on pyroclastic soils from Central and

Southern Itlay (Cecconi et al., 2005) ..................................................... 38 Figure 2.18 Volumetric collapse for some pyroclastic soils: a) Olivares et

al., 2003; b) Bilotta et al., in press .......................................................... 40 Figure 2.19 Undrained triaxial tests on pyroclastic soils from Southern

Italy (Picarelli, 2001) ................................................................................ 41 Figure 2.20 Results of undrained ring shear tests on pyroclastic soils from

Japan (Wang et al., 2002) ........................................................................ 41 Figure 2.21 Differences for undisturbed and remoulded pyroclastic soils:

a) saturated shear strength for some pyroclastic soils of

Southern Italy (Bilotta et al., 2005), b) soil water characteristic

curves for a volcanic soil located in Hong Kong (Ng & Pang,

2000) ........................................................................................................... 42 Figure 2.22 Grain size distributions of the Rio Blanco tephra deposit

versus distance from the Rincon de la Vieja volcano (Costa

Rica) (Kempter et al., 2000).................................................................... 43 Figure 2.23 In-situ conditions for some pyroclastic deposits in eastern

Mexico (Capra et al., 2003) ..................................................................... 44 Figure 2.24 Thickness of tephra deposits versus distance from volcanoes

in western United States (Hoblitt et al., 1987) .................................... 44 Figure 2.25 Worldwide diffusion of volcanoes (http://terra.rice.edu/

plateboundary) .......................................................................................... 45 Figure 2.26 Worldwide diffusion of pyroclastic soils according to: a) WRB

Classification (FAO et al., 1998), b) US Soil Taxonomy (Soil

Survey Staff, 1999) ................................................................................... 46 Table 2.27 Common uses of pyroclastic soils (Sigurdson et al., 2000) ................ 47

Figure 2.28 Hazard related to pyroclastic products: a) danger to aircraft

along North Pacific air routes, b) roof collapse, c) pollution

(Nael et al., 1997) ..................................................................................... 49 Figure 3.1 Catastrophic landslides in Italy (Canuti, 2000) ..................................... 52

Figure 3.2 Flow-like mass movements in the world (Perov et al., 1997) ............ 52 Figure 3.3 Scheme for a flow-like mass movement (modified by Fell et al.,

2000 and Hungr et al., 2001) .................................................................. 54

Figure 3.4 Intensities of flow-like mass movements in volcanic soils and

non (Crosta et al, 2005) ........................................................................... 56

Figure 3.5 Recurrence of landslides due to: a) volcanic activity (Guida,

2003), b) weathering (Glade et al., 2005). ............................................. 56

5

Figure 3.6 Panoramic view of some flow-like mass movements occurred

in Sierra Norte (Mexico) on October 1999 (Capra et al., 2003) ...... 58 Figure 3.7 Spatial distribution of several rainfall induced flow-like mass

movements in Japan (Chigira et al., 2002) ........................................... 59 Figure 3.8 Some triggering mechanisms for some rainfall induced flow-like

mass movements in Japan (Chigira et al., 2002) ................................. 60 Figure 3.9 Earthquake-induced flow-like mass movements in Japan

(Fukuoka et al., 2004).............................................................................. 62 Figure 3.10 Weathering-induced flow-like mass movements in Japan

(Yokota et al., 1999) ................................................................................ 64 Figure 3.11 Man-made induced flow-like mass movements in Italy (Cairo

& Dente, 2003) ........................................................................................ 65

Figure 3.12 A huge earthquake-induced flow-like mass movement from El

Salvador (Las Colinas): different proposed classifications ................ 67

Figure 3.13 Huge rainfall-induced flow-like mass movements from

Southern Italy: different proposed classifications .............................. 68

Table 3.14 A comparison of different landslide classifications with

reference to flow-like mass movements .............................................. 69 Figure 4.1 Some triggering mechanisms for rainfall-induced phenomena ......... 73

Figure 4.2 Models for rainfall-induced triggering mechanisms ............................ 74 Figure 4.3 Some examples of black-box models: rainfall intensity/duration

thresholds for shallow landslide (Terlien, 1998) ................................. 75 Figure 4.4 An example of geological models over large areas at scale

1:100000 (Moreiras, 2005) ...................................................................... 77 Figure 4.5 Examples of physical models: a) centrifuge tests (Take et al.,

2004), b) flume tests (Eckersley et al., 1990) ....................................... 79 Figure 4.6 Examples of physical models: a) full-scale flume test (Morikawi

et al., 2004), b) tests on natural slope (Ochiai et al., 2004) ............... 81

Figure 4.7 Some examples of physically-based model: comparison between

the results obtained through the SHALSTAB and TRIGRS

codes (Savage et al., 2004) ..................................................................... 83 Figure 5.1 Pyroclastic soils and some recent flow-like mass movements in

Campania region (Southern Italy) (Cascini, 2004).............................. 92 Figure 5.2 The Somma-Vesuvius volcano ............................................................... 93 Figure 5.3 Eruptive activity of Vesuvius volcano................................................... 95

Figure 5.4 Areas covered by pyroclastic products of Vesuvius volcano

during the eruptions occurred (a) 18000 years ago, (b) 8000

years ago, (c) 3800 years ago, (d) 79 A.D., (e) 1631 (Cioni et al.,

1999) .......................................................................................................... 96

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Figure 5.5 Pyroclastic soils of Vesuvius volcano originated by (a) air-fall

deposition and (b) flow deposition ....................................................... 97 Figure 5.6 Stratigraphy of some pyroclastic deposits originated from the

472 AD Vesuvius eruption (Mastrolorenzo et al., 2002) ................... 98 Figure 5.7 Pyroclastic soils over different settings in Campania region: 1)

Isopach lines of the pyroclastic products; 2) carbonate bedrock;

3) tuff and lava deposits; 4) flysch and terrigenous bedrock. ........... 99

Figure 5.8 Examples of different mountain basins involved by flow-like

mass movements: a) Sarno Mounts; b) Lattari Mounts ...................100

Figure 5.9 Victims caused in Campania region during the last centuries

(O.U. 2.38, 1998) ....................................................................................101 Figure 5.10 The study area (Pizzo d’Alvano massif) ............................................105

Figure 5.11 Rainfall recorded in the neighbourhood of Pizzo d’Alvano

massif .......................................................................................................106

Figure 5.12 Some examples of the flow-like mass movements occurred in

(a) Sarno, (b) Siano, (c) Bracigliano, (d) Quindici. ............................107

Table 5.13 Intensity of flow-like mass movements (data from Cascini,

2004) .........................................................................................................107 Figure 5.14 Triggered and total mobilised volumes (Cascini, 2004) ..................107

Table 5.15 Velocities of unstable masses and their effects (modified after

Faella & Nigro, 2003) ............................................................................108

Figure 5.16 Damages produced by the May 1998 event: a) traces on

external walls; b) failure of corner columns, c) maisonary

building impacted by a flow-like mass movement, d) failure of

ground floor columns and building translation (Faella et al.,

2003) .........................................................................................................109 Figure 5.17 Hydrological evaluation of rainfall, occurred before May 1998,

producing or not landslides in pyroclastic covers of Campania

Region (Rossi et al., 1998) ....................................................................110 Figure 5.18 Rainfall threshold in Campania region (De Vita & Piscopo,

2002) .........................................................................................................111 Figure 5.19 Mobility function for the Pizzo d’Alvano massif (Sirangelo &

Braca, 2004) ............................................................................................112 Figure 5.20 A geological model for the Pizzo d’Alvano massif (Pareschi et

al., 2000)...................................................................................................112

Figure 5.21 Main morphological and morphometric parameters of flow-

like mass movements (Di Crescenzo & Santo, 2005) ......................113

Figure 5.22 Settings for the main source area types for 1998 landslides

(Guadagno et al., 2005) .........................................................................114

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Figure 5.23 Some geomorphological schemes for Pizzo d’Alvano massif:

1) summital paleosurfaces landscape, 2) border slopes, 3)

piedmont zones, 4) plains (Brancaccio et al., 1999) ........................ 116

Figure 5.24 Predicted time series of stored/mobilised sediment and related

process in ZOB (a) and in a typical first/second-order channel

(b) of the Pizzo d’Alvano massif from 4000 years b.p. (Guida,

2003) ........................................................................................................ 116

Figure 5.25 Hydrogeological factors for the Pizzo d’Alvano massif (De

Vita & Piscopo, 2002) ........................................................................... 117

Figure 5.26 Hydro-geomorphological model for the Pizzo d’Alvano massif

(modified form Cascini et al., 2000) ................................................... 118 Figure 5.27 Physical-based models for rainfall threshold in the Campania

region (Fiorillo & Wilson, 2004) ......................................................... 119 Figure 5.28 Findings of some physically-based models (Frattini et al., 2004) . 120

Figure 5.29 Some schemes for geotechnical analyses performed by: a)

Cascini et al. (2003), b) Guadagno et al. (2003), c) Crosta & Dal

Negro (2003) .......................................................................................... 121 Figure 5.30 Different classifications for May 1998 flow-like mass

movements ............................................................................................. 122

Figure 5.31 A scheme for the study area: the geo-environmental system ........ 123 Figure 5.32 Followed multidisciplinary approach combining geology,

geotechnical engineering and geomechanics ..................................... 124 Figure 6.1 Pizzo d’Alvano massif: basin involved (1) and not involved (2)

into the May 1998 events (Cascini et al., 2004) ................................ 128 Figure 6.2 1 Simplified geological map of Pizzo d’Alvano massif (1:25000):

1) dolomized limestones, 2) microcrystalline limestone 3)

calcarenites and calcilutites, 4) calcarenites, 5) calcarenites and

calcilutites, 6) dolomized limestones, 7) calcarenites and

calcilutites (modified after Cascini et al., 2006) ................................ 129 Figure 6.3 Simplified geomorphological map of Pizzo d’Alvano massif

(1:25000): 1) colluvial deposits in karst areas, 2) zero order

basins, 3) ancient landslides zones, 4) alluvial fans (modified

after Cascini et al., 2006). ..................................................................... 131 Figure 6.4 Stratigraphical and hydrogeological elements within the

limestone bedrock (modified after Cascini et al., 2006). ................. 132

Figure 6.5 Structural and hydro-structural conditions for Pizzo d’Alvano

massif: a) schematic geological section, b) scheme of an

hydrowedge, c) summital portion of an hydrowedge (modified

after Cascini et al., 2006) ...................................................................... 133

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Figure 6.6 Pizzo d’Alvano massif: hydro-structural map (modified after

Cascini et al., 2006) ................................................................................134 Figure 6.7 Pyroclastic cover thickness map: a) h=0 m, b) h<0.5 m, c)

0.5m<h<1m, d) 1m<h<2m, d) 2m<h<5m, e) h<5m

(modified after Cascini et al., 2006) ....................................................135

Figure 6.8 Geological hillslope models: A) Sarno model, B) Quindici

model, C) Bracigliano model, 1) summit, 2) nose, 3) inner

gorge, 4) talus-debris slope, 5) alluvial fan (a. ancient, b. recent),

6) main channel, 7) transient channel, 8) head valley, 9) zob,

10) litho-morphological scarp. .............................................................137 Figure 6.9 Simplified “morphological map”: 1) concavities, 2) open slopes,

3) gullies ...................................................................................................139

Figure 6.10 Different source areas detected in-situ with elongated (a),

pseudo-triangular (b), anastomised (c), graped (d), linear (e, f)

shapes. ......................................................................................................140 Figure 6.11 Relevant factors for triggering mechanisms .....................................141

Figure 6.12 Triggering mechanisms for the study area ........................................142 Figure 6.13 Distribution of triggering mechanisms inside Pizzo d’Alvano

massif .......................................................................................................144

Figure 6.14 Some examples of triggering mechanisms induced by bedrock

outlets according to: a) Johnson & Sitar (1990) and b)

Montgomery et al. (1997). .....................................................................145 Figure 6.15 Examples of triggering mechanisms induced by upper outlets:

a) Guadagno et al. (2005), b) Budetta & de Riso (2004) ..................146 Figure 6.16 Examples of triggering mechanisms induced by impact of

unstable soil masses (Sassa et al., 1997) ..............................................147 Figure 6.17 Examples of “avalanche” phenomena for (a) snow (Jamieson

& Stethem, 2002), and (b) for volcanic ashes on Mars (Gerstell

et al., 2004) ..............................................................................................147 Figure 6.18 Some effects of mountain roads during storms: a) preferential

corridors for water and sediment, b) run-off phenomena from

an upslope track, c) network of gullies and rills, d) road runoff

(Siedle et al., 2004) .................................................................................149 Figure 6.19 Some examples of gullies evolution through erosion and

landslide: a) Vandaele et al. (1996), b) Futai el al. (2004). ................150

Figure 6.20 An example of in-situ investigations for the hillslopes facing

the town of Bracigliano (Cascini, 2004) .............................................151

Figure 6.21 Location of the in-situ investigations inside the study area ...........152

9

Figure 6.22 An example of different topographical reconstruction for a

basin facing the town of Sarno: a) DTM (1:5000), b) elevation

contour lines (1:2000) (modified after Cascini, 2004)...................... 153

Figure 6.23 Typical stratigraphical profiles (Cascini & Sorbino, 2003)............. 153 Figure 6.24 In-situ soil suction measurements (Cascini & Sorbino, 2003) ...... 154

Figure 6.25 Pyroclastic soils from the sample area .............................................. 156 Table 6.26 Available data set ................................................................................... 156

Figure 6.27 Grain distribution size (Bilotta et al., 2005) ..................................... 157 Figure 6.28 Soil water characteristic curves (Bilotta et al., 2005; Sorbino &

Foresta, 2002) ......................................................................................... 157 Figure 6.29 Results of some direct shear tests on unsaturated pyroclastic

soils (Bilotta et al., 2005) ...................................................................... 158

Figure 6.30 Peak shear strength versus net vertical stress for different

saturation degree (Srf): a) soils class “A”, b) soils class “B”

(Bilotta et al., 2005) ............................................................................... 159 Figure 6.31 Shear strength with suction (Bilotta et al., 2005) ............................. 160

Figure 6.32 Suction controlled triaxial tests at different net vertical stress

and suction (Bilotta et al., 2005) .......................................................... 161 Figure 6.33 Suction effect on the stiffness: (a) triaxial modulus E50; (b)

oedometer modulus Eed (Bilotta et al., in press) ............................... 161 Figure 7.1 Methods utilised for the geomechanical modelling .......................... 164

Table 7.2 Parameters utilised for the geomechanical modelling ........................ 165 Figure 7.3 Simulated in-situ stress conditions: a) open slope, b) concavity ..... 166

Figure 7.4 In-situ evidences for triggering mechanism “M1”: examples

from the basins n. 1 (a), 14 (b) facing the town of Sarno ............... 167

Figure 7.5 Reference schemes for the triggering mechanism “M1” ................. 168 Figure 7.6 Tuostolo sample basin: a) plan view, b) stratigraphical

reconstruction ........................................................................................ 169

Figure 7.7 In-situ measured rainfall and bedrock outlet flows assumed as

hydraulic boundary conditions ............................................................ 169

Figure 7.8 Simulated pore water pressure regime inside the slope section

of Figure 7.6. .......................................................................................... 170

Figure 7.9 Simulated unstable areas for different cases, considering (a) and

not considering (b) the presence of the bedrock outlet .................. 171 Figure 7.10 Computational schemes at massif scale (numbers specify the

depths in metres) ................................................................................... 172 Figure 7.11 Pore water pressure induced by rainfall for some schemes (1, 2,

8) among those depicted in figure 7.10. ............................................ 173

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Figure 7.12 Pore water pressure regime induced by rainfall and bedrock

outlet ........................................................................................................174 Figure 7.13 Ratio of the shear strength to the shear stress acting on planes

parallel to the ground surface ...............................................................175 Figure 7.14 Slope instability scenarios simulated through limit equilibrium

method .....................................................................................................175 Figure 7.15 Slope safety factor reduction versus pore water pressure

increase ....................................................................................................176 Figure 7.16 Deformations and effective vertical stress variations induced

by a bedrock outlet for the scheme 1 of Figure 7.10 .......................177 Figure 7.17 Pore water pressure and plastic strains induced by a bedrock

outlet ........................................................................................................178

Figure 7.18 3D analysis of plastic strains induced by a bedrock outlet .............179 Figure 7.19 Findings of the hydro-geomorphological model set up by

Cascini et al. (2000) ................................................................................180 Figure 7.20 Instability scenarios outlined for the basin N. 3, with

SHALSTAB code: (1) ZOBs, (2) observed and (3) simulated

source areas .............................................................................................180 Figure 7.21 Possible slope instability scenarios related to different

stratigraphical settings ...........................................................................181 Figure 7.22 Slope instability scenarios outlined with distinct methods: a)

limit equilibrium analysis; b) uncoupled FEM analysis; c)

coupled FEM analysis ...........................................................................181

Figure 7.23 Comparison of computed pore water and vertical effective

stress outlined with (a) uncoupled and (b) hydro-mechanical

coupled approaches ...............................................................................182 Figure 7.24 Flow-like mass movements analysed through physical models:

a) centrifuge tests (Take et al., 2004), b) flume tests (Eckersley,

1990) .........................................................................................................183 Figure 7.25 In-situ evidences for triggering mechanism “M2”: examples

from the basin n. 3 (a) and basin n. 9 (b) of Pizzo d’Alvano

massif .......................................................................................................184

Figure 7.26 Some relevant in-situ evidences: a) karstic conduct (Guadagno,

2005); b) impact induced landslides; c) impact not causing

failure .......................................................................................................185

Figure 7.27 Reference schemes for the triggering mechanism “M2” ................185 Figure 7.28 Computational schemes for the triggering mechanism “M2”:

a) water inflow; b) hydrostatic pore water pressure .........................186

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Figure 7.29 Pore water pressure increase due to upper water inflows for

different stratigraphical settings. ......................................................... 187 Figure 7.30 Pore water pressure increase due to fracture filling phenomena

for different stratigraphical settings: a) deep fracture, b)

superficial fracture ................................................................................. 187

Figure 7.31 Slope stability conditions computed via Limit Equilibrium

Method: a) water inflow, b) fracture filling ....................................... 188

Figure 7.32 Effects of upper outlet (a) and fracture filling (b) simulated

through uncoupled stress-strain analyses .......................................... 189

Figure 7.33 Schemes for loading impact over still in place soils ........................ 190 Figure 7.34 Drained loading versus safety factor ................................................. 191 Figure 7.35 FEM analyses for drained loading a) case 1, b) case 2 ................... 192

Figure 7.36 Typical slope stability scenarios due to an undrained impact: a)

before, b) just after load application, c) after rapid load removal.. 193

Figure 7.37 Effects of an undrained loading impact over mean effective

stresses computed through an uncoupled approach ....................... 193

Figure 7.38 Pore water pressures and plastic strains computed through an

hydro-mechanical coupled approach ................................................. 194 Figure 7.39 3D effects induced by an undrained impact loading over an

open slope ............................................................................................... 194 Figure 7.40 Stress and strains evolution, in a plane parallel to the ground

surface, due to load applied at the upper limit of an open slope ... 195

Figure 7.41 Instability scenarios for different initial lateral stress y –

increasing from case (a) to (c) – .......................................................... 196 Figure 7.42 Geotechnical analyses for pore water pressures induced by

upper outlets in some pyroclastic covers (Calcaterra et al., 2004) . 197 Figure 7.43 Correlation between scarps and source areas (modified after

Guadagno et al., 2005) .......................................................................... 198


from the basins n. 35 (a), 36 (b), 38 (c) facing the town of

Quindici................................................................................................... 200 Figure 7.45 Differences between natural scarps (a) and artificial cuts (b) ........ 201

Figure 7.46 Complex environmental setting in the basin n. 35 facing the

town of Quindici: 1) man-made elements, 2) bedrock scarp ......... 201

Figure 7.47 Reference schemes for the triggering mechanism “M3” ............... 202 Figure 7.48 Contributing area index: a) definition (Pack et al., 1998), b) an

example for an hillslope landscape (Tucker et al., 1998) ................. 202

12

Figure 7.49 Qualitative evaluation of different mountain roads

arrangements through the evaluation of the topographic index ....203 Figure 7.50 Superficial and groundwater circulation related to the presence

of a mountain road (Ziegler et al., 2004) ............................................204 Figure 7.51 Considered stratigraphical schemes ...................................................205

Figure 7.52 Computational schemes for zones (a) corresponding and (b)

contiguous to the bends of trackway ..................................................205

Figure 7.53 Pore water pressure induced by rainfall and a local ponding

condition over different stratigraphical schemes ..............................206

Figure 7.54 Limit equilibrium analyses for the mechanism “M3” in

correspondence the bands of trackways ............................................206 Figure 7.55 Limit equilibrium analyses for the mechanism “M3” for zones

contiguous to the bends of trackways ................................................207 Figure 7.56 Displacement field obtained through uncoupled FEM analyses ..207

Figure 7.57 Displacement versus time for the unstable mass .............................208 Figure 7.58 Geotechnical analyses for slope instability inside upwards slope

at morphological scarps (Guadagno et al., 2003) ..............................209 Figure A.1 Morphological landslide classification (Blong, 1973) .......................238 Figure A.2 Slope movement types and processes (Varnes, 1978) ......................238

Figure A.3 Geotechnical classification (Hutchinson, 1988) ................................239 Figure A.4 Landslide types and processes (Cruden & Varnes, 1996) ...............240

Figure A.5 Glossary for forming names of landslides (Cruden & Varnes,

1996) .........................................................................................................241

Figure A.6 Slope movements: a) scheme for landslide characterization, b)

different stages, c) involved materials (Leroueil et al., 1998) ..........242

Figure A.7 Classification of flow-like mass movements (Hungr et al., 2001) ..243 Figure A.8 Classification of flow-like mass movements: a) debris flow, b)

flow slides, c) rock avalanches, d) mudslides (Hutchinson,

2004) .........................................................................................................244 Figure B.1 Main volcanic complexes in the Campania region (Southern

Italy): 1) Vesuvius, 2) Phlegrean fields, 3) Roccamonfina ..............247 Figure B.2 Roccamonfina volcanic complex .........................................................249

Figure B.3 The Phlegraean Fields (Rosi et al., 1999; DeVita et al., 1999) .........251 Figure B.4 Diffusion of soils originated by the Phlegraean Fields (Rosi et

al., 1999)...................................................................................................252

Figure B.5 Stratigraphic conditions of soils originated by the Phlegraean

Fields (DeVita et al., 1999) ...................................................................252

Figure B.6 Soil originated by the Phlegraean Fields (Rosi et al., 1999) .............253 Figure B.7 The Somma-Vesuvius complex............................................................253

13

Figure B.8 The Mount Somma: a) inner view, b) outside view ......................... 254 Figure B.9 Historical evolution of Vesuvius volcano due to its explosive

activity (Cioni et al., 1999) .................................................................... 256

Figure C.1 The 4-5 May 1998 meteoric event (data from Meteosat) ................ 258 Figure C.2 Measured rainfall values at different rain gauges (Cascini et al.,

in press) ................................................................................................... 259 Figure C.3 Pizzo d’Alvano massif: basin involved (1) and not involved (2) .... 260

Figure C.4 Examples of flow-like mass movements .......................................... 261 Figure C.5 Different estimation of total mobilised volumes .............................. 261

Figure C.6 Examples of detailed evaluation of mobilised volumes ................. 261 Figure C.7 Estimation of unstable masses velocity (Faella & Nigro, 2003) ..... 262 Figure C.8 Victims caused by flow-like mass movements .................................. 263

Figure C.9 Examples of damages induced by flow-like mass movements in

(a) Sarno, (b) Bracigliano, (c) Siano. ................................................... 264

14

ABSTRACT

Flow-like mass movements are among the most severe natural hazards,

and they often cause catastrophic consequences when involving pyroclastic

soils that are widespread all over the world, also in very high populated regions.

In the last decades, these soils received great attention due to their peculiar

mechanical features and to the occurred flow-like mass movements whose

characteristics significantly differ according to regional, seasonal and local

features, soil properties and boundary conditions.

Generally, their analysis is based on both classifications and models aimed

to capture, under different assumptions and simplifications, the essential

features of the triggering, post-failure and propagation stages. However,

available classifications do not always provide a unique conceptual framework

for the above phenomena and sector-based approaches often introduce drastic

simplifications eventually disregarding relevant factors.

Due to the variety of flow-like mass movements, the attention is focused

on rainfall-induced phenomena and, in particular, on the modelling of

triggering mechanisms to be considered a fundamental step towards the

landslides hazard assessment. At this regard, several valuable approaches are

available and they can be usefully classified into black-box, geological, physical,

physically based and geotechnical models. Anyway, these models must be

systematically checked through the application to well documented case

histories, whose back-analyses represent an important step for the setting up of

more enhanced computational tools.

To this aim, a study area from Southern Italy is addressed, where in May

1998 huge flow-like mass movements caused 160 victims. The occurred

landslides were among the most catastrophic in the last centuries and a wide

literature is available. Anyway, several interpretations were proposed using

different sector-based approaches with several yet open questions.

Within this scientific debate, the thesis particularly address the relevant

triggering factors and mechanisms. To provide a significant contribution, a

15

multidisciplinary approach is used because it seems the only one that allows to

analyse the occurred phenomena at both massif, site and REV scales.

Therefore, on the basis of an extensive data set, concerning both in-situ

conditions and soil properties, an accurate landslides characterization is

performed in order to achieve insights on the triggering mechanisms. These last

are recognised as strictly related to the bedrock morphological and hydro-

geological features, as well as to the past and actual processes involving

pyroclastic covers. In particular, from the analysis of the available information,

six typical triggering mechanisms are recognised and mapped all over the study

area. They are characterised by different landforms and intensity in terms of

mobilised volume and travel distance; moreover, they are not casually

distributed on the massif.

Then, geomechanical modelling is performed for the most relevant

triggering mechanisms. The analyses are carried on the basis of a saturated-

unsaturated groundwater modelling whose results are used to outline significant

slope instability scenarios. In particular, the proposed triggering mechanisms

are validated with the aid of limit equilibrium method, uncoupled and hydro-

mechanical coupled stress-strain analyses.

The obtained results are compared with the findings of other models and

discussed within the available literature. Certainly, multidisciplinary approaches

must to be strongly encouraged since they also allow some general remarks for

rainfall-induced flow-like mass movements, whose analysis is an urgent need

due to the diffusion and catastrophic consequences of these phenomena.

16

SOMMARIO

Le colate rapide rappresentano alcuni tra i più rilevanti pericoli natuali e

risultano particolarmente distruttive quando coinvolgno i terreni di origine

vulcanica che sono diffusi nel mondo, anche in aree fortemente urbanizzate.

Negli ultimi decenni, questi suoli hanno ricevuto una crescente attenzione per le

loro peculiari proprietà meccaniche e per i fenomeni franosi che in essi hanno

sede, le cui caratteristiche possono significativamene cambiare in relazione a

fattori regionali, stagionali e locali, alle proprietà dei suoli ed alle condizioni al

contorno.

Generalmente, l’analisi di tali fenomeni è basata sia sulle classifiche che su

modelli che riproducono, sulla base di differenti assunzioni e semplificazioni, le

caratteristiche essenziali delle fasi di innesco, post-rottura e propagazione.

Comunque, le classifiche disponibili non sempre delineano un quadro di

riferimento unitario ed approcci monodisciplinari spesso introducono drastiche

semplificazioni che finiscono per trascurare aspetti talora rilevanti.

Per la molteplicità dei fenomeni di colata, la tesi si concentra sui fenomeni

indotti da eventi meteorici ed, in particolare, sulla modellazione dei meccanismi

di innesco che deve ritenersi un passo fondamentale per la valutazione della

pericolosità da frana. A tal rigurado, numerosi e validi approcci sono disponibili

e possono essere utilmente classificati in modelli a scatola chiusa, geologici,

fisici, fisicamente basati e geotecnici. Comunque, tali modelli devono essere

sistematicamente validati attraverso la loro applicazione a casi di studio ben

documentati, per i quali le analsi a ritroso costitusicono un passo importante

per lo sviluppo di più avanzati strumenti computazionali.

A tal fine, si è presa in esame un’area dell’Italia meridionale, dove nel

maggio 1998 importanti colate produssero 160 vittime. Tali eventi risultarono

tra i più catastrofici degli utlitmi secoli ed una ampia letteratura è disponibile al

riguardo. Tuttavia, le numerose interpretazioni dei fenomeni occorsi non hanno

ancora fornito una risposta conclusiva ed il dibattito scientifico risulta

particolarmente serrato e ricco di spunti.

17

Nell’ambito di tale dibattito si inquadra la tesi che affronta, in particolare, i

meccanismi di innesco, tema per il quale si è ritenuto di dover privilegiare un

approccio multisciplinare, l’unico in grado di consentire una analisi dei

fenomeni occorsi a scala di massicio, locale e di elemento di volume (REV).

Sulla base di una rilevante banca dati riguardante sia le condizioni in sito

che le proprietà dei terreni, si è quindi condotta una accurata caratterizzazione

delle frane finalizzata all’inividuazione dei meccanismi di innesco che sono

risultati strettamente legati alle caratteristiche morfologiche ed idrogeologiche

del substrato ed ai processi passati e recenti che coinvolgono le coltri

piroclastiche. In particolare, l’analisi di tutte le informazioni disponibili ha

consentito di individuare e cartografare sei principali meccanismi di innesco che

differiscono per caratteristiche geomorfiche, entità dei volumi mobilitati e

distanze percorse dalle masse instabili, risultando distribuiti non casualmente

all’interno del massiccio.

Successivamente, per i principali meccnaismi di innesco, si è condotta una

modellazione (saturo-parzialmente saturo) del regime delle acque sotterranee

che ha fornito gli elementi di partenza per le analisi di stabilità che si sono

svolte con i metodi dell’equilibrio limite e con modelli tensio-deformativi, anche

considerando l’accoppiamento idro-meccanico tra le fasi costituenti il terreno.

I risultati conseguiti sono stati confrontati con le risultanze di altri modelli

ed inqaudrati nell’ambito della letteratura disponibile. Il confronto sembra

fortemente incoraggiare il ricorso all’approccio multidisciplinare che consente,

tra l’altro, alcune considerazioni di carattere generale per la modellazione delle

colate indotte da eventi meteorici, la cui analisi riveste una rilevanza notevole

per la diffusione di tali fenomeni le cui conseguenze sono spesso catastrofiche.

18

ACKNOWLEDGEMENTS

To finish a work is the beginning of acknowledging people for their help

along the path and I would need much more than few words

First of all, thanks to prof. Leonardo Cascini that “triggered” my interest

for Geotechnics and environmental issues. His constant and confident advises

encouraged and accompanied me towards fascinating research challenges.

“Muchas gracias” to prof. Manuel Pastor, head of the “Sector de

Ingeniería Computacional” (CEDEX, Madrid), for the important thematic

discussions on numerical modelling. His contagious enthusiasm was really

stimulating to develop the research topics.

Special thanks to prof. Giuseppe Sorbino for the valuable suggestions and

interesting discussions about Soil Mechanics. His friendly support was always

“effective” and significantly improved this work.

Thanks to prof. Domenico Guida that turned my eyes to the understanding

of natural landscapes. Learning from environment is always appealing.

Thanks to the friends of the Civil Engineering Department, Settimio,

Michele, Vito, Carlo, Vincenzo, Alessandra, Pasquale for the shared experiences.

Thanks to prof. Pablo Mira for his kind availability and to the researchers

of “Centro de Estudio y Tecnica Aplicada” (CEDEX), Maribel, Elena,

Bouchra, Valentina for their friendly support during my stay in Madrid.

Special thanks to Josè Antonio Fernandez Merodo for his advices on

computing issues and his infinite helping hand for finite element analyses.

I am fully grateful to my family, for their unreserved support in everything

I do. Thanks to my father that would be proud to see these happy days. Thanks

to my mother that always encourages me in pursuing my wishs. Thanks to Don

Biagio for his support and reproaches, always a true blessing from the sky.

19

ABOUT THE AUTHOR

Sabatino Cuomo graduates in Civil Engineering at the University of

Salerno with 110 cum laude/110 and a special mention of the Jury. During the

PhD course he develops research topics related to the analysis and modelling of

the triggering for flow-like mass movements. These phenomena are worldwide

diffused and often catastrophic when involving pyroclastic soils. To deepen

these issues, he undertakes training and research activities at the University of

Salerno and in leading national and international research Institutions such as

CUGRI (Consorzio inter-Universitario per la Previsione e Prevenzione dei

Grandi RIschi), CERIUS (Centro di Eccellenza per la previsione e prevenzione

del Rischio idrogeologico dell’Università di Salerno), and CEDEX (Centro de

Estudio y Experimentacion de Obras Publicas, Madrid). In particular, the

activities performed at the CUGRI and CERIUS allowed to mainly deepen

environmental issues while at the CEDEX it was possible to use enhanced

mathematical models for the simulation of flow-like mass movements.

Sabatino Cuomo si laurea in Ingegneria Civile presso l’Università degli

Studi di Salerno con voto 110 e lode/110 e menzione speciale della

Commissione. Durante il Corso di Dottorato si dedica a tematiche di ricerca

connesse alla analisi e modellazione dell’innesco di frane veloci. Tali fenomeni

sono diffusi nel mondo e risultano spesso catastrofici quando coinvologno

terreni di origine vulcanica. Per approfondire tali argomenti, svolge attività di

formazione e ricerca presso l’Università di Salerno, e in prestigiosi Istituti di

ricerca nazionali ed internazionali, quali il CUGRI (Consorzio inter-

Universitario per la Previsione e Prevenzione dei Grandi RIschi), il CERIUS

(Centro di Eccellenza per la previsione e prevenzione del Rischio idrogeologico

dell’Università di Salerno), il CEDEX (Centro de Estudio y Experimentacion

de Obras Publicas, Madrid). In particolare, le attività svolte presso il CUGRI ed

il CERIUS hanno consentito l’approfondimento di rilevanti tematiche

territoriali, mentre quelle sviluppate presso il CEDEX hanno reso possibile il

ricorso ai più avanzati modelli matematici per la simulazione delle colate rapide.

20

1 INTRODUCTION

Flow-like mass movements involving volcanic soils are among the most

dangerous natural hazards, often causing catastrophic consequences, because of

their weak warning signals, long travel distances, high velocities and huge

involved volumes. Unfortunately, they are widespread all over the world and

can be triggered by several causes such as rainfall, earthquake, weathering,

human activities or their combination. Moreover, first and post-failure stages

can significantly differ according to regional, seasonal and local features,

triggering factors and mechanisms, soil properties and boundary conditions. As

a consequence, the understanding and analysis of such phenomena are an

urgent need and relevant efforts are necessary in order to obtain significant

results.

At the present, the analysis of flow-like mass movements is developed

with the aid of approaches based on both classifications and models aiming to

capture, under different assumptions and simplifications, the essential features

of the first-failure, post-failure and propagation stages. However, available

classifications do not always allow a unique conceptual framework for the

above phenomena and a sector-based modelling can disregard some relevant

factors, also with reference to a particular class of phenomena as, for instance,

those induced by rainfall.

The thesis is aimed to provide a contribution towards an advanced

analysis and modelling of rainfall-induced triggering mechanisms, which

represents a fundamental step in the landslide hazard assessment procedures.

In particular, Chapter 2 concerns the general features and geotechnical

characteristics of pyroclastic soils. Some discussions are also proposed on both

their worldwide diffusion and uses.

Chapter 3 deals with flow-like mass movements occurring in pyroclastic

soils that are usually characterised by intensities greater than those recorded in

21

non-volcanic soils. For these phenomena, their general features and

classifications are summarised and some relevant case histories are analysed.

Chapter 4 addresses the modelling of the triggering stage for these

phenomena with particular reference to those induced by rainfall. To this aim,

an overview is proposed about the main triggering mechanisms and models

provided by literature. Among the available approaches, the geomechanical

modelling is deepened with particular reference to the modelling of pore water

pressure and instability conditions.

Chapter 5 presents a relevant case study of flow-like mass movements in

pyroclastic soils induced by rainfall. The environmental conditions, the origin

and features of the involved soils are described in detail. Moreover, focusing

the attention on a sample area, the occurred phenomena are analysed and the

yet open questions are discussed. Referring to the studies available in literature,

the followed multidisciplinary approach is then presented.

Chapter 6 concerns the landslide characterisation inside the sample area,

obtained on the basis of an extensive data set concerning the in-situ conditions

and soils mechanical properties. Particularly, different triggering mechanisms

are proposed according to the available in-situ evidence.

Chapter 7 discusses the geomechanical analyses, performed through

different methods, in order to validate the detected triggering mechanisms. In

particular, for three of them, the numerical analyses justify the in-situ evidence

and outline the predisposition and triggering factors for all the occurred

phenomena.

Finally, in Chapter 8, on the basis of the results obtained for the sample

area, some general discussions are proposed within the general framework of

Chapters 2 – 4. Then, the research developments are presented and the

conclusions are outlined.

23

2 PYROCLASTIC SOILS

“In the afternoon my mother drew his attention to a cloud of

unusual size and appearance. The cloud was rising from a mountain,

at such a distance we couldn’t tell which, but afterwards learned that

it was Vesuvius. I can best describe its shape by likening it to a pine

tree. It rose into the sky on a very long "trunk" from which spread

some "branches". I imagine it had been raised by a sudden blast,

which then weakened, leaving the cloud unsupported so that its own

weight caused it to spread sideways. Some of the cloud was white, in

other parts there were dark patches of dirt and ash.”

from Letters of Pliny the Younger to the Historian Tacitus

Volcanoes are widespread all over the world and they are often feared for

their devastating eruptions that produce pyroclastic flows or clouds and the

ejection of volcanic materials over large areas.

Nevertheless, many volcanic regions are very populated due to the

peculiar properties of volcanic soils that, in the centuries, resulted in several

uses strengthening the practical and economic relationship between mankind

and volcanoes.

Among the products of volcanic eruptions, pyroclastic soils originate from

air-fall deposition of volcanic fragments. These soils, in the last decades, have

received an increasing attention, in the scientific literature, due to their special

features that are hereafter discussed at different scales also referring to their

origin and diffusion.

Chapter 2 ________________________________________________________________________________________________________________________________________

24

2.1 GENERAL FEATURES

2.1.1 Origin

Pyroclastic fragments are produced by three main processes consisting in

volcanic explosive eruption, wind transportation and air-fall deposition.

Explosive eruptions occur, during the magma rising, as a result of the

changes in pressure and high viscosity of magma, when dissolved volatile

components lead to the formation and subsequent explosion of bubbles. In

other case, under certain conditions, the transfer of heat from magma to water

can result in the explosive conversion of water to steam and to the

fragmentation of magma. In both cases, expanding gas violently shatters solid

rocks and magma that, after the ejection into the air, solify into fragments of

volcanic rock and glass (Myers, 2004) (Fig. 2.1). Generally, the concept of

volcanic explosivity index is utilised and two principal quantities are referred to:

magnitude (mass of material erupted) and intensity (eruption column height)

(Kobayashi & Okuno, 2002). In relation to these factors, different names have

been addressed to famous volcanic eruptions and are commonly used such as,

for instance, “hawaiian”, “strombolian”, “sub-plinian”, “plinian”, “vulcanian”

(Fig. 2.2, 2.3, Tab. 2.4).

During an explosive eruption, three main spatially overlapping regions can

be addressed: the jet region, the plume region and the umbrella-cloud region.

In the jet region, the material first emerges from the vent at high

velocities. For several hundreds to thousands of meters above the vent, the

eruption mode is essentially controlled by the source conditions related to

magma fragmentation process.

Once the magma is fragmented into small pieces, volcanic plumes are

produced, consisting in mixtures of volcanic molten or solid particles, gases,

and air. They are injected into the atmosphere, to a height determined by the

thermal flux, rising up to altitude as great as 50 km above the Earth’s surface,

and for periods up to some hours. The features of the plume region are

essentially controlled by the composition of erupted magma, the amount and

nature of volatile components, the rate of magma discharge, and the geometry

Pyroclastic soils ________________________________________________________________________________________________________________________________________

25

of the source vent. The plume height, in turn, controls the dispersal of volcanic

ejecta.

Inside the umbrella-cloud region, the eruption plume spreads laterally, in

response to atmospheric stratification and wind. In general, downwind plumes

seem to be oval in cross section, with their long axis horizontal. In plan view,

they often consist of downwind-elongated lenses of gas and ash, that become

detached from the volcano when the eruption ceases (Fig. 2.5).

Figure 2.1 Scheme of a volcanic eruption (Myers et al., 2004)

Figure 2.2 Explosivity index (Kobayashi & Okuno, 2002)

Chapter 2 ________________________________________________________________________________________________________________________________________

26

a) b)

Figure 2.3 Some examples of explosive eruptions: a) 1996 Popocatepelt (Mexico)

(Granados et al., 2000), b) 1990 Radoubt (Alaska) (Nael et al., 1997)

Description Volume of

ejected material

Plume Height

Eruption Type

Duration

Total eruptions Given this

VEI

Example VEI

Non-Explosive variable <100m Hawaiian variable 699 Kilauea (1983 to

present) 0

Small <.001 km3 100-

1000m Hawaiian/

Strombolian <1 hr 845 Nyiragongo (1982) 1

Moderate .001-.01 km3 1-5km Strombolian/

Vulcanian 1-6 hrs 3477 Colima (1991) 2

Moderate/Large .01-.1 km3 3-15km Vulcanian/

Plinian 1-12 hrs. 869 Galeras (1924) 3

Large .1-1 km3 10-

25km Vulcanian/

Plinian 1-12 hrs. 278 Sakura-Jima (1914) 4

Very Large 1-10 km3 >25 km

Plinian/ Ultra-Plinian

6-12 hrs. 84 Villarrica (1810) 5


Plinian/ Ultra-Plinian

>12 hrs. 39 Vesuvius (79 AD) 6


Ultra-Plinian >12 hrs. 4 Tambura (1812) 7

Very Large >1,000 km3 >25 km

Ultra-Plinian >12 hrs. 0 Yellowstone Caldera (2 million years ago)

8

Table 2.4 Examples of explosive eruptions (Sigurdson et al., 2000)

Wind transportation and rain-out of clasts through the atmosphere

produce pyroclastic air-fall deposits. These last form concave-upwards cone of

volcanic ejecta that thin away from a point of maximum thickness, usually close

to or coincident with the eruptive vent. Thickness of such deposits are

quantified through isopach lines that usually form more or less regular ellipses,

elongated in the downwind direction (Fig. 2.5, 2.6). In particular, the direction

http://www.pbs.org/wgbh/nova/vesuvius/deavolume.html



http://www.pbs.org/wgbh/nova/vesuvius/deaplume.html


http://www.pbs.org/wgbh/nova/vesuvius/deatype.html


http://www.pbs.org/wgbh/nova/vesuvius/deaduration.html

http://www.pbs.org/wgbh/nova/vesuvius/deatotal.html




http://www.pbs.org/wgbh/nova/vesuvius/deaexample.html

http://www.pbs.org/wgbh/nova/vesuvius/deavei.html

http://www.pbs.org/wgbh/nova/vesuvius/vei0.html










27

of elongation away from the vent defines the dispersal axis of the deposit.

Sometimes, different spatial distributions can be recognised, for instance, due

to the elongation of the umbrella cloud in different directions at different levels

in the atmosphere as well as because of changing wind directions during the

course of an eruption. Several examples of air-fall deposits are described in

literature, clearly highlighting the variability in both ejected volumes and

covered areas. For instance, the materials produced during the May 18, 1980

eruption of Mount st. Helens were deposited over a 57’000 square kilometres in

the Western United States (Fig. 2.6, 2.7).

Figure 2.5 Examples of hysopach from (a) Fuego volcano (Guatemala),

(b) Spurr volcano (Alaska), (c) Nebraska (United States) (Riley et al., 2003)

Inside the deposition areas, the ejected materials can have different sizes.

So, the terms blocks or bombs usually refer to the fragmental volcanic products

greater than 64 mm in size; particles smaller than 2 mm are normally referred to

as ashy particles; the intermediate class is referred to as lapilli. If a lapilli or

bomb size fragment contains abundant vescicles (gas-bubble cavities), it is

usually called pumice (Fig. 2.8).

Chapter 2 ________________________________________________________________________________________________________________________________________

28

Figure 2.6 Ash transported by wind (Neal et al., 1997)

Figure 2.7 Volcanic ash fall from some ancient and modern eruptions

in the western United States (Kenedi et al., 2000)


29

a) b)

c) d)

Figure 2.8 Pyroclastic ejecta: a) ash, b) lapilli, c) blocks and bombs d) pumice

Soon after the air-fall deposition, soil formation process begins, in relation

to five main factors such as parent material, climate, topography, vegetation and

time. The parent material of pyroclastic soils are the above described volcanic

clasts ejected during the eruption. Climate, in which these soils can form, vary

in a wide range as directly connected to the position of volcanic apparata.

Natural forces such as heat, rain, ice, snow, wind, sunshine, and other

environmental forces, break down parent material and affect the velocity of soil

formation processes.

As far as topographic conditions, pyroclastic soils generally overlay

mountainous or hilly areas (i.e. in the neighbouring of the volcanic apparata)

but can also mantle very large areas far from the source in a variety of

landscapes (also intermountain valleys, flat areas, etc.).

The location of soils inside a landscape can certainly affect the climatic

processes affecting them. Soils at the bottom of a hill will get more water than

soils on the slopes, and soils on the slopes that directly face the sun will be drier

than soils on slopes that do not. Also, mineral accumulations, plant nutrients,

Chapter 2 ________________________________________________________________________________________________________________________________________

30

type of vegetation, vegetation growth, erosion, and water drainage are

dependent on topographic relief. (Fig. 2.9).

Natural vegetation associated with pyroclastic soils is strictly related to

their common acidic characteristics and it can consist into forests (Pacific

Northwest and Alaska), dense grass (south central Alaska and Japan), conifer

forest or tundra vegetation, mixed vegetation of grass and forest (northern

California, central Washington, Oregon) shrubby or grass vegetation.

All of the above factors assert themselves over time, often hundreds or

thousands of years. Soil profiles continually change from weakly developed to

well developed over time. The formation process has different velocity in

relation to the climate and landscape stability. For instance, the erosional

dissection of an ash deposit at Pinatubo volcano in the Philippines created, in

only five months, the intricate pattern shown in Figure 2.10. Generally,

pyroclastic soils are found on young parent materials in humid climates; the

drier and/or cooler the environment, the longer it takes for such soils to form;

moreover the older and more stable the landscape, the more weathering will

have taken place (examples Japan, Alaska, etc.).

Figure 2.9 A scheme for soil formation process


31

Figure 2.10 Early stages of soil formation process: five months after

the 1991 Pinatubo volcano eruption (Newhall et al., 1991)

2.1.2 Soil taxonomy

Pyroclastic soils did not receive the necessary attention among the soil

scientists until the middle of 20th century when they were included, for the first

time, in an international soil classification system (Takahashi & Shoji, 2002).

Nowadays, these soils are referred to as andisoils in the US Soil Taxonomy (Soil

Survey Staff, 1999) and as andosols in the WRB Classification (FAO et al.,

1998).

The names “andosols” and “andisols” are derived from “Ando soils”

whose etymology is dark (An) and soils (do) in Japanese, connotative of soils

with thick, dark surface horizons and acid in reaction. Anyway, the literature

provide other terms such as, for instance, pyroclastic soils, tephra, andosols,

andisoils, ashy soils, or simply volcanic soils.

The term “pyroclast” is derived from Greek “pyro”, meaning fire, and

“klastos”, meaning broken; thus pyroclastic carry the connotation of "broken

by fire" in relation to the magma fragmentation process.

Chapter 2 ________________________________________________________________________________________________________________________________________

32

The term “tephra” (Greek, for ash) was introduced by Thorarinsson

(1944, 1954) to describe volcanic ash and coarser detritus that were projected

through the air. It usually refers to particles erupted into the air and fallen back.

The term “ashy soils” is commonly used to denote the same soils. This

term is not to be confused with “volcanic ash” or tephra that instead indicate

rocks pulverized into dust (or sand) by volcanic activity not yet experiencing

any soil formation process. Anyway, volcanic ash is not the product of

combustion, it is hot near the volcano, but it is cool when it falls at greater

distances; it is hard, extremely abrasive and quite corrosive, and it does not

dissolve into the water.

Finally, such soils are often referred to as “volcanic soils”, without any

more information or, conversely, they are addressed with terms related to

typical particle size of the deposits.

In the thesis, air-fall volcanic soils are referred to as “pyroclastic soils”

including ashy and pumice soils.

2.2 GEOTECHNICAL CHARACTERISTICS

Due to their origin, pyroclastic soils are characterised by peculiar features

at micro, REV (Representative Elementary Volume) and macro scales. Here,

only some features are reviewed that strongly affect the mechanical behaviour

and the uses of these soils.

2.2.1 Micro scale

As far as the micro scale characteristics, it can be observed that the clasts

have blocky planar surfaces if the magma fragmentation occur in a brittle state,

due to the contact with external water or high strain rates during fragmentation.

Fluidal clasts are, instead, produced by magmas in hot fluid conditions; finally

chipped or rounded edges are typical of pyroclastic flow deposits (Fig. 2.11).


33

For instance, in Figure 2.12 some SEM images are proposed concerning distal

fallout materials originated respectively by the 1974 Fuego volcano eruption

(Guatemala), by the 1992 Mount Spurr eruption (Alaska) and by a miocenic

eruption in Nebraska (United States). In this figure, it can be seen that the

particles can be vescicular or not also for the same eruption.

The presence of vescicularities inside the clasts is strictly related to the

degassing phenomena during explosive magma fragmentation events.

Vesciluraties, for instance, are quite uniform for high viscous magmas while

wider range of vescicularities are typical of magmas that progressively change

their portion inside the vent.

Figure 2.11 Products of volcanic eruptions: (a, b) Volcanic clasts

from 1992 Spurr eruption (Alaska), (c) Angular glass bubble-wall shards

from 1974 Fuego eruption (Guatemala), (d) Bubble-wall shards

from the Ash Hollow deposit in Nebraska (United States) (Riley et al., 2003)

Connections among internal voids have been observed by many Authors

such as, for instance, Whitman & Sparks (1986), Pellegrino (1967), Esposito &

Guadagno (1998). In Figure 2.12, some examples are proposed concerning

inter-particles connected pores for a pyroclastic soil of Southern Italy.

Chapter 2 ________________________________________________________________________________________________________________________________________

34

Moreover, the phenocryst content in juvenile clasts is highly influenced by the

residence time and conditions in the magma chamber. Together with the clasts,

also wallrock lithics are present and their presence is strictly related to the

features of the conduit walls, to the interactions with aquifers and, above all, to

the eruptive intensity.

The primary minerals consist in volcanic glass, phenocrystal, feldspar,

silica minerals and all the iron and magnesium rich minerals together with

minor content of allophone-like materials, imogolite, zeolite, non-crystalline

silicate clays. Their aluminium-rich elemental composition, the highly reactive

nature of their colloidal fractions and their high surface area result in unique

chemical properties. Generally such soils exhibit pHs ranging from 5 to 7 (weak

acid range). In turn, chemical and mineralogical characteristics are reflected in

their physical properties and biological activities and affect positively the

utilization of these soils. These features change during the soil formation

process and also due to eventual weathering phenomena. In particular,

formation of noncrystalline materials and accumulation of organic matter are

the dominant pedogenic processes occurring in most soils formed in volcanic

materials (Shoji et al., 1993; Ugolini & Dahlgren, 2002). Time and climate

combine to determine the relative degree of weathering and pedogenetic

development. Moreover, in some cases, intermitted addition of volcanic ash

juvenates soil developmental processes, maintaining such soils in relatively

stable chemical conditions.

Figure 2.12 SEM photographs showing (a) the surface of a flow pumice and (b)

its internal voids (Esposito & Guadagno, 1998).


35

2.2.2 REV scale

The previous discussed features and site conditions (geological processes,

pore water pressure, stress levels) strongly influence the physical and

mechanical characteristics of these soils at REV scale.

Some unique physical properties are directly visible to the eye and sensible

to the touch. Surface soils are rich in humus and dark in colour, soil clods are

light, fluffy and easy to break into small pieces. Water from volcanic areas is

mostly transparent due to the poorly dispersible clays in the soils at a neutral

pH range. These properties are suitable for growing various upland crops

(Shojii & Takahashi, 2002).

As far as the physical properties, it can be observed that presence of

organic matter, amorphous materials, porous glass and intra-particles cavities

result in very small bulk density (Fig. 2.13). This last strongly decreases with

increasing humus content and only organic soils have lower bulk densities than

pyroclastic soils (Nanzyo, 2002). On the other side, air-fall deposition process

makes possible very high porosity values (n = 0.5 – 0.7) (Wesley, 2001; Bilotta

et al., in press; Nanzyo, 2002). These characteristics make indeed the soils very

light, as testified by the total unit weight values, usually ranging from 4 kN/m3

to 15 kN/m3. Moreover, all the previous features are characterised by a strong

variability related to eruption and deposition conditions. This aspect is clearly

outlined in Figures 2.14 and 2.15 for some pyroclastic soils of southern Italy,

involved in huge landslides. In this sense, the detection of homogenous soil

classes, with reference to both physical and mechanical properties, can be an

important step to solve engineering problems (Fig. 2.14, 2.15).

Figure 2.13 Bulk density versus humus (Nanzjo, 2002)

Chapter 2 ________________________________________________________________________________________________________________________________________

36

Figure 2.14 Dry unit weight (a) and void ratio (b) for some pyroclastic soils

(Bilotta et al., 2005)

0

20

40

60

80

100

120

140

0.001 0.01 0.1 1 10 100

grain size (mm)

per

centa

ge

by w

eigh

t (%

)

Silt Sand Gravel

Ashy soils

Pumices

0

10

20

30

40

50

60

70

0.3 0.4 0.5 0.6 0.7 0.8

Porosity, n

fre

qu

en

cy

(%

)

0

10

20

30

40

50

60

70

0.3 0.4 0.5 0.6 0.7 0.8

Porosity, n

fre

qu

en

cy

(%

)

c) d)

a) b)

Figure 2.15 Variability of some features for pyroclastic soils: a) grain size

distributions (Bilotta et al., 2005) b) porosity (Picarelli et al., 2001),

c) friction angle and d) cohesion (Crosta & Dal Negro, 2003)


37

Referring to the mechanical properties, it must be observed that in the

variety of landscapes where pyroclastic soils are present, they often lie in

unsaturated conditions (Bommer et al., 2000; Cascini & Sorbino, 2003;

Evangelista et al. 2002; Chigira et al., 2002; Olivares & Picarelli, 2003). As a

consequence, steep slopes or high cuts can be observed but their stability

conditions are highly dependent by the suction regime. In turn, negative pore

pressures change during the hydrological year, depending mainly from the

climate and soil features. For instance, in the Tierra Blanca deposits in el

Salvador, Bommer et al. (2002) measured suction values up to 400-500 kPa. In

some pyroclastic covers in Southern Italy, where huge flow-like mass

movements occurred, Cascini & Sorbino (2003) measured suctions ranging

between 65 kPa and 0 kPa, over a three years period, inside a 60 km2 area (Fig.

2.16a); similar values were reported by Scotto et al. (2004) and Damiano (2004)

for some smaller zones, not far from the previous one, affected by flow-like

mass movements (Fig. 2.16b, c).

The suction regime and its temporal fluctuations, strongly influence the

shear strength of these soils. For instance, Nicotera (2000) performed an

extensive laboratory program on the above quoted “pozzolana” of Naples

district (Southern Italy). In particular, the Author performed three different

kinds of direct shear tests on: a) natural water content samples, b) saturated

samples in the consolidation stage, c) natural water content samples wetted

during the shear stage at peak strength. The wetting of samples in

correspondence of peak strength produces a sudden and relevant decrease of

strength and an abrupt settlement. Moreover, in the first part of shear tests, the

sample initially subjected at lower values of vertical stresses tends to dilate,

while the samples subjected to higher values of vertical stresses contract. After

saturation, however, all the samples have a contracting behaviour. Cecconi et al.

(2005) performed the same laboratory tests on pyroclastic soils located in the

Central part of Italy originated by the Colli Albani complex, comparing the

obtained results with those reported by Nicotera (2000). Indeed, very similar

behaviour were observed (Fig. 2.17) while some differences arose from the

initial grain size distribution and void ratio as well as in relation with their intact

or remoulded state. Hormdee et al. (2005) performed similar tests on the above

quoted “Shirasu”, obtaining interesting relationships between the shear stress,

the vertical displacement and horizontal shear displacement curves.

Chapter 2 ________________________________________________________________________________________________________________________________________

38

Figure 2.16 In situ negative pore pressures for some pyroclastic covers

of Southern Italy: a) at massif scale (Cascini & Sorbino, 2003);

b,c) at site scale (respectively Scotto et al., 2005; Damiano, 2004)

Figure 2.17 Direct shear tests on pyroclastic soils from

Central and Southern Itlay (Cecconi et al., 2005)


39

Moreover, some peculiar mechanical behaviours of pyrocalstic soils are

related to their metastable structure (Ng & Pang, 2000; Sorbino & Foresta,

2002). For instance, Pellegrino (1967), Olivares et al. (2003) and Bilotta et al.

(2006) observed volumetric collapse phenomena induced by wetting during

laboratory tests. All these tests addressed the behaviour of pyroclastic soils

originated from Somma-Vesuvius volcano, that are widespread inside a large

territory of Southern Italy and they are often involved in huge landslides.

Particularly, in conventional oedometer tests, Pellegrino (1967) observed the

collapse of “pozzolana”, a pyroclastic soil widespread in the Naples district

(Southern Italy), when the unsaturated samples were inundated by water under

different values of vertical net stress. Similar behaviour was observed by

Olivares et al. (2003b) in suction controlled triaxial tests, for some pyroclastic

soils of Cervinara district (Southern Italy) involved in huge landslides, in 1999

(Fig. 2.18a). In this case, the collapse starts for a saturation degree very close to

unity (95%) and the on-set of failure is observed for a nil suction value,

essentially, in undrained conditions. Bilotta et al. (2006) performed oedometer

suction controlled tests on pyroclastic soils involved in the catastrophic

landslides occurred in 1998 (Sect. 5.2) (Fig. 2.18b). In such a case, the tests

showed a collapsible behaviour also for low vertical stress values, and

decreasing collapse settlement with increasing vertical net stress. Similar results

were also obtained by Yasufuku et al. (2005) on a non-plastic volcanic sandy

soil named “Shirasu”, widespread in some districts on Japan. In this case, the

Authors stressed that the volumetric collapse settlements decrease with

increasing shear stress level during the infiltration process.

Due to their metastable structure, pyroclastic soils can experience

liquefaction phenomena. For instance, Olivares and Picarelli (2003) performed

both drained and undrained tests on undisturbed saturated samples from some

pyroclastic deposits of Southern Italy (Fig. 2.19). For these soils, the measured

stress paths showed a contractive behaviour in drained tests, while positive

induced excess pore pressures were observed in undrained tests. Moreover, the

undrained stress strain curve evidences the unstable behaviour of soil that is

characterized, after the peak of strength, by a progressive shear strength

decrease due to a continuous pore pressure increase (Fig. 2.19). During

undrained ring-shear tests performed on some pyroclastic soils of Japan, Wang

et al. (2002) observed excess pore-pressure upon shearing with the effective

Chapter 2 ________________________________________________________________________________________________________________________________________

40

stress and shear resistance finally decreasing to near zero (Fig. 2.20). From this

effective stress path, it can be recognized that the shear resistance reaches its

peak strength before reaching the failure line, thus showing a sort of collapse

phenomenon due to failure of the soil structure.

suction decrease (from 87kPa to 0)

p-ua=40kPa; q=80kPa

-2

-1

0

1

2

3

4

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l&

vo

lum

etr

icstr

ain

[%]

0.4

0.45

0.5

0.55

0.6

de

gre

eo

f sa

tura

tio

n,

Sr

Sr

a

v

r


p-ua=42kPa; q=80kPa

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l & v

olu

metr

ic s

train

[%

]

0.4

0.5

0.6

0.7

0.8

0.9

1

degre

e o

f satu

ratio

n, S

r

a

Sr

drained wetting

undrained failure

v

r

b)

a)


p-ua=40kPa; q=80kPa

-2

-1

0

1

2

3

4

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l&

vo

lum

etr

icstr

ain

[%]

0.4

0.45

0.5

0.55

0.6

de

gre

eo

f sa

tura

tio

n,

Sr

Sr

a

v

r


p-ua=42kPa; q=80kPa

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l & v

olu

metr

ic s

train

[%

]

0.4

0.5

0.6

0.7

0.8

0.9

1

degre

e o

f satu

ratio

n, S

r

a

Sr

drained wetting

undrained failure

v

r


p-ua=42kPa; q=80kPa

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l & v

olu

metr

ic s

train

[%

]

0.4

0.5

0.6

0.7

0.8

0.9

1

degre

e o

f satu

ratio

n, S

r

a

Sr

drained wetting

undrained failure

v

r

b)

a)

1.4

1.5

1.6

1.7

1.8

1.9

2

1 10 100 1000 10000

v-ua [kPa]

h [

cm

]

2

1

collapse

creep

3 4

5

6

s = 0 kPa

s = 50 kPa

saturation

saturation

saturation

test num. specimen e0 t collap h collap

hour cm

1 7_04 2.25 -------

2 8_04 2.35 -------

3 12_03 2.73 29 0.0632

4 11_04 2.29 54 0.0437

5 10_04 2.38 22 0.0234

6 remoulded 1.63 45 0.0001

1.4

1.5

1.6

1.7

1.8

1.9

2

1 10 100 1000 10000

v-ua [kPa]

H [

cm]

saturated suction 50 remould Creep Serie11

2

1

1.4

1.5

1.6

1.7

1.8

1.9

2

1 10 100 1000 10000

v-ua [kPa]

H [

cm]


2

1

1.4

1.5

1.6

1.7

1.8

1.9

2

1 10 100 1000 10000

v-ua [kPa]

H [

cm]


2

1

a)

b)


p-ua=40kPa; q=80kPa

-2

-1

0

1

2

3

4

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l&

vo

lum

etr

icstr

ain

[%]

0.4

0.45

0.5

0.55

0.6

de

gre

eo

f sa

tura

tio

n,

Sr

Sr

a

v

r


p-ua=42kPa; q=80kPa

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l & v

olu

metr

ic s

train

[%

]

0.4

0.5

0.6

0.7

0.8

0.9

1

degre

e o

f satu

ratio

n, S

r

a

Sr

drained wetting

undrained failure

v

r

b)

a)


p-ua=40kPa; q=80kPa

-2

-1

0

1

2

3

4

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l&

vo

lum

etr

icstr

ain

[%]

0.4

0.45

0.5

0.55

0.6

de

gre

eo

f sa

tura

tio

n,

Sr

Sr

a

v

r


p-ua=42kPa; q=80kPa

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l & v

olu

metr

ic s

train

[%

]

0.4

0.5

0.6

0.7

0.8

0.9

1

degre

e o

f satu

ratio

n, S

r

a

Sr

drained wetting

undrained failure

v

r


p-ua=42kPa; q=80kPa

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000 6000 7000

time [min]

axia

l & v

olu

metr

ic s

train

[%

]

0.4

0.5

0.6

0.7

0.8

0.9

1

degre

e o

f satu

ratio

n, S

r

a

Sr

drained wetting

undrained failure

v

r

b)

a)

1.4

1.5

1.6

1.7

1.8

1.9

2

1 10 100 1000 10000

v-ua [kPa]

h [

cm

]

2

1

collapse

creep

3 4

5

6

s = 0 kPa

s = 50 kPa

saturation

saturation

saturation

test num. specimen e0 t collap h collap

hour cm

1 7_04 2.25 -------

2 8_04 2.35 -------

3 12_03 2.73 29 0.0632

4 11_04 2.29 54 0.0437

5 10_04 2.38 22 0.0234

6 remoulded 1.63 45 0.0001

1.4

1.5

1.6

1.7

1.8

1.9

2

1 10 100 1000 10000

v-ua [kPa]

H [

cm]


2

1

1.4

1.5

1.6

1.7

1.8

1.9

2

1 10 100 1000 10000

v-ua [kPa]

H [

cm]


2

1

1.4

1.5

1.6

1.7

1.8

1.9

2

1 10 100 1000 10000

v-ua [kPa]

H [

cm]


2

1

a)

b)

Figure 2.18 Volumetric collapse for some pyroclastic soils:

a) Olivares et al., 2003; b) Bilotta et al., in press


41

Figure 2.19 Undrained triaxial tests on pyroclastic soils

from Southern Italy (Picarelli, 2001)

Figure 2.20 Results of undrained ring shear tests on pyroclastic soils

from Japan (Wang et al., 2002)

As previously stated, pyroclastic soils can be found not only in a primary

deposition state but also in a remoulded state due to erosion and mass wasting

processes along the hillslopes. As a consequence, they can be characterised by

quite different mechanical features as highlighted, for instance, by Bilotta et al.

(2005) that performed direct shear tests, in unsaturated and saturated

conditions, on undisturbed and remoulded samples of the above mentioned

soils of Campania region (Southern Italy) (Fig. 2.21a). For the shear stress-

horizontal displacement curves, a strain-hardening behaviour was systematically

detected in these tests for vertical stress values higher than 30 kPa. Remoulded

specimens, on the contrary, show a strain-softening behaviour, almost

regardless of the vertical stress applied, resulting in peak shear strength values

Chapter 2 ________________________________________________________________________________________________________________________________________

42

higher than those attained by the undisturbed specimens. Moreover, the

obtained results clearly outline that the saturated shear strength is higher for

remoulded specimens. The considerable differences in shear strength are

certainly related to the soil porosity decrease induced by remoulding. At it

concerns the hydraulic properties, Ng & Pang (2000) stressed the differences

between undisturbed and remoulded specimens, for a volcanic soil located in

Hong Kong where relevant landslides occurred in 1997 (Sun & Campbell,

1998) (Fig. 2.21b). In particular, soil water characteristic curves showed an

hysteresis loop considerably smaller for natural specimens, that have also a

slightly lower air-entry value and a higher rate of desorption, in comparison to

recompacted soil specimens.

Figure 2.21 Differences for undisturbed and remoulded pyroclastic soils:

a) saturated shear strength for some pyroclastic soils of Southern Italy

(Bilotta et al., 2005), b) soil water characteristic curves for a

volcanic soil located in Hong Kong (Ng & Pang, 2000)


43

2.2.3 Macro scale

At macro scale, pyroclastic soil deposits are characterised by an evident

decrease in both maximum grain size and thickness moving from proximal to

distal areas. The grain size characteristics of entire fall deposits is directly related

to processes of fragmentation and eruption. For instance, Kempter et al. (2000)

studied the grain size features of the “Rio Blanco” tephra deposit in Costa Rica

highlighting that the proximal samples are very poorly sorted, while medial and

distal deposits show a progressive increase in sorting efficiency.

On the other hand, thickness and stratification is strictly connected to

wind transportation. In particular, most deposits have striking morphological

features, as usually characterised by multiple sequences of horizons. The nature

of the stratification and the fluctuations in grain size offer insight into the

changing intensity of the parent eruptions, particularly for very proximal

sections (Fig. 2.22).

Figure 2.22 Grain size distributions of the Rio Blanco tephra deposit versus

distance from the Rincon de la Vieja volcano (Costa Rica) (Kempter et al., 2000)

Most fall deposits consist of an alternation of coarser and finer grained

intervals. Sharp bedding planes, dividing units of contrasting grain size, imply

intermittent volcanic fallout while deposits lacking well-defined bedding planes

are originated by fluctuating intensity of the eruption as well as by wind shifts

Chapter 2 ________________________________________________________________________________________________________________________________________

44

and/or changes in the inclination of the eruption column or jet (Fig. 2.3).

Several examples are provided in literature and Figure 2.23 shows the in-situ

conditions of some pyroclastic covers, in eastern Mexico, involved in huge

landslides on 1999 (Capra et al., 2003). Finally, it’s well known that the

pyroclastic deposits can extend up to hundreds of kilometres (Fig. 2.24); their

thicknesses generally decrease with the distance from the volcano, according to

the eruption style (Hoblitt et al., 1987).

Figure 2.23 In-situ conditions for some pyroclastic deposits

in eastern Mexico (Capra et al., 2003)

Figure 2.24 Thickness of tephra deposits versus distance from volcanoes

in western United States (Hoblitt et al., 1987)


45

2.3 WORLDWIDE DIFFUSION

Pyroclastic deposits are widespread all over the world but they are

concentrated in narrow zones, corresponding to the main borders of tectonic

plates, where the most of active volcanoes are located (Fig. 2.25, 2.26).

Figure 2.25 Worldwide diffusion of volcanoes

(http://terra.rice.edu/ plateboundary)

As a result, the distribution of pyroclastic soils follows the Circum-Pacific

Ring of Fire and the active tectonic zone along the western coast of both North

and South America. They are widely diffused in Japan, the Philippine Islands

and Indonesia, across Papua New Guinea, and Pacific islands to New Zealand.

Another large area of pyroclastic soils exists along the Rift Valley in Africa

through Yemen, Ethiopia Kenya, Rwanda and Burundi. Other areas of

pyroclastic soils associated with active volcanism exist in the Cameroons, West

Indies, southern Italy, the Canary Islands, and Iceland. In China, pyroclastic

soils are found in north-eastern and southern provinces, Inner Mongolia,

Chapter 2 ________________________________________________________________________________________________________________________________________

46

Xinjiang, Hainan, and Taiwan. In Alaska, pyroclastic deposits are widespread

and abundant especially in the southern regions. Pyroclastic soils also exist in

Washington, Oregon, California, and Idaho, all of which are affected by the 17

active volcanoes in the Cascade Range. Finally, pyroclastic soils can be also

found in Arizona, southern portions of British Columbia and Alberta of

Canada. Totally, they cover more than 1.5 million km2 or about 1% of Earth’s

land surface (Sigurdson et al., 2000). Other estimations refer to a covered area

of 0.1 million km2 corresponding to about 0.7% of Earth’s land surface (Soil

Survey Staff, 1999).

a)

b)

a)

b)

Figure 2.26 Worldwide diffusion of pyroclastic soils according to: a) WRB

Classification (FAO et al., 1998), b) US Soil Taxonomy (Soil Survey Staff, 1999)


47

These volcanic regions are continuously evolving and every year

remarkable volumes of new volcanic material is erupted (6-8 km3). In particular,

some explosive volcanoes are capable to produce relevant quantities of

pyroclastic material. From any volcanic eruption, it can result huge quantities of

pyroclastic materials ranging from less than 0.01 km3 up to 100-1’000 km3 (Tab.

2.4), covering areas up to 57’000 km2, as for the famous case of Saint Helen

eruption in 1980.

From ancient times, the special characteristics of such soils resulted in a

variety of uses, related to the eruptive style as well as chemistry and mechanical

properties of volcanic materials (Tab. 2.27).

Volcanic product Color Typical grain-size Common uses

Raw products

Basaltic scoria Black to red Coarse aggregate < 3 cm Road construction, use, in cinder blocks moderate insulator

Basaltic lava Black/grey Massive Construction, decorative purposes, moderate insulator

Rhyolite ash Light gray/brown

Fine aggregate < 2 mm Abrasiver,, creation of perlite, a good refractory, insulator

Pumice Light gray/brown

Aggregate 0.2 – 10 cm Absorbent, abrasives, good insulator

Silicic ignimbrite Light to dark brown

Massive Decorative uses, contruction, poor to moderate insulator

Rhyolite lava Brown to gray black

Massive Decorative uses, contruction, poor to moderate insulator

Obsidian Clear black Small lenses or tears (cm) Decorative uses, cutting implements, poor insulator

Native sulfur Yellow Microcristalline Chemical additive, component need to “vulcanize” rubber

Bentonite clays Light brown < 0.005 mm Additive to drilling muds, good insultator and sealant

Man-made products

Perlite White to light gray

Corse aggregate < 3 cm Absorbent, insulator, lightweigth concrete

Cinder concrete Gray Blocks (man-made) Construction, insulation

Table 2.27 Common uses of pyroclastic soils (Sigurdson et al., 2000)

Most pyroclastic soils are a good growth medium for plants because they

are a very permeable due to their low bulk density and high porosity. Volcanic

soils are fertile and nowadays many countries use volcanic ash as a soil

supplement in agriculture. Moreover, pyroclastic soils are relatively inexpensive

Chapter 2 ________________________________________________________________________________________________________________________________________

48

and ideal for construction and manufacturing. Prehistoric uses consisted in

construction materials such as massive stone, concrete and cinder blocks,

tunnelling, mining. Before the industrial age, volcanic rocks were used for a

wide range of purposes and provide a tradable commodity and a basis for local

economies. Some modern uses are additives (bentonite, pumice) and aggregates

(Perlite, Cinder, refractories). For instance, pumice has a relatively high

strength, yet in some cases a low enough density to float on water, and provides

excellent insulation and absorption characteristics.

The use of volcanic materials has increased as additional applications have

been found, strengthening the practical and economic relationship between

mankind and volcanoes. Although crude, these estimates give a sense of the

significant positive impact that volcanic materials have on our everyday life. As

a consequence, the neighbourhood of volcanoes often result in very populated

regions.

Unfortunately, volcanic activity and pyroclastic soils originate serious

natural hazards both during volcanic eruptions and later. During the eruptive

episodes, serious dangers are represented by pyroclastic flow and lahars in the

neighbourhood of volcanoes and, sometimes, also at relevant distances due to

the high mobility of volcanic products during these phenomena. Serious

dangers, during explosive eruptions, are related to the air-fall transportation of

volcanic ejecta. Air fly are strongly threatened by volcanic plumes that can

reach high altitude spanning over large areas (Fig. 2.28a). Moreover, ash

deposition provokes pollution, health problems and, eventually, the failure of

buildings due to the heavy deposits on the roofs (Fig. 2.28b, c).

On the other hand, delayed hazards are directly related to peculiar

mechanical features of pyroclastic soils (Sect. 2.2) that are systematically

involved into flow-like mass movements, as discussed in the following chapter.


49

a)

b) c)

Figure 2.28 Hazard related to pyroclastic products: a) danger to aircraft along

North Pacific air routes, b) roof collapse, c) pollution (Nael et al., 1997)

Flow-like mass movements in pyroclastic soils ________________________________________________________________________________________________________________________________________

51

3 FLOW-LIKE MASS MOVEMENTS IN

PYROCLASTIC SOILS

The landslide begins with only faint rumblings,

the devastation it leaves in its wake are enormous.

How does one recover from a personal and emotional landslide?

How do I go on knowing that never again will I hold my child in my arms?

How do I go on knowing that an alternate destiny awaited me?

Alfred B. Mitchell

Landslides represent a global issue as they occur all over the world

(Ayala, 2002) and in many countries, together with floods, they generate a yearly

loss of property larger than that from any other natural disaster (Reichenbach et

al., 1997). According to Canuti (2000), the average values of victims per year

ranges between 50 (in USA) and 735 (in Andean countries) while the average

cost of losses vary between 1-2 billions of euro for (USA) and 4-6 Billions of

euro (Japan). For instance, in Italy, a nation-wide project aimed at collecting

information on landslides and floods for the period 1918 - 1990 inventoried

about 11.000 huge landslides (Guzzetti et al., 1994, 2000).

Among landslide, flow-like mass movements often result in catastrophic

events as in the cases recorded in Venezuela (1999), in China (1999), in Japan

(1998) and El Salvador (2001) (Pastor et al., 2002). Similar phenomena

systematically occur in many parts of Italy (Fig. 3.1), with analogous effects, as

dramatically testified by the events occurred in Valtellina (1963) and in

Campania region (1954, 1998) (Fig. 3.2).

In the following, after a preliminary overview about general features of

flow-like mass movements, some relevant case studies are discussed.

Chapter 3 ________________________________________________________________________________________________________________________________________

52

Figure 3.1 Catastrophic landslides in Italy (Canuti, 2000)

Figure 3.2 Flow-like mass movements in the world (Perov et al., 1997)


53

3.1 GENERAL FEATURES

Flow-like mass movements (Hutchinson, 2004) are characterised by long

travel distances (up to tens of kilometres) and high velocities (in the order of

metres/second) strictly related to mechanical and rheological properties of the

involved materials. For this type of landslides, indeed, the usual terminology

(IAEG 1990; WP/WL1, 1991) is not exhaustive. This is essentially due to the

difficulties in applying morphological terms since the displaced mass completely

vacates the space above the rupture surface and moves down the slope. As a

consequence, for such landslides it’s very important to recognise different

zones inside their traces and conceptually distinguish their stages.

At this regard, Fell et al. (2000) defined “source” the space between the

rupture surface and the original ground. The strip of land traversed by the

moving mass down-slope from the source is referred to as “path”, while the

“accumulation zone” coincides, in this case, with the “deposit”. The word

“run-out” (or “travel distance”) is used to describe the distance between the toe

(or the crown) of the source and the toe of the deposit. According to Hungr et

al. (200x), three main zones can be recognised, using a geomorphological

terminology: the source area (which refers to the starting zone of the material),

the transport zone and the depositional area (Fig. 3.3b). Adopting a

geotechnical terminology, three different stages can be defined: pre-failure,

failure and post-failure stages (Leruoeil 2004) (Fig. 3.3a). Moreover, other terms

as triggering and propagation are provided in literature. In the following, three

main stages will be referred to: triggering, post-failure and propagation.

In the present thesis, triggering is assumed as the whole process leading a

soil mass to experience “indefinite” or “large” irreversible deformations

compared to those usually occurring during the pre-failure stages. The

triggering stage can be related to a number of factors such as rainfall,

earthquake, weathering, human activity or a combination of them. It surely

represents a fundamental issue, since it determines the initial unstable volumes

in relation to local and site conditions, soil behaviour and boundary conditions.

Post-failure stage leads unstable masses to high mobility compared to their

mechanical properties. Such stage is usually governed by a number of

mechanisms such as, for instance, the liquefaction phenomena (Musso &

Chapter 3 ________________________________________________________________________________________________________________________________________

54

Olivares, 2004; Wang et al., 2002) that can occur both in static and dynamic

conditions. Static liquefaction can occur along the sliding surface or within the

sliding zone during the rise in pore-water pressure, which reduces shear

resistance by decreasing the effective normal stress (Bishop, 1973). On the

other hand, dynamic liquefaction is induced by cyclic loading, as it has been

pointed out since 1964, after the catastrophic effects of Niigata earthquake

(Japan).

Finally, propagation stage includes the movement of the unstable masses

from the source area up to the rest. This stage eventually includes erosion

processes and fluidization phenomena along the followed path. Erosion

process can be extremely important since the amount of material added along

the path of travel may also exceed, in some cases, the original volume of the

landslide (Costa & Williams, 1984; Jibson, 1989; Sassa et al., 1997; Wieczorek et

al., 2000; Egashira et al., 2000; Cascini, 2004). On the other hand, fluidization is

generally considered as the mechanism that continuously re-arrange the soil

structure (Musso & Olivares, 2004). This phenomenon occurs during the

movement of unstable mass and it should be included in the propagation stage

of flow-like mass movements.

Fr

displacement

F

Fd

t0

dt t

Fr

Fd Triggering

t0

Fr

FdPost-failure

dt

Fd

Propagation

DtDt

Fr

Fr

Fd Triggering

t0

Fr

FdFr

Fd Triggering

t0

Fr

FdPost-failure

dtFr

FdPost-failure

dt

Fd

Propagation

DtDt

Fr

Fd

Propagation

DtDt

Fr

Fr

displacement

F

Fd

dt

t0

t

Fr

FdFr

Fd

FdFd

Triggering

t0

Fr

FdPost-failure

dtFr

FdPost-failure

dt

Propagation

DtDt

SOURCE

PATH

DEPOSITION

b)

a)

Figure 3.3 Scheme for a flow-like mass movement

(modified by Fell et al., 2000 and Hungr et al., 2001)


55

With reference to the previous scheme of flow-like mass movements, it’s

useful to observe that it’s difficult to state a clear correspondence between the

“zones” and the “stages” of these phenomena. Indeed, triggering and post-

failure, although quite distinct, can occur inside similar or overlapping areas. On

the other hand, the propagation stage can be, in some extents, more easily

related to the path of unstable masses along mountain channels or gullies.

Anyway, in some cases, the distinction between the source and path zones (and

in turn between triggering and propagation stages) is a crucial issue in order to

assess and manage the associated landslide risk.

With reference to flow-like mass movements, some features need to be

discussed more in details. For instance, these phenomena affect both artificial

and natural slopes, involving volumes that, according to Crosta et al. (2005),

can reach values up to 1000 m3 in terrestrial, sub-aerial settings, and also greater

values for submarine and extraterrestrial landslides. Moreover, they often occur

over large areas, in form of multiple events as, for instance, in Central America

in 2001 (Bommer et al., 2000), in Italy in 1998 (Cascini, 2004) and in Japan in

1998 (Chigira, 2002). They typically occur in topographic hollows, but they are

also found on flat planar slopes (Cannon, 1982; Anderson & Sitar, 1995) and

they involve a variety of soils, including colluvial and residual soils (Legros,

2002; Crosta, 2005). Anyway, it must be stressed that such phenomena are

characterised by greater intensity when involving volcanic soils since they

generally travel over longer distances than those provided by non-volcanic soils

(Fig. 3.4). This is strictly related to the mechanical and rheological properties of

pyroclastic soils as well as to the features of pyroclastic deposits, as discussed in

the previous chapter.

Finally, such phenomena can be recurrent in the same areas (Fig. 3.5). In

particular, eruptive episodes and landslide can alternate in the centuries because

the volcanic activity periodically ensures the production of new parent materials

(air-fall fragments) that are later mobilised during slope failures (Guida, 2003).

Similar time trends are related to the weathering processes and landslide

phenomena. In fact, weathering phenomena are capable to provide soils from

altered rocks that are mobilised through landslide events leaving intact rocks

once again subjected to weathering phenomena (Glade et al., 2005).

Chapter 3 ________________________________________________________________________________________________________________________________________

56

Figure 3.4 Intensities of flow-like mass movements

in volcanic soils and non (Crosta et al, 2005)

a)

b)

Figure 3.5 Recurrence of landslides due to:

a) volcanic activity (Guida, 2003), b) weathering (Glade et al., 2005).


57

3.2 CASE HISTORIES

Many catastrophic flow-like mass movements in pyroclastic soils are

systematically recorded in several regions of the world due to both their

mechanical properties of the soils and their worldwide diffusion (Chapter 2).

Generally, several factors are responsible for the first-failure stage of these

phenomena and the most common are rainfall, earthquake, weathering and

human activities.

Examples of huge rainfall-induced phenomena were systematically

recorded in Japan (Wang et al., 2002), Mexico (Capra et al., 2003), Colombia

(Terlien, 1996), Italy (Cascini, 2004), New Zealand (Ekanayake & Philipps,

2002), Micronesia (Harp, 2000).

Among the most relevant cases, those occurred in Mexico can be surely

mentioned (Alcantara-Ayala, 2004; Capra et al., 2003) since approximately 3000

“mass movements” - consisting of rock and soil slides and slips, debris flows

and avalanches - were triggered by heavy rains during October 1999 (Fig. 3.6).

In particular, Alcantara-Ayala (2004) describes a rainfall-induced event occurred

in the Sierra Norte, after 2 day of intense rainfall (300 mm equal to 42% of the

annual mean rainfall quantity). Here a “slide” evolved into a “mudflow” causing

109 victims. The failed mass had a maximum length of 100 m with a mean

depth to the slide surface of 4.4 m, and an involved volume of 7350 m3. For

this case, hydrological analyses and simplified stability analyses, performed in a

GIS system, outlined the relevant role played by the ratios between event

rainfall and, respectively, antecedent rainfalls and the mean annual rainfall. On

the other hand, on a regional basis, the geological setting of the area and

deforestation were also recognised as important factors influencing landsliding.

For the same area, Capra et al. (2003) address a single landslide occurred

in a volcanic deposit that caused approximately 150 deaths. This type of

movement affected only the sequence composed of the clay-rich paleosols

interbedded with the pumice/scoria ashfall. The thickness of the material

involved in such events ranged from a minimum of 0.4 m up to 1.5 m. In

particular, the sliding surface corresponded with the transition to the lower,

Chapter 3 ________________________________________________________________________________________________________________________________________

58

clayrich paleosol layer, which separated the pumice/scoria ashfall sequence

from the basal ignimbrite. On the basis of a simple infiltration model, set up for

typical stratigraphical sections, the Authors demonstrated that some particular

intercalation of paleosols (Sect. 2.2) and ashfall units, with different hydraulic

conductivities, caused the formation of perched water tables. In this scenario, a

saturated zone forms in correspondence of those low-permeability soils and

propagates upwards. When the failure occurs along the slip surface, due to the

contracting behaviour of the involved soils, positive pore pressures develop

upward, in the non-saturated part of the soil, liquefying the whole mass to form

the debris flow.

Figure 3.6 Panoramic view of some flow-like mass movements occurred

in Sierra Norte (Mexico) on October 1999 (Capra et al., 2003)

Huge events were induced by rainfall also in Japan, on August 1998. For

instance, Chigira (2002) describe the events recorded in the Fukushima district

(Fig. 3.7). Here, disastrous floods and landslides occurred because of a

cumulative rainfall exceeding 1200 mm (compared to an average August

precipitation of 250 mm). Three types of landslides were recognised, strongly

related to the hydrogeological structures originated through both the geological

and geomorphological history of volcanic materials (Fig. 3.8). Landslides of the


59

first type occurred along the edges of small plateaus, where permeable,

horizontal beds rest on exposed impermeable rocks. The second type of

landslide was favoured by weakly welded tuff with a weathering profile that

includes a clearly defined weathering front above which the tuff was greatly

deteriorated and exfoliated in layers. Finally, the third type of landslide involved

a slide of materials that fill depressions, according to a common mechanism

induced by rainstorms when concentration of infiltrating groundwater occurs

along buried hollows (Montgomery et al., 2000). During the same meteoric

event, other landslides were triggered. Wang et al. (2002) discuss a “long-runout

landslide” occurred in loamy volcanic-ash/pumice layer and deposited in a

nearby rice paddy. In an observation pit dug, placed in the middle part of the

landslide deposit, the sliding zone was detected just above the deflected rice

plants, and it was confirmed that grain crushing occurred in the sliding zone.

On the basis of this in-situ evidence, the Authors essentially related the post-

falilure stage of the movement to the metastable structure of the involved soils.

Figure 3.7 Spatial distribution of several rainfall induced flow-like mass

movements in Japan (Chigira et al., 2002)

Chapter 3 ________________________________________________________________________________________________________________________________________

60

Figure 3.8 Some triggering mechanisms for some rainfall induced flow-like mass

movements in Japan (Chigira et al., 2002)

Intense rainfalls induced landslides in Colombia, on May 1993 (Terlien,

1997; 1998). In the study area, the thickness of the pyroclastic deposits varies

from 15m, on the flat hilltops, to less than 1m on slopes steeper than 45° (Sect.

2.1). These pyroclastic deposits are built up of a large number of ashy layers

with grain sizes ranging from sand to silt (Sect. 2.1, 2.2). Here, the landslides are

related to the geological and geomorphological setting as well as to the wet

climate (2’000 mm of precipitation for year). The most of the shallow

translation landslides have a slip surface between 0.50 m and 6.0 m, while most

landslides deeper than 6 m were recognised as “flow slides”, on the basis of in-

situ measurements and hydrological and geotechnical studies. Three different

typologies of rainfall induced landslides were recognised, respectively triggered


61

by the saturation of the most superficial layers, formation of perched water

table, and rising of basal groundwater table.

Other relevant events were recorded in Lantau Island, the largest outlying

island of the territory of Hong Kong. Here, a severe rainstorm on 4–5

November 1993, induced more than 800 slope failures on natural terrain.

Detailed field investigations were carried out to study the failure modes, in

relation with various influencing factors (Fuchu et al., 1999). It was found that

the occurrence of slide debris flows has a close relationship with bedrock

geology, slope gradient, vegetation cover and micro landform. The failure

modes of slide-debris flows may be classified into translational slides and

rotational slides, and the former are predominant. Analysis of the hydrological

response of colluvial slopes during the rainstorm indicated that the majority of

the failures were caused by the development of a perched water table in the

thin surface layer of colluvium of volcanic origin due to infiltration during the

heavy rain.

Significant examples of huge flow-like mass movements triggered by

earthquakes are those recorded in Japan (Fukuoka et al., 2004), California (Harp

& Jibson, 1996), Central America (Boomer et al., 2002; Jibson & Crone, 2001).

In particular, Fukuoka et al. (2004) refer to an earthquake, with a moment

magnitude of 7.0, that triggered several landslides in Japan, not causing any

deaths or missing persons while some structures were damaged and landslides

were triggered; among these, the Tsukidate landslide originated on a gentle

slope of approximately 10 degrees (Fig. 3.9). The landslide was triggered in the

filling of a gully, which was buried for the purpose of developing residential

ground some decades before. The displaced landslide mass travelled a long

distance of about 130 m and, finally, spread and deposited on a horizontal rice

paddy, thus showing some typical characteristics of rapid long travelling flow

phenomenon. In-situ penetration test revealed that the filling material is

composed of pyroclastic deposits with very low shear resistance. Moreover,

standing groundwater existed at the source area and it saturated the filling

material; on the other hand, undrained ring-shear test results revealed that the

filling material was highly liquefiable. It was so concluded that, due to seismic

loading, a certain excess pore-water pressure built up within the saturated

Chapter 3 ________________________________________________________________________________________________________________________________________

62

sliding surface, which then led to the failure of the slope. After failure, high

excess pore-water pressure resulted in a great reduction in the shear resistance

and rapid movement.

b)

a)

c)

Figure 3.9 Earthquake-induced flow-like mass movements in Japan

(Fukuoka et al., 2004)

Earthquake induced flow-like mass movement are very common also in

pyroclastic deposits of Central America (Boomer & Rodriguez, 2002). They

occur predominantly in Guatemala and El Salvador but also in Nicaragua and

Costa Rica. Here, natural slopes close to vertical, and even up to tens of meters

high are systematically observed. The ability of these slopes to remain stable at

inclinations close to 90° is also ensured by high in-situ suction values (Sect. 2.2)

that, according to Bommer et al. (2002), is strongly affected by the rainfall

regime. For instance, the San Salvador earthquake of 3 May 1965 occurred at

the very end of the dry season whereas the earthquake of 10 October 1986

occurred very close to end of the rainy season. The locations of the two

earthquakes is similar, but the area affected by landslides was as much as five

times greater in the 1986 earthquake. Moreover, the total number of landslides

was significantly greater, despite the smaller magnitude of the latter (Rymer and


63

White, 1989). Similarly, it seems that the earthquake that occurred on 2

October 1878 in the eastern area of Jucuapa - Chinameca, triggered more and

larger landslides (including one on the Cerro El Tigre that killed 14 people)

than the triple event that struck the same area on 6 - 7 May 1951, again at

opposite ends of the rainy season (Meyer-Abich, 1952; Ambraseys et al., 2001).

The mechanisms of triggered landslides also appear to be influenced by the

ground-water conditions: the 1965 San Salvador earthquake only triggered soil

slides, whereas the 1986 event triggered soil slides and also significant slumps

and flows (Rymer, 1987).

Examples of weathering induced instability phenomena are provided by

Yokota & Iwamatsu (1999) that present some cases occurred in the hilly areas

of Kyushu Island (Japan) (Fig. 3.10). Here, frequent failures along steep slopes

composed of soft degradable pyroclastic rocks, seem to be caused by

weathering of rocks as individual failures are very shallow and tend to repeat

over time. The main weathering processes – taking place especially in the

shallow portion of slopes, where the strongest pore water pressure fluctuations

occur – produce the volcanic glass changes into clay minerals such as allophane

and halloysite. As a consequence, physical and mechanical properties change

due to porosity increases and both dry density and strength decrease with time.

Anyway, in some cases, the observed slope instability phenomena are related to

both weathering and rainfall as discussed by Chigira et al. (2002) with reference

to the above mentioned pyroclastic soils named “Shirasu”. Chigira et al. (2002)

observe that “Shirasu” weathers quickly as rainfall penetrates the ground

surface and the increasing thickness of the weathered zone leads to failure.

Conversely, strongly welded ignimbrite generally does not suffer from shallow

landslides because it is dense and resistant to weathering.

Chapter 3 ________________________________________________________________________________________________________________________________________

64

Figure 3.10 Weathering-induced flow-like mass movements

in Japan (Yokota et al., 1999)

Finally, Cairo & Dente (2003) describe a flow-like mass movement clearly

triggered by human activity in 1982, in Monteforte Irpino (Southern Italy) (Fig.

3.11). Here, during the construction of some cottages, cuts and levelling in

pyroclastic covers were carried out. The material resulting from excavation was

pushed as far as the edge of the gully lying below, sideways to the area under

construction. As a result, a flat slope of pyroclastic soil was formed at the

bottom of the gully. In order to hold this material a retaining wall was

constructed and after a three-months interruption, the scaffolding was

removed. As a consequence, a sudden “flow slide” was triggered in the

pyroclastic flat slope, causing the wall to be swept away and a workman to be

killed. On the basis of the available data, geotechnical analyses confirmed the


65

anthropogenic activity as main triggering factor for this event. However, the

human activity does not always correspond to the temporal occurrence of slope

instability conditions. This is, for instance, the cause of topographic

modifications that influence sub-superficial groundwater, eventually, causing

erosion processes, failure and/or slope instability phenomena (Luce, 2001;

Siedle et al., 2004).

Figure 3.11 Man-made induced flow-like mass movements

in Italy (Cairo & Dente, 2003)

Chapter 3 ________________________________________________________________________________________________________________________________________

66

3.3 REMARKS ON LANDSLIDES CLASSIFICATION

The variety of triggering factors, the extremely variable environmental

conditions (Sect. 2.1) and the different mechanical and rheological properties

(Sect. 2.2) of the involved soils result in a number of mechanisms characterising

both the first and post-failure stages (Sect. 3.2). Consequently, an unambiguous

classification of flow-like mass movements, occurring in pyroclastic soils (Sect.

3.2), is a difficult issue and some insights can be outlined referring to the

contributions of Dana (1862), Sharpe (1938), Terzaghi (1950), Skempton

(1953), Hutchinson (1968), Blong (1973), Hatano & Oyagi (1977), Varnes

(1978), Sassa (1985), Pierson & Costa (1987), Hutchinson (1988), Cruden &

Varnes (1996), Coussot & Meunier (1996), Dikau & Brunsden (1996), Leroueil

& Locat (1998), Hungr et al. (2001), Shooder (2003), Hutchinson (2004), Jacob

(2005). An extensive discussion of these works is over the aims of the thesis,

while some remarks are here proposed with reference to some key aspects of

flow-like mass movements, deepened in the following section. Further details

on the referred classifications are reported in the Appendix A.

In particular, the above mentioned classifications are different for the

utilised criteria that are, for instance, the type of movement, type and size of

involved material, underlying geology, age of movement, degree of activity,

geographic type, climatic type, velocity of movements, morphology,

geotechnical properties. Generally, they give a good framework for the most of

the landslide typologies, while the less accepted terminology is just that

concerning flow-like mass movements. At this regard, Hutchinson (1986) noted

that “flow-like motion subsequent to fluidization is a neglected and little-

understood group of movements with confusing terminology”. This appears

evident when referring to well documented phenomena triggered in pyroclastic

soils. Hereafter, two relevant cases are quoted, occurred in Central America and

Southern Italy respectively in 2001 and 1998. The first was a single earthquake-

induced event while in the second case, rainfall caused more than one hundred

events in few hours (Fig. 3.12, 3.13). Due to the peculiar features and in-situ

conditions of the involved soils (Sect. 2.1, 2.2), these phenomena were

characterised by particular features that lead some Authors to provide different

interpretations and, as a consequence, also a different terminology was utilised.


67

Slide, landslide Boomer et al. (2002)

Rapid earth-flow Jibson & Crone (2001)

Fast-moving flow-like landslide Crosta et al. (2005a, 2005b)

Rapid and long travelling soil flow Konagai et al. (2002, 2004)

Flowslide Evans et al. (2004)

Flowslide Pastor et al. (2002, 2003)

Figure 3.12 A huge earthquake-induced flow-like mass movement

from El Salvador (Las Colinas): different proposed classifications

Going further into details, it must be observed that classifications use a

hierarchy of descriptors to form branching structures or fix some classes based

on various attributes. At this regard, Hungr et al. (2001) observed that a

taxonomic structure is difficult to achieve. On the other hand, Hutchinson

(2004) argued that “notwithstanding the recent progress made by Hungr et al.

(2001), problems still remain in the area”. The yet unsolved questions are

related to many factors. For instance, Hutchinson (1988) observed that

“landslides exhibit an initial failure stage followed by a run-out and a central

problem in classification is what weight to give to each of these two, often

contrasting stages”. Similarly, Crozier (1986) argued that the answer to this

dilemma depends on “whether the principal interest rests with analysing the

condition of failure or the results of movement”. At this regard, it must be

stressed that the landslide classifications of Varnes (1978), Hutchinson (1988)

and Cruden & Varnes (1996) and, in particular, the classification reviews

Chapter 3 ________________________________________________________________________________________________________________________________________

68

Landslide Del Prete et al. (1998), Cascini et al. (2000), Calcaterra & Santo (2004), Frattini et al. (2004)

Landslide-flowslide Olivares & Damiano (2004)

Debris slide Calcaterra et al. (2004)

Debris avalanche Revellino et al. (2004)

Debris flow

Guadagno et al. (1999), Pareschi et al. (2000), Fiorillo et al. (2001), Pareschi et al. (2002), Budetta (2002), D’Ambrosio et al. (2003), Aleotti et al. (2003), Zanchetta et al. (2004), Fiorillo & Wilson (2004), Revellino et al. (2004)

Multiple debris flow Calcaterra et al. (2000)

Mudflow Martino & Papa (2003), Sirangelo & Braca (2004)

Flowslide-debris flow Guida (2003)

Flowslide Cascini & Sorbino (2003), Cascini et al. (2003), Olivares et al. (2003), Musso & Olivares (2003), Picarelli et al. (2004), Bilotta et al. (2005)

Soil slip Montrasio & Valentino (2004)

Figure 3.13 Huge rainfall-induced flow-like mass movements

from Southern Italy: different proposed classifications

proposed for the flow-like mass movement by Hungr et al. (2001) and

Hutchinson (2004) are capable to give an useful conceptual framework mainly

with reference to the post-failure and propagation stages (Tab. 3.14) On the

contrary, they do not help in clearly recognising and distinguishing the

triggering mechanisms, environmental conditions and processes leading to

failure in the source areas.

Actually, to achieve a more shared classification of these phenomena,

some important factors could be taken into account and, among those, the

change in type of movement during the motion from the source area to the

transportation-deposition area (Hatano & Oyagi, 1977) is a primary issue. Such

concept was already considered by Cruden & Varnes (1996) who proposed that

name of a landslide can become more elaborate as more information about the

movement become available. In particular, the suggested sequence could


69

provide a progressive narrowing of the focus of the descriptors first by

landslide, continuing with parts of the movement and finally defining the

material involved. In this way, also the amount of available data should be

better emphasised because it strongly influences the landslide characterisation,

analysis and interpretation. Certainly, in a research context, detailed

classifications, based on mechanics and the principles of physics may be needed

(Hutchinson, 2004). At this regard, a relevant example is provided by Pastor al.

(2003) that pointed out the close connection between modelling and

classification. In particular, the Authors classified landslide phenomena on the

basis of both their features and equations to be solved towards the analyses of

their propagation stages.

In conclusion, it can be observed that, up to now, the wide literature on

landslides classification has defeated one of its principal purpose: the provision

of clear and unambiguous terminology. Moreover, many studies do not refer to

any of the above classifications while using different definitions to better point

out some features of the analysed phenomena.

Following the above discussion, hereafter, in agreement with the

contribution of Hutchinson (2004), long run-out landslides are termed “flow-

like mass movements”, to be conceptually subdivided into triggering, post-

failure and propagation stages, coherently with the discussion of Section 3.1.

Varnes (1978) Hutchinson (1988) Hungr et al. (2001) Hutchinson (2004)

“Flows” “Debris movement of

flow-like form” “Landslides of the

flow type” “Flow-like mass

movements”

Wet sand, silt flow Flow slide Sand, silt flow slide

Flowslide Rapid earth flow Flow slide (clay) Clay flow slide

Loess flow Flow slide (loess) Loess flow slide

Dry sand flow - Dry sand flow

Earth flow Mudslide Earth flow Mudslide

- Mudflow Mud flow

Debris flow Debris avalanche Hillslope debris flow Debris avalanche

Debris flow Debris flow Debris flow

- Hyperconcentrated flow Debris flood

Rock avalanche Sturzstroms Rock avalanche Rock avalanche

Table 3.14 A comparison of different landslide classifications with

reference to flow-like mass movements



71

4 MODELLING OF RAINFALL-INDUCED

TRIGGERING MECHANISMS

“The laws of Nature are written in the language of mathematics ...

the symbols are triangles, circles and other geometrical figures,

without whose help it is impossible to understand a single word.”

Galileo Galiei

The features of flow-like mass movements provide serious difficulties

towards their complete modelling. As a consequence, only few pionerisitc

contributions are available to reproduce the whole phenomena, within an

unitary framework. For instance, this is the case of the mathematical model

proposed by Pastor et al. (2003) that address the triggering and propagation

stages respectively in a langrangian and eulerian reference system. On the

contrary, several models are already capable to interpret the triggering, post-

failure and propagation stage.

For the triggering stage, these models are based on statistical (Caine,

1980), geomorphological (Corominas et al., 2003), hydrological (Montgomery et

al., 1997), or geotechnical approaches (Rahardjio et al., 2001). The post-failure

stage is generally addressed through laboratory tests (Sassa et a., 2004) and/or

flume tests (Wang et al., 2003) and numerical modelling with the aid of

advanced constitutive models (Pastor et al., 2003). Finally, the propagation

stage is usually deepened through empirical (Corominas, 1996) or numerical

modelling (Pastor et al. 2003) and, in some extents, through small-scale

laboratory tests (Ochiai et al., 2004).

Due to the variety and complexity of flow-like mass movements (Chapter

3), a comprehensive analysis of these phenomena is beyond the potentialities of

this work, also referring only to pyroclastic soils. Therefore, the only rainfall-

Chapter 4 ________________________________________________________________________________________________________________________________________

72

induced landslides are addressed in the following. In particular, the attention is

focused on the analysis of the triggering stage to be considered a fundamental

step towards the evaluation of the landslides intensity (Fell, 1996) and

consequently for the hazard assessment (Cascini et al., 2005). The main reasons

of this choice, as well as the available models to analyse such triggering

mechanisms, are hereafter discussed.

4.1 RAINFALL-INDUCED TRIGGERING MECHANISMS

Rainfall is commonly known as one of the main landslide triggers

(Rahardjo et al., 1988; De Vita et al., 1998) and some significant case histories

have been discussed in Sect. 3.2. Moreover, in many regions of the world,

rainfall-induced landslides, causing significant erosional process, strongly

influence the land-use practice (Anderson & Sitar, 1995) and often pose grave

threats to life and property. As a consequence, the study of rainfall-induced

landslide mechanics is an important issue for landslide research (Hengxing et al.

2003), also due to the variety of triggering mechanisms discussed in the

scientific literature.

For instance, with reference to natural slopes, Brand et al. (1984), Johnson

& Sitar (1995), Gasmo et al. (2000) highlighted that most slope failures are

caused by the infiltration of rainwater into the slope. However, considering that

the temporal occurrence of landslides and movement activities is controlled by

rainfall patterns with different durations (Van Ash, 1999), it must be observed

that rainfall infiltration results in several effects such as raising main water table

and increasing soil unit weight as well as pore water pressure increasing and soil

shear strength reduction. Interesting discussions on the above topics are

provided by Alonso et al. (1995), Rahardjo et al. (2001), Iverson et al. (1997),

Guzzetti (1998), Tsaparas et al. (2002).

Focusing the attention on rainfall duration, the induced processes can be

usefully grouped into two main classes, here respectively termed “short” and

“long” processes (Fig. 4.1). The first class comprises surface run-off (Van Dine,

1985; Takahashi, 1991; Montgomery & Dietrich, 2002); particular groundwater

Modelling of rainfall induced triggering mechanisms ________________________________________________________________________________________________________________________________________

73

flow patterns caused by the stratigraphic setting and/or anthropogenic

structures and roads (Wolle & Hachich, 1989; Ng & Shi, 1998); increase of

saturation degree in unsaturated soils (Futai et al., 2004); filling of deep rills

(excavated by erosion processes) (Deere & Patton, 1972). In the second group

includes phenomena such as, for instance, raising of the water table (Leroueil,

2004; Dietrich & Montgomery, 1998) and groundwater supplies provided by

artesian conditions or hidden springs (Lacerda, 2004). Finally, first-time slides

triggered by rainfall, impacting on in-place soils, are also considered the main

cause of further instabilities through undrained loading (Hutchinson &

Bhandari, 1971) and/or liquefaction phenomena (Sassa, 1985; Hungr et al.,

2001). All the above processes often combine their effects and they result into

several triggering mechanisms in the following referred to as “rainfall-induced

triggering mechanisms”.

Notwithstanding the several types of landslide originated by different

responses to rainfall process, some general considerations are possible. For

shallow landslides, slope stability conditions are mainly dominated by direct

effects of rainfall resulting in transient pore pressure regimes often to be added

to water run-off and/or soil erosion processes. Conversely, deep seated

landslides are mostly affected by the rising of main water table. In the

following, the attention is essentially focused on the available models to

simulate and assess these triggering mechanisms.

LONG PERIODS (DAYS, MONTHS)

Raising of water table (Leroueil, 2004; Dietrich & Montgomery, 1998)

Groundwater supplies provided by artesian conditions or hidden springs

(Johnson & Sitar, 1990; Lacerda, 2004)

SHORT PERIODS (HOURS, DAYS)

Particular groundwater flow patterns caused by the stratigraphic setting

(Wolle & Hachich, 1989; Take et al., 2004)

Increase of saturation degree in unsaturated soils

(Futai et al., 2004)

Water filling of deep rills excavated by erosion processes

(Deere & Patton, 1972)

Water supplies in correspondence of bends of trackways

(Au, 1998; Fuchu, 1999)

Figure 4.1 Some triggering mechanisms for rainfall-induced phenomena

Chapter 4 ________________________________________________________________________________________________________________________________________

74

4.2 AN OVERVIEW OF THE AVAILABLE MODELS

The different models are available in the scientific literature can be

reviewed with particular reference to both the extension of the study area and

used spatial scales. To this aim, the models have been here classified into the

following main groups: the black-box models, the geological models, the

physical models, the physical based models, the geotechnical models (Fig. 4.2)

BLACK-BOX MODEL

regional scale (1:25000) (Brand et al., 1984; Wilson & Wieczorek, 1995)

local scale (1:5000) (Caine, 1980; Rossi & Chirico, 1998)

GEOLOGICAL MODEL

regional scale (1:25000) (Corominas et al., 2003; Van Westen et al., 2003)

local scale (1:5000) (Pareschi et al., 2000; Guida, 2003)

PHYSICALLY-BASED MODEL

local scale (1:5000) (Dietrich & Montgomery, 1998; Savage et al., 2003)

GEOMECHANICAL MODEL

site scale (1:2000) (Darve & Laouafa, 2000; Rahardjo et al., 2001)

PHYSICAL MODEL

natural slopes (Ochiai et al., 2004)

flume tests (Wang & Sassa, 2001; Olivares & Damiano, 2004)

centrifuge tests (Take et al., 2004)

Figure 4.2 Models for rainfall-induced triggering mechanisms

The black-box models correlate, by means of empirical or statistical tools,

some features of the rainfall event - such as its intensity and/or duration - to

the instability occurrence, eventually considering the antecedent rainfall. These

models are usually utilised to evaluate instability conditions over an

homogeneous area at medium and regional scale (1:25000 and smaller) (Crozier


75

& Eyles, 1980; Brand et al., 1984; Finlay et al., 1997; Wilson & Wieczorek,

1995; Wilson, 1997; Sandersen et al., 1996). In addition, some applications at

large scale (1:5000) can be also found in literature (Caine, 1980; Rossi &

Chirico, 1998; Sirangelo & Braca, 2004).

These empirical methods do not provide a theoretical framework for

understanding how hydrologic processes influence the location, timing, and

rates of landslides or for anticipating how landslide hazards might change in

response to changing climate or land use (Iverson, 2000). Anyway, the

identification of regional thresholds for the initiation of slope movements or

widespread flooding may play a renewed role in helping the risk mitigation (Fig.

4.3). Examples of rainfall thresholds are provided for the San Francisco Bay

region by Keefer et al. (1987) and Wilson & Wieczoreck (1995); for Hong

Kong by Brand et al. (1984) and Premchitt et al. (1994); for Japan by Onodera

et al. (1974); for Honolulu by Wilson et al. (1992); and for Southern Italy by

Rossi et al. (1998).

Figure 4.3 Some examples of black-box models: rainfall intensity/duration

thresholds for shallow landslide (Terlien, 1998)

The geological models aim to recognise landslide-prone areas (Ri & Liang,

1978; Hansen, 1984; Soeters and van Westen, 1996 daIverson2000) considering

the topographic and/or morphometric conditions, the watershed shape and the

Chapter 4 ________________________________________________________________________________________________________________________________________

76

drainage network typology, the presence of particular morphological structures,

litology, vegetation cover type and land use. In this sense, geological models are

generally oriented over small scales and they can also refer to long time periods

if attempting to reconstruct the overall slope evolution processes, as discussed

in the following examples.

Moreiras (2005) proposed a qualitative landslide susceptibility zonation

map (scale 1:100,000) by overlapping thematic maps of conditioning factors for

a study area in Argentina (Fig. 4.4). In this case, geomorphological surveys and

field observations, together with a landslide inventory map, enabled the

identification of 300 historical or prehistoric landslides.

Santacana et al (2003) presented a GIS-aided procedure for shallow

landslide susceptibility mapping at medium scale (scale 1:22000). To this aim,

13 parameters, related to the slope geometry, are derived from the digital

elevation model (DEM). Vegetation cover and thickness of superficial

formations are obtained from photo interpretation and field work. The

susceptibility assessment is based on multivariate statistical techniques

(discriminant analysis).

Van Westen et al. (2003) underlined the importance of geomorphological

expert knowledge in the generation of landslide susceptibility maps, using GIS

supported indirect bivariate statistical analysis. In particular, for a test area,

detailed geomorphological maps (scale 1:5000) were generated at different

levels of complexity. Other factor map were also collected, such as lithology,

structural geology, superficial materials, slope classes, land use, distance from

streams, roads and houses. Six different combinations of factor maps were

evaluated, with varying geomorphological input. Success rates were used to

classify the weight maps into three qualitative landslide susceptibility classes.

The resulting six maps were compared with a direct susceptibility map, made by

direct assignment of susceptibility classes in the field. With these analyses, the

Authors indicated that the use of detailed geomorphological information in the

bivariate statistical analysis can considerably improve the overall accuracy of the

final susceptibility map.


77

10 km

Figure 4.4 An example of geological models over large areas

at scale 1:100000 (Moreiras, 2005)

The physical models provide information on the occurrence of slope

failure through the direct observation of artificially induced instability

phenomena, provideing some insights on the features of flow-like mass

movements during their triggering, post-failure and propagation stage. At this

regard, several models are based on centrifuge and flume tests.

Examples of centrifuge tests are provided in Take et al. (2004) who

analyzed two possible triggering mechanisms respectively consisting in static

liquefaction and localised transient pore water pressures (Fig. 4.5a). The tests

were performed with a highly instrumented centrifuge model with an height of

200 mm and a length of about 600 mm. Sandy soils from Hong Kong were

utilised and arranged according to a layered stratigraphical setting capable to

induce particular transient pore water pressure regime. The obtained results

showed that static liquefaction is unlikely to occur if the model fill is

unsaturated and the depth to bedrock large, as the high compressibility and

Chapter 4 ________________________________________________________________________________________________________________________________________

78

mobility of air in the unsaturated void spaces allows the model fill slope to

accommodate wetting collapse without initiating undrained failure. In contrast,

high-speed failures with low-angle run-outs are shown to be easily triggered, in

model fill slopes, from initially slow moving slips driven by localised transient

pore water pressures arising from constricted seepage and material layering.

The obtained results showed that static liquefaction occur, both for dense and

loose soils, as a consequence of induced failure.

Laboratory flume tests were performed by Eckersley (1990) (Fig. 4.5b). In

this case, flowslides were initiated on a laboratory scale in 1 meter high slopes

of loose coking coal by saturation of the base and slow pore pressure increase.

Deep-seated and often retrogressive compound sliding were observed as the

typical triggering mechanisms. In particular, the failure initiated under

essentially static drained conditions with mobilised effective strength

parameters f’ considerably less than the steady state value. The subsequent

displacements were accompanied by rapid generation of excess pore water

pressures in thin shear zones and loss of shear strength. This process resulted

in a sudden acceleration of the sliding mass to form a rapidly flowing mass

which decelerated and stopped with a very flat final profile. The experiments

demonstrated that static liquefaction is a consequence of failure initiation rather

than the cause, and imposed vibrations or undrained loading are not

prerequisites. The direct observation of pore water pressures and stresses in

actual flowslides allowed also important confirmation of concepts known

previously only by inference from other laboratory tests under highly idealized

conditions.

Similar experiments were performed by Wang & Sassa (2001), using a

small flume, to trigger rainfall-induced landslides through the sprinkling of

water. The tests, carried out on a silica sand, showed that the failure mode

depend on the initial density and, greatly, on the grain size. In fact, flowslides

were initiated in the tests on finer silica sand, whereas retrogressive sliding

occurred in the tests on a coarser silica sand. Results of tests on mixtures of

silica sand, with different contents of loess by weight, showed that the existence

of fine-particle soil (loess) could significantly change the flow behaviour of a

landslide mass during motion. This also suggested the possible existence of a

mechanism that maintains high pore pressures during motion for these soils. In


79

a)

b)

Figure 4.5 Examples of physical models: a) centrifuge tests (Take et al., 2004),

b) flume tests (Eckersley et al., 1990)

addition, the Authors concluded that grain size and fine-particle contents

can have a significant impact on the mobility of rainfall-induced landslides.

Full-scale landslide experiments were also performed by Morikawi et al.

(2004) by means of a 23 m long and about 8 m high flume, consisting of three

parts: an upper 30° slope section, a lower 10° slope section, and a horizontal

section at the foot of the slope (Fig. 4.6a). The flume was sprinkled at a

constant intensity of 100 mm/h. The landslide occurred first in the upper slope

about 154 min after the sprinkling started, following a creep movement within

Chapter 4 ________________________________________________________________________________________________________________________________________

80

41 min. The sliding mass slid to a stop in about 5 s, compressing soils in the

lower gentle slope and horizontal sections. The dynamic process related to slide

movement and the fluctuation of subsurface water pressures during failure were

measured and analyzed. Sequential visual observations provided a clear record

of the slip surface during failure. The rapid increase of subsurface water

pressure in the slope and horizontal soil layers was also recorded during failure.

In this case, it was inferred that the increased water pressures in the upper slope

resulted from collapse of loose soil structure during shearing in the translational

slide; on the contrary those in the lower portion of the slope and horizontal

sections resulted from a mix of soil compression and shearing by the sliding

mass.

Finally, Ochiai et al. (2004) performed an experiment to induce a flow-like

mass movement by artificial rainfall on a natural slope in Japan (Fig. 4.6b). The

experimental slope was 30 m long, 5 m wide, and the average slope gradient

was 33°. The induced landslide (14 m long and 1.2 m deep) evolved into a

“debris flow”, with a travel distance up to 50 m. The entire landslide movement

was recorded by digital video cameras, allowing qualitative interpretation of the

shape of the landslide and definition of failure initiation and deposition times.

The soil-surface movements were measured by tracing the three-dimensional

displacement of targets on the experimental slope using image analysis.

Formation of the sliding surface was detected by soil-strain probes. Strain

probes, inserted in the middle or lower parts of the mass that failed, showed a

sliding surface at depths of between 1.10 - 1.20 m and 0.60 - 0.70 m,

respectively. The tensiometer showed a rapid increase in pore water pressure

after about 290 minutes from the start of sprinkling. This almost coincided with

the time when the strain was first observed on the sliding surface. Due to the

information collected during the test it was so possible to characterise the entire

phenomenon during the triggering, post-failure and propagation stages.

From the above discussion, it is evident that the physical models, among

their main advantages, allow the observation of slope behaviour, the

measurements of deformations and pore water pressure variation during the

whole landslide phenomena. On the contrary, some limitations are strictly

related to the differences between the in-situ conditions and test setting

especially with reference to the imposed boundary conditions. Moreover, it

seems that the scientific debate about static liquefaction is still open.


81

a)

b)

Figure 4.6 Examples of physical models: a) full-scale flume test (Morikawi et al.,

2004), b) tests on natural slope (Ochiai et al., 2004)

A completely different approach rely on mathematical models (physically

based models and geotechnical models) that certainly represent powerful tools

as they can be used to investigate a wide variety of different scenarios;

moreover, they can provide information and results at any location within the

analysed domain. Conversely, mathematical modelling still presents some inner

limitations, related to the complexity of including all the involved processes

and/or soil features and, in some extent, due to the computational capability

(Barbour & Khran, 2004).

In particular, physically based models are oriented to simulate the physical

processes leading to instability. They investigate the slope hydrological response

referring to a simplified slope stability analysis using different approaches to

compute pore pressure regime. For instance, simplified hydrological “tank

models” are utilised in a code proposed by Wu & Slide (1995). Steady state pore

pressure conditions are considered by Dietrich & Montgomery (1998). In this

model, the hillslope is primarily divided into cells of finite area for which the

Chapter 4 ________________________________________________________________________________________________________________________________________

82

upslope drainage area is detected according to the topographic setting; then the

phreatic surface depths are evaluated via a steady-state seepage analysis by

combining rainfall intensity and soil saturated conductivity. In some cases, the

Richard’s equation is integrated along the depth, as suggested by Iverson

(2000). In some cases, transient pore water pressure are computed as suggested

by Baum et al. (2004) while in other cases unsaturated conditions are taken into

account (Savage et al., 2003) (Fig. 4.7). On the basis of the obtained results,

stability analyses are generally performed referring to the indefinite slope

configuration and often implemented in a GIS platform in order to analyse the

instability conditions over large areas (Van Westen, 2000, 2004; Fabbri et al.,

2003). Indeed, the stability analysis are generally addressed at regional and/or

local scales, although they could also be utilised at site scale (1:2000 or larger),

and often implemented, according to the risk theory (Fell et al., 2005; Cascini et

al., 2005), including the probability of occurrence of rainfall (Pack et al., 1998).

In conclusion, these models are particularly useful to analyse slope stability

conditions over large areas, with acceptable computational times, even though

their restrictive assumptions, in several cases, provide unrealistic and/or

misleading scenarios.

The geotechnical models are aimed to rigorously simulate the physical

processes causing the slope failure and, can be considered a powerful tool for

slope failure simulation since they allow the assessment of unstable volumes

that are strictly related to the landslide intensity (Fell, 1996). However, these

models need careful evaluations on stratigraphic conditions, soil mechanical

properties and pore pressures regime as discussed, for instance by Burland

(1996) and Morgenstern (2000). In particular, the reconstruction of ground

profile rely on in situ investigations and eventually field monitoring. The soil

behaviour characterisation includes in-situ and laboratory investigations to be

performed respectively at site and REV (Representative Elementary Volume)

scales. Depending on the investigations at REV scales, simple (Bishop, 1955;

Morgenstern & Price, 1965; Rahardjo et al., 2001) or more advanced

constitutive models (Darve & Laouafa, 2000; Pastor et al., 2003a) can be used

in order to properly simulate the slope instability phenomena. Generally,

geotechnical models address site scale problems even though applications at

local scale can be also pursued, through the use of computational schemes

capable to interpret the typical slope conditions over large areas.


83

a) b)

c) d)

Figure 4.7 Some examples of physically-based model: comparison between

the results obtained through the SHALSTAB and TRIGRS codes

(Savage et al., 2004)

In conclusion, all the above approaches certainly provide useful tools

towards the understanding, prediction and modelling of the triggering stage of

flow-like mass movements. However, each of them systematically select some

aspects of rainfall-induced phenomena with reference to a preferential spatial

Chapter 4 ________________________________________________________________________________________________________________________________________

84

and time scale. Consequently, the use of only a model among the previous one

could not adequately address some rainfall induced phenomena. This is

essentially related to the features of such phenomena that, as discussed in

Section 4.1, involve several mechanisms acting over different spatial and

temporal scale and, sometimes, combine and overlap their effects. In these

cases, rather than any sector based approach, it could be particularly effective

an integrated approach aimed to join the potentialities of some available

models. In such a way, the results obtained through independent methods can

be profitably compared towards an adequate assessment of both predisposing

and triggering factors.

Within this framework, the attention is here focused on geomechanical

modelling that appears as the most rigorous approach towards the assessment

of the triggering mechanisms of rainfall-induced phenomena. Here

“geomechanical models” (Darve & Laouafa, 2000) are intended as models that

mainly address the soil mechanical behaviour instead of being oriented to

geotechnical practice.

4.3 GEOMECHANICAL MODELLING

4.3.1 Pore water pressure modelling

The evaluation of pore water pressure regime certainly represents a

fundamental issue, as discussed in Section 4.1. Among the several factors

affecting the pore pressure regime inside superficial covers, a key role is played

by the slope angle, the depth and the stratigraphical arrangement as well as their

variation along the hillslopes. At this regard, it must be observed that pore

water pressure variations can occur, in drained conditions, due to the direct

effects of rainstorms, water supplies coming from the bedrock and/or from

natural or anthropogenic discontinuities. On the other hand, the propagation

and/or impact of unstable masses on still in place soils are capable to induce


85

pore water pressure excess, mainly, in undrained conditions. For all these

phenomena, uncoupled or coupled approaches can be utilised.

With reference to the available literature, it must be observed that, often,

pore water pressure values are computed disregarding soil deformability. Within

this framework, some Authors deepened theoretical and numerical issues

related to pore pressure both in saturated and unsaturated conditions. For

instance, Zhan & Ng (2004) proposed analytical solutions for the analysis of

rainfall infiltration mechanism in unsaturated soils under simple hypotheses,

while Karthikeyan et al. (2001) discussed the problem of numerical oscillations

in finite element seepage analyses for unsaturated soils. Other enhancements

were proposed by Griffiths & Fenton (1993) that presented a stochastic finite

element seepage analysis, to include variability in soils properties while Ng et al.

(2001) presented, for an interesting case study, some 3D seepage analyses.

The relevance of coupled approaches, that provide a better insight about

the mechanical behaviour of multiphase materials was discussed by Schrefler &

Sanavia (2002). Indeed, in this field, some pioneristic mathematical models

were proposed by Biot (1941) and Biot (1955) for linear elastic materials. These

contributions were followed by further developments (Zienkiewicz et al.; 1980,

1990, 2000) through the extension of the theory to non-linear materials and

large deformation problems. It is also worth mentioning the work of Lewis and

Schrefler (1998), Coussy (1995) and de Boer (2000). Among the different

alternative ways which can be used to describe the coupling between solid

skeleton and pore fluids, Pastor et al. (2003) chose an approach close to

mixture theories, capable to provide a general mathematical framework which

can be used for both initiation of failure and propagation of catastrophic

landslides.

4.3.2 Remarks on failure and instability in geomaterials

The evaluation of pore water pressure regime, through any of the previous

approaches, is a fundamental step towards the modelling of triggering

mechanisms. At this regard, it is useful to discuss the concepts of failure and/or

instability that are commonly utilised to assess slope safety. Referring to failure,

Chapter 4 ________________________________________________________________________________________________________________________________________

86

some Authors (on the basis of laboratory tests and/or boundary value

problems) observed that it can be “localized” or “diffuse”, depending on both

stress limit condition and flow rule (Darve, 2002; Pastor, 2003).

Localized failure mechanism consists of a clearly defined surface where

shear strain concentrates on a narrow band. Many landslides, presenting a clear

failure surface, like rotational and translational landslides, fall in this category.

Localized failure presents important mathematical and numerical difficulties.

From a mathematical point of view, the inception of a shear band is

characterized by a discontinuity in the strain field, which can evolve towards a

discontinuity in the displacements at a later stage. Much effort has been

devoted during the past years to better understand this phenomenon. The

problem is ill posed for elastoplastic materials, and the results obtained in

numerical models depend on the mesh size and alignment. Comprehensive

discussions of these topics are provided by Vardoulakis & Sulem (1995), Pastor

& Tamagnini (2002), Mira (2002), Fernandez Merodo et al. (2004).

Diffuse failures do not present such clear surfaces, especially for very

loose or metastable soils with a strong tendency to compact under shearing.

One paramount feature is that effective stresses approach zero, and the material

behaves like a viscous fluid in which buildings can sink, as it happened during

the 1966 earthquake of Niigata in Japan. High pore pressures development may

cause liquefaction of the soil, and liquefaction – or conditions close to it – may

have played an important role on failure of slopes such as those of Aberfan in

South Wales, Anhui in China, Jupille in Belgium and Rocky Mountains coal

mine dumps. When this failure mode takes place in a slope, the mass of

mobilized soil can propagate downhill, evolving into flow slides or mudflows.

Of course, initial localized mechanisms evolving into a diffuse mode, as soil

masses liquefy, can also be observed (Pastor et al., 2003; Take et al, 2004).

Beside to the concept of failure, it’s also important to take into account

the issue of instability (or not controllability) as stated, for instance, by Darve

(2002) and Nova (2003). In fact, the not-associated flow rule exhibited by many

soils is responsible to produce phenomena not explainable with classical plastic

limit condition. In some cases, the observed soil behaviour testifies instability

conditions before that limit state is approached within usually considered safe

regions. Typical examples of such instabilities are the liquefaction in undrained


87

tests, shear and compactation banding Nova (2002). Other famous examples

are provided by failure in moderate slope that cannot be explained with the

Mohr-Coulomb criterion. At this regard, it must be noted that, according to the

experimental evidence, the granular materials may become unstable inside the

failure surface (Lade et al., 1997). Many studies deepened these phenomena,

from a mathematical point of view, during the instability conditions as in the

case of granular materials (Darve, 2002).

In the following, the concept of failure is referred to and the most

common methods utilised for slope stability analysis are briefly summarised.

4.3.3 Modelling of the triggering stage

The assessment of the safety level of natural and man-made earth slopes

constitutes one of the most frequent problems faced by geotechnical engineers

(Espinoza et al., 1992). As a consequence, several approaches are available and

the choice of suitable methods for safety analysis is an important issue.

Referring to deterministic methods, the most widespread are the analytical

methods (e.g. Limit Equilibrium Method) and numerical (e.g. Finite Element

Method) or a combination of them. A comprehensive review of these

approaches in slope stability analysis is beyond the scope of the thesis, while

further details can be found in Fredlund (1984) and Espinoza et al. (1992).

Among the analytical methods, the infinite slope method is very common

in “geotechnical practice” owing to its simplicity and it is generally used to

analyse shallow landslides (Iverson, 2000). Such analyses requires that driving

and resisting forces vary only as functions of a coordinate directed normal to

the surface of a planar slope. As a result, infinite slope analyses require no

additional assumptions to make them statically determinate. Concerning the

effects of pore pressure regime, the simplest assumption is that groundwater

flows parallel to the ground surface (Taylor, 1948; Skempton & DeLory, 1957).

This assumption is unnecessarily restrictive, but it obviates the need of pore

pressure data if the depth of the water table is known. A more general

assumption is that, for instance, pore pressures are constant along the potential

Chapter 4 ________________________________________________________________________________________________________________________________________

88

failure plane (Haefeli, 1948; Graham, 1984). The use of this assumption

requires knowledge of the pore pressure on the failure plane as well as of the

water table depth. This method resulted very attractive due to the possibility of

easily implementing it in GIS aided code. Anyway, it can present relevant

limitations, due to the assumptions on both the geometrical setting and the

pore pressure regime.

To consider complex geometries as well as soil and pore water pressure

variations, the limit equilibrium methods referring to not-planar slip surfaces

are certainly more appropriate than the previous one. These methods utilize

only equilibrium equations for potential unstable masses bordered by user-

defined slip surfaces. Despite several inner assumptions, limit equilibrium

method represents a powerful tool, because it is capable to give a clear idea of

the slope stability conditions through the computed safety factors F. According

to Bishop (1955) F is the ratio of the actual soil shear strength to the minimum

shear strength required to prevent failure. According to Duncan (1996) F is the

factor by which the soil shear must be divided to bring a slope to the verge of

failure. In this category, a number of methods have been proposed and widely

applied (Fellenius, 1936; Bishop, 1955; Janbu, 1954; Morgenstern & Price,

1965; Spencer, 1967; Sarma, 1973). However, although the concepts are

basically the same, the nature of the hypotheses used to represent the internal

forces distinguishes the different procedures, leading to different expressions of

the factor of safety. Hence, the choice of the appropriate procedure, with a

good understanding of its advantages and limitations, is often a cumbersome

task for the engineer (Espinoza et al., 1992). In fact, the factor of safety

resulting from the limit equilibrium methods is not uniquely determined, due to

the intrisic indeterminate characteristics of these methods. As a matter of fact,

the potential slip surfaces are assumed as input data rather than to be results of

the analysis. In this sense, the slip surface with the minimum safety factor

should be detected selecting a “proper” shape and location of the potential slip

surfaces. To look for the minimum safety factor, some optimization techniques

have been suggested, for instances, by Malkami et al. (2001) and Khran (2004)

that provide particularly efficient criteria for homogeneous slopes.

As observed by Leruoeil (2004), the above methods can provide

satisfactory results, when well calibrated with failure cases and, in this sense,

they require great engineering judgment. On the other hand, some


89

shortcomings can be outlined. Among these, must be surely included the

assumptions about the stress distribution and impossibility to take into account

stress-strain behaviour of soil and soil stress history. Moreover, no indication is

provided where yield starts or how it develops (Farias & Naylor, 1999).

Conversely, some numerical approaches are capable to take into account

deformations, progressive failure and soil-water mechanical coupling. Among

these, particularly relevant are the contributions of Potts (1997), Dawson (2000)

and Pastor et al. (2003), based on finite element method. In these analyses, a

key issue is the detection of failure conditions and subsequent mechanism. In

some cases, the arise and accumulation of inelastic deformation are assumed as

a revealing factor of the on-going failure mechanism. In other cases, the

effective mean or deviatoric stress evolution in time is referred to detect the

failure conditions (Pastor et al., 2003). For instance, Dawson (2000) proposed

to detect failure conditions through the “unbalanced nodal forces” criterion.

Anyway, for any finite element analysis, a key issue is represented by a

proper selection of the constitutive models. In this field, a wide literature is

available and exhaustive reviews were proposed, for instance, by Viggiani et al.

(2002), Jommi (2000), Wheeler & Karube (1996). In particular, some models

address the unsaturated conditions that are quite common in natural slopes

affected by flow-like mass movements. Among these models, relevant

contributions were provided, for instance, by Alonso (1990), Toll et al. (1996),

Cui & Delage (1996), Tamagnini & Pastor (2004). At this regard, it is worth

mentioning that each constitutive model can reproduce some features of the

observed soil behaviour. In this sense, the choice of an “adequate” constitutive

models must be essentially related to the amount and quality of information

gathered through in-situ and laboratory tests. Moreover, the potentialities of

models including advanced constitutive models are strictly related to the level

of knowledge on the analysed phenomena.

91

5 THE CASE STUDY

We must accept responsibility for a problem before we can solve it.

We cannot solve a problem by saying "It's not my problem," and

hoping that someone else will solve it for us. We can solve a problem

only when we say "This is my problem and it's up to me to solve it."

M. Scott Peck

As discussed in the previous chapters, several methods are available to

simulate the triggering stage of rainfall-induced landslides. However, due to the

generally used sector-based approaches, such methods must be systematically

checked through severe applications to well-documented case histories.

Moreover, the back-analysis of relevant case histories is also a fundamental step

towards the setting up of more advanced computational tools and/or

alternative approaches capable to better address the main features of these

phenomena.

To this aim, a case study from Southern Italy is here addressed. The

significance of this case study relies on the following reasons: i) the occurred

phenomena were among the most catastrophic landslide events recorded in the

Southern Italy, in the last centuries; ii) due to their high intensity, the events

were addressed by a number studies, even though different interpretations have

been proposed by several Authors; iii) an extensive data set is available, much

more over the average, concerning both the in-situ conditions and soil

properties; iv) finally, within the Southern Italy, there is a wide area threatened

by a similar natural hazard.

Chapter 5 ________________________________________________________________________________________________________________________________________

92

5.1 SOUTHERN ITALY

5.1.1 Pyroclastic soils in the Campania region

Pyroclastic soil deposits are widely diffused in Southern Italy over an area

of about 3000 km2, where more than 200 towns are located (Fig. 5.1). They are

related to the Late Quaternary - Holocene explosive activity of Somma-

Vesuvius volcano and, subordinately, to Campi Flegrei and Roccamonfina

volcanic apparata (App. B). In the following, only some remarks on Somma-

Vesuvius volcanic activity are reported, while several Authors provide

remarkable contributions on Somma-Vesuvius (Lirer 1997, 2001; Cioni 1999;

Bertagnini 1991), Campi Flegrei (Dellino et al., 2001, 2004; Nunziata 1991,

2004; Piochi 2005; Signorelli 2001) and Roccamonfina (Giordano 1998, Cole

1992, 1993; Vacca et al., 2003) volcanic apparata.

Figure 5.1 Pyroclastic soils and some recent flow-like mass movements

in Campania region (Southern Italy) (Cascini, 2004)

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93

The Somma-Vesuvius volcano, famous for the destruction of the Roman

city of Pompeii in AD 79, is located on the coast of the Bay of Naples, about

10 km to the east of the city, at a short distance inland from the shore (Fig. 5.2).

It is the only active volcano on the European mainland, although its

activity is currently limited to little more than steam from vents at the bottom

of the crater. Anyway, it has erupted many times and it is today regarded as one

of the most dangerous volcanoes in the world because the area around the

mountain is extremely densely populated, with more than two million people

living in the district and on the volcano’s slopes. As discussed in Section 2.3, the

Figure 5.2 The Somma-Vesuvius volcano

Chapter 5 ________________________________________________________________________________________________________________________________________

94

the main natural hazards generally consist in tephra fall, volcanic gases,

pyroclastic flow, pyroclastic surge, lava flows (App. B). To these ones it must

be added the risk of huge landslide occurring in pyroclastic soils mantling the

surrounding reliefs, as discussed in Section 3.1.

Going further into details, it can be observed that Vesuvius is a distinctive

"humpbacked" mountain, consisting of a large cone partially encircled by a

large secondary summit, Monte Somma (actually the remains of a huge ancient

cone destroyed in a catastrophic eruption, probably the famous one of AD 79).

The height of the main cone is constantly modified by eruptions but presently

stands at 1281 m. Layers of lava, scoriae, ashes, and pumice make up the

mountain.

The activity of Somma-Vesuvius volcano was mainly effusive and

suborditanately explosive (Sect. 2.1), with low intensity events (Fig. 5.3, 5.4).

The eruptions were occurred mainly at the central crater, increasing it up to an

estimated height of 1600 – 1900 meters a.s.l. (Cioni et al., 1999). The activity

involved also the lateral vents, aligned along the faults and fractures system, as

testified by the dikes exposed along the caldera slopes and by the cones along

the volcano slopes and in the surrounding plains.

The most ancient Plinian eruption (18300 years b.p.) was the “Basal

Pumice” eruption (Arnò et al., 1987; Andronico et al., 1995; Bertagnini et al.,

1998; Cioni et al., 1999) that caused both the collapse of the Somma Mount

edifice and the caldera formation (Fig. 5.3, 5.4). Then, the history of the

volcano was essentially characterized by three main plinian eruptions,

respectively Mercato (8000 years b.p.), Avellino (3800 years b.p.) and Pompei

(AD 79), and by several sub-plinian events.

After the AD 79 plinian eruption, two further sub-plinian eruptions

occurred, respectively in the 472 (Rosi and Santacroce, 1983; Arnò et al., 1987;

Lirer et al., 2001; Mastrolorenzo et al., 2001) and in 1631 (Arnò et al., 1987;

Rolandi et al., 1993b; Rosi et al., 1993) and open conduct activity, with low

intensity, during the I, III, V, VIII, X, XI centuries, in 1632 and 1944

(Andronico et al., 1995; Cioni et al., 1999; Arrighi et al., 2001). In the meantime,

the open conduct activity produced great quantities of lava that covered almost

completely the south-eastern and south western slopes of the volcano, that is

quiescent since 1944 (Fig. 5.3, 5.4).

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95

Figure 5.3 Eruptive activity of Vesuvius volcano

The above eruptions varied greatly in severity, often characterised by

explosive outbursts of the kind dubbed Plinian after the Roman writer who

observed the AD 79 eruption. On occasion, the eruptions have been so large

that the whole southern Europe has been blanketed by ashes; in 472 and 1631,

Vesuvian ashes fell on Constantinople (now known as Istanbul) over 1,600 km

away (Fig. 5.4).

All the plinian eruptions began with a phase of vent opening, followed by

the formation of sustained eruptive column and phases characterised by

pyroclastic flows and/or pyroclastic surges. The sustained eruptive columns

attained maximum heights up to 30 km and they produced air-fall deposits over

large areas, characterised by volumes ranging between 1.5 and 4.4 km3.

Chapter 5 ________________________________________________________________________________________________________________________________________

96

c)

a)

b)

e)

d)

Figure 5.4 Areas covered by pyroclastic products of Vesuvius volcano

during the eruptions occurred (a) 18000 years ago, (b) 8000 years ago,

(c) 3800 years ago, (d) 79 A.D., (e) 1631 (Cioni et al., 1999)

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97

The pyroclastic flows involved the slopes of the volcano and the

surrounding planes, with travelled distances up to 20 km from the source and

volumes ranging between 0.25 and 1 km3. In the proximal areas, thick breccia

deposits can be recognised, essentially related to the caldera collapse (Fig. 5.3,

5.4).

Among the Vesuvius’ sub-plinian eruptions, the events occurred in 472

and 1631 were characterised by the alternation of phases with sustained

eruptive column and pyroclastic flows (and/or surges). The first ones were

characterised by heights less than 20 km producing air-fall deposits whose aerial

extension was lower than those related to the plinian deposits. The pyroclastic

flows were distributed over distances not exceeding 10 km from the source.

The plinian and sub-plinian eruption deposits have dispersal axes with

directions ranging from N50° (Avellino eruption) to N150° (AD 79 eruption).

The isopachs, concerning heights of fallen material equal to 20 cm, associated

to the plinian “Pomici di base” and “Mercato” eruptions, cover respectively

areas of 2600 and 1150 km2, while the sub-plinian ones, occurred in the 472

and 1631, are distributed over areas of 1000 and 400 km2.

Figure 5.5 Pyroclastic soils of Vesuvius volcano originated by

(a) air-fall deposition and (b) flow deposition

The air-fall deposition phenomena produced thick deposits constituted by

ashy and pumice levels, covering several reliefs belonging to the Appenine

chain. These deposits were heavily modified by alteration, erosion and colluvial

processes. In particular, weathering processes, described in the Section 2.2, lead

Chapter 5 ________________________________________________________________________________________________________________________________________

98

to the formation of paleosols that are alternating to ashy and pumice layers (Fig.

5.5). On the other hand, colluvial and mass wasting processes resulted into

remoulding of these soils strongly affecting their mechanical features (Sect. 2.2).

Finally, in some cases, rainfall was capable to induce floods, hyperconcentrated

fluxes and flow-like mass movements. As a matter of fact, in the area where

these pyroclastic deposits originated, several and extremely variable in-situ

conditions can be recognised as discussed in Section 2.1. Consequently, the

reconstruction of typical stratigraphical conditions requires great efforts and,

sometimes, the presence of experts belonging to different disciplines. For

instance, for the deposits originated from the 472 AD eruption, their

stratigraphical conditions are quite different moving from the volcano slopes,

along the alluvial planes up to the distal topographic highs (Fig. 5.6). Indeed,

the recognition of typical soil classes, on the basis of their mechanical features,

it is often a fundamental issue to adequately set up the models discussed in

Section 4.3.

Figure 5.6 Stratigraphy of some pyroclastic deposits originated from

the 472 AD Vesuvius eruption (Mastrolorenzo et al., 2002)

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99

5.1.2 Environmental settings

The pyroclastic deposits deriving from explosive activity of Somma-

Vesuvius volcano (and subordinately of Campi Flegrei and Roccamonfina

volcanic apparata) (App. B) essentially cover three main distinct geo-

environmental contexts (Fig. 5.7) (Cascini et al., in press; Calcaterra et al., 2004;

O.U. 2.38, 1998).

0 20km

Napoli

Avellino

Monti PicentiniIschia

Procida

Capri

10

2 3 41

VesuvioCampi Flegrei

Roccamonfina

Salerno

Caserta

Benevento

Monti Lattari

Monti di Sarno

50 cm25 cm

80 cm

60 cm

20 cm

50 cm

Figure 5.7 Pyroclastic soils over different settings in Campania region:

1) isopach lines of the pyroclastic products; 2) carbonate bedrock;

3) tuff and lava deposits; 4) flysch and terrigenous bedrock.

The first context coincides with large sectors of the Campanian Apennine

chain, constituted by Mesozoic-Cenozoic carbonate rocks, where Salerno and

Lattari mounts, Sarno and Avella-Partenio mounts and Caserta and Cancello

hills are located. The air-fall pyroclastic soils lye upon these reliefs generally in

thin covers (0.1 – 5.0 m), depending on the prevailing wind directions, as

discussed in the previous section.

Chapter 5 ________________________________________________________________________________________________________________________________________

100

The second context corresponds to the Phlegraean district, including the

town of Naples, and the islands of Ischia and Procida; this context is

characterised by a bedrock of Late Pleistocene volcanic tuffs and lavas overlaid

by pyroclastic deposits which can reach thicknesses of several metres

(Calcaterra et al., 2004).

Finally, the third context, mainly located at the north-west of the Sarno

and Picentini Mounts, includes flysch and terrigenous hills. Here, thin

pyroclastic deposits (< 2.0 m) have been generally recognised.

The previous environmental settings are quite different for both the

geological, hydrogeological, geomorphological features of the reliefs as well as

for the characteristics of the pyroclastic covers, especially as it concerns their

age and thickness. Moreover, also inside the same environmental setting, some

macroscopic differences can be easily recognised. Focusing, for instance, on the

carbonate landscape, it can be observed that: i) depending on the eruption

intensity and prevailing wind directions, the pyroclastic deposits were originated

by many eruptions (e.g. Sarno and Lattari mounts, Cancello hills) or few

eruptions (e.g. Salerno mounts and Avella-Partenio mounts, Cancello hills) (Fig.

5.7); ii) the reliefs are characterised by relevant heights (e.g. Sarno, Lattari and

Avella-Partenio mounts) or low heights (e.g. Salerno mounts, Cancello and

Caserta hills) (Fig. 5.7); iii) the mountain watershed can have a low drainage

order (e.g. Sarno mounts, Cancello hills) or high drainage order (e.g. Lattari and

Salerno mounts) (Fig. 5.7, 5.8).

0.1 km 2.0 km

b)a)

Figure 5.8 Examples of different mountain basins involved by

flow-like mass movements: a) Sarno Mounts; b) Lattari Mounts

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101

5.1.3 Flow-like mass movements in the Campania region

Notwithstanding all the above differences, the pyroclastic deposits are

systematically involved in flow-like mass movements as in many regions of the

world (Sect. 3.1, 3.2). The O.U. 2.38 (1998) identified about 800 events

occurred during the last centuries causing casualties and huge damages (Fig.

5.9). Most of these phenomena involved the first two environmental contexts

as discussed also by other Authors (Brancaccio et al., 1999; Calcaterra et al.,

2004; Cascini et al., 2000; Cascini & Ferlisi, 2003; Celico & Guadagno, 1998; Di

Crescenzo & Santo 1999; Esposito et al., 2003; Olivares & Picarelli, 2001). On

the contrary, events of smaller intensity generally occurred where pyroclastic

covers lye on tuff and flysch hills.

Napoli

Salerno

Avellino

Figure 5.9 Victims caused in Campania region during the last centuries

(O.U. 2.38, 1998)

During the last years several studies dealt with the flow-like mass

movements that threaten the Campania region. Most of them were based on

models described in Section 4.4.2. Particularly, with reference to the whole

region, the hydrological models (Fiorillo & Wilson, 2004; Rossi et al., 1998)

Chapter 5 ________________________________________________________________________________________________________________________________________

102

tried to outline rainfall threshold aimed to protect people living inside the risk

areas. Geological approaches (Guadagno et al., 2005; Di Crescenzo & Santo,

2005) addressed morphological and morphometric features towards the

assessment of both the susceptibility and features of flow-like mass

movements. Finally, geotechnical analyses (Guadagno et al., 1999; Calcaterra et

al., 2004) stressed the relevance of geometric discontinuities and hydraulic

boundary conditions for the instability of pyroclastic soil slopes.

Other studies focused the attention on the carbonate environment. For

instance, with reference to the Lattari Mounts and Amalfi coast, Esposito et al.

(2002, 2003, 2004) deepened the flow-like mass movements occurred in 1924

and 1954 that are also discussed in Bordiga (1924), Lazzari (1954), Penta et al.

(1954). For this area, the performed analyses are based on black-box models

essentially relating the occurrence of events to the rainfall intensity over

different time periods.

Many other contributions focused on the landslides occurring at the

Sorrento Peninsula (Montella, 1841; Ranieri, 1841; Calcaterra et al., 1997; Mele

and Del Prete, 1999). Among these studies, Di Crescenzo and Santo (1998,

1999, 2004) described some landslides triggered by the January 1997 rainfall

event. Using a geological approach, they observe that all these events occurred

along structural slopes, regularised by rectilinear parallel recession processes

(Young, 1972; Brancaccio et al., 1978) and often inside small incisions (i.e.

ZOB, Guida, 2003) where pyroclastic covers reach the maximum depths.

Moreover, the Authors underlined that, despite a number of possible geo-

environmental conditions inside the source areas, a relevant element is

represented by the stratigraphical setting and, in particular, by the presence of

pumice layers always found along the sliding zone. For one of the previous

cases, using a geological approach, Calcaterra & Santo (2004) stressed the

importance of both the stratigraphical setting and groundwater regime related

to direct rainfall and water flowing from the carbonate bedrock. On the other

hand, geotechnical analyses performed by Calcaterra et al. (2004) suggested that

infiltration from the upper lateral boundary of the pyroclastic cover is more

effective as triggering mechanism than infiltration from ground surface.

Some other Authors deepened the events occurred on the Avella-Partenio

mounts, in December 1999. In particular, Fiorillo et al (2001) utilised an

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103

hydrological approach outlining the exceptional characteristics of the storm.

However, the only rainfall is not capable to explain the triggering of this event,

since similar storms didn’t have the same consequences. At this regard, the

Authors suggested that a possible important role has been played by

anthropogenic activities especially those related to the creation of cuts and

trackways. On the other hand, Montrasio & Valentino (2003) and Damiano

(2004) performed laboratory flume tests, outlining that failure occurs in fully

saturated conditions and that the triggering stage features strictly depend from

soil porosity values. For the same area, Olivares et al. (2003) performed

geotechnical analyses aimed to simulate pore water pressure regime inside the

pyroclastic cover. In particular, they obtained a good agreement between

measured and simulated suction values for superficial and intermediate layers,

while for deeper layers, the obtained results highlighted the importance of

hydraulic boundary condition at the base of the pyroclastic cover. Finally, with

reference to the Avella and Cancello Hills, Pareschi et al. (2002) set up a

geological model addressing both geomorphic and slope data. In particular, for

slope instability, the Authors assumed the slope angle distribution, the order

and shapes of drainage basins as relevant factors.

Inside the carbonate environment, one of the most investigated area is

surely represented by the Sarno Mounts where in 1986 a landslide involved the

town of Palma Campania. With reference to this case study an interpretative

model was proposed by Celico et al. (1986), Guadagno et al. (1998), Guadagno

1991), successively used for other analogous phenomena. This model stressed

the importance of groundwater inside the bedrock, eventually capable to induce

high pore pressure values at the base of the pyroclastic covers. In the same area,

on May 5th–6th, 1998 several flow-like mass movements occurred along the

slopes of Pizzo d’Alvano massif involving the urban centres at the toe of the

relief. Several scientific contributions focused on such events because of their

destructiveness and due to the relevant number of landslides occurred in a

short time period. With the aid of the models described in Section 4.2, the

triggering stage was analysed by several Authors whose contributions are

discussed in the next Section. Moreover, for the same flow-like mass

movements, Revellino et al. (2005) and D’Ambrosio et al. (2003) deepened the

propagation stages of the occurred phenomena.

Chapter 5 ________________________________________________________________________________________________________________________________________

104

From the analysis of the above contributions, it must be observed that,

inside the Campania region, the occurred flow-like mass movements are

characterised by extremely variable features, in relation to both the involved

environmental settings (Sect. 5.1.2) and the critical rainfalls. At this regard, it

must stressed that instability phenomena occur during the whole hydrological

year, with highest frequency during spring and autumn when critical rainfall are

different for intensity and duration (Rossi et al., 2005). Referring to their

modelling, the occurred phenomena were deepened by means of different

sector-based approaches that outlined several predisposition and triggering

factors. As a consequence, a comprehensive analysis of flow-like mass

movements occurring inside the whole Campania region appears yet a

challenging topic. On the contrary, it seems more useful to focus the attention

on a study area, the Pizzo d’Alvano massif, that on May 1998 was threatened by

catastrophic flow-like mass movements, for which an extensive data set is

available.

5.2 THE STUDY AREA “PIZZO D’ALVANO MASSIF”

5.2.1 The May 1998 event

On 5-6 May 1998, after a period of heavy rainfall, hundreds of flow-like

mass movements occurred inside an area of about 60 km2, along the slopes of

the Pizzo d’Alvano massif, involving the Bracigliano, Quindici, Sarno and Siano

towns (Fig. 5.10). A comprehensive description of the events occurred inside

the study area is provided by Cascini (2004) and Cascini et al. (2000, 2003,

2006a, 2006b). Based on these studies, here, only some relevant features are

recalled that will be deepened in the following and further detailed in the

Appendix C.

The rainfall event began at 2.00 a.m. on Monday May 4, 1998 with

moderate intensity (cumulated rainfall of about 15 - 20 mm) until 8.00 a.m.,

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105

with total rainfall values lower than 150 mm over a 48-hour period (O.U.2.38,

1998a; SIMN 1998a, b) (Fig. 5.11). Statistical analysis of hourly and daily

rainfall, as well as cumulated rainfall related to periods of time ranging from 2

to 240 days before the event, provide some elements about the rainfall return

period: i) on an annual scale, the 4-5 May rainfall return period is below 30

years and rainfall with duration ranging from one hour to 240 days is always

below the highest data recorded so far; ii) on the contrary, with reference to the

spring, the simultaneous occurrence of various independent phenomena

(rainfall in previous months, 9 consecutive rainy days, considerable rainfall

during the last two days) represents an extraordinary event.

Vesuvius

Pizzo d’Alvano

~ 10 km

Figure 5.10 The study area (Pizzo d’Alvano massif)

Chapter 5 ________________________________________________________________________________________________________________________________________

106

NAPOLI

CASERTABENEVENTO

AVELLINO

SALERNO

40

6080

120

Pizzo d'AlvanoVesuvio

100

a) b)

Figure 5.11 Rainfall recorded in the neighbourhood of Pizzo d’Alvano massif

Within the 16 hours from 11.00 a.m. on May 5th till 2.30 a.m. on May 6th,

the Pizzo d’Alvano massif was affected by huge landslides which developed

into large flow-like mass movements along the valleys and the channels below,

down to the highly urbanized piedmont areas (Fig. 5.12). The flow-like mass

movements occurred in almost all the basins of the massif, with the triggering

zones located primarily in the upper part of those basins; in fact, 36 out of a

total of 47 basins were affected by landslides of different intensity (Tab. 5.13).

Multiple slope instability phenomena developed in most of these zones and

progressively involved larger portions of the slope, according to mechanisms

and time sequences not easily identifiable after the event. Post-failure

phenomena resulted in flow-like mass movements and, as the unstable masses

travelled downslope, their initial volume increased mainly due to the erosion of

the soil in the gullies below and, in some cases, through the incorporation of

minor slides mobilised along the flanks of the gullies. The total involved

volume was about 3’000’000 m3 (O.U. 2.38, 1998) and in many cases, unstable

masses eroded further materials along the gullies increasing the initial masses up

to 40% (Fig. 5.14). Finally, the run-out distances ranged between few hundreds

meters up to distances higher than 2 km, with velocities that at base of gullies

were estimated about 20 m/s (Faella & Nigro, 2003).

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107

a) b)

c)

d)

Figure 5.12 Some examples of the flow-like mass movements occurred in

(a) Sarno, (b) Siano, (c) Bracigliano, (d) Quindici.

Town Inhabitants Mobilised Volumes (m3)

Victims Buildings destroyed

Buildings heavily damaged

Buildings slightly damaged

Bracigliano 5105 424832 6 2 7 2

Qundici 3023 1379513 11 19 154 46

Sarno 31509 1124980 137 126 195 66

Siano 9265 262706 5 5 34 10

3192031 159 152 390 124

Table 5.13 Intensity of flow-like mass movements (data from Cascini, 2004)

Figure 5.14 Triggered and total mobilised volumes (Cascini, 2004)








Chapter 5 ________________________________________________________________________________________________________________________________________

108

The damages produced by the events (classified and reported at 1:5000

and 1:2000 scales) were essentially related to the impact of the unstable

materials on walls perpendicular to the flow path and/or to the presence of

structures inside the expansion and deposition of the flow-like mass

movements (Tab. 5.13, 5.15). In the former case, many masonry structures

were seriously damaged while structures with reinforced concrete frameworks

were not seriously weakened by damage to their brick outside walls, with the

exception of direct impacted structures case (Fig. 5.16). In the latter case,

basements and ground floors were often filled with pyroclastic soils with rare

cases for the upper floors where, instead, frames and some partition walls were

damaged. One particularly significant case shows that the impact forces were

able to completely cut off the structural connections between the foundations

and the parts above ground-level, thus causing the translation of the building

over a distance of 50 m.

Survey Velocity Effects

Sarno town

1 V 13.8 m/s r.c. column failure

2 14.4 m/s V 20.7 m/s r.c. column failure



5 3.2 m/s V 11.6 m/s brick external wall failure


7 V 5.6 m/s tuff masonry walls failure

8 V 1.5 m/s stone masonry walls failure



11 V 10 m/s tuff masonry walls failure due to only hydrostatic pressure

12 V 10 m/s global collapse of ground-floor columns

Quindici town




4 3.1 m/s V 13.1 m/s tuff external wall failure

Table 5.15 Velocities of unstable masses and their effects

(modified after Faella & Nigro, 2003)


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109

Figure 5.16 Damages produced by the May 1998 event: a) traces on external

walls; b) failure of corner columns, c) maisonary building impacted

by a flow-like mass movement, d) failure of ground floor columns

and building translation (Faella et al., 2003)

5.2.2 The scientific debate

Due to the relevance of the occurred phenomena, many contributions can

be found in literature aimed to deepen several aspects of the triggering stage.

However, the scientific debate is yet open and several unsolved questions are

hereafter discussed with reference to the proposed modelling and classification

that are briefly summarised according to the schemes adopted in Section 3.3

and Section 4.2.

Chapter 5 ________________________________________________________________________________________________________________________________________

110

As it concerns the modelling, some studies were based on black-box

models (Sect. 4.2) often addressing the Pizzo d’Alvano massif together with

other similar reliefs. For instance, with reference to the territory of Campania

region, Rossi et al. (1998) plotted 2-day cumulative rainfall (Pe) versus

antecedent cumulative rainfall computed from the start of the rainy season (Pa)

(Fig. 5.17). However, Fiorillo & Wilson (2004) observed that the distance

between the envelopes of the storms which failed and triggered “debris flows”

indicates the uncertainty of the lower envelope as a hydrological threshold.

Figure 5.17 Hydrological evaluation of rainfall, occurred before May 1998,

producing or not landslides in pyroclastic covers

of Campania Region (Rossi et al., 1998)

De Vita (2000) performed some analyses of typical rainfall patterns to

reconstruct a relationship between rainfall and “debris flow” occurrence and to

highlight empirical hydrological thresholds (Fig. 5.18). In particular, the Author

correlated antecedent rainfall and daily rainfall of major storms to determine

the minimum amount of daily rainfall needed to trigger debris flows. The

results of such research highlighted, for instance, that empirical hydrological

thresholds set up for the Lattari and Salerno Mountains seem to be different

from those pointed out for the Sarno Mountains. In particular, daily rainfall

higher than 50 mm is needed in the Lattari mounts and Salerno area, while

debris flows can occur for daily rainfall lower than 32 mm, following high

antecedent rainfall in the Sarno area.

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111

Figure 5.18 Rainfall threshold in Campania region (De Vita & Piscopo, 2002)

With reference to the same phenomena, Sirangelo and Braca (2002, 2004)

developed an hydrological model to predict “debris-flow” initiation (Fig. 5.19).

This model needs a preliminary calibration, based on the historical data, and

makes use of exponential functions. In particular, the Authors applied the

model to the sample area – on the basis of hourly precipitation data available

from a real time rain gauge installed immediately after the catastrophic event

occurred on May 1998 – highlighting its capability to provide relatively few

false alarms.

The described models do not take into account geological elements that

according to other Authors are relevant predisposing factors for the triggering

stage of the instability phenomena.

For instance, with reference to the basins facing the town of Sarno,

Pareschi et al. (2000) recognised the drainage network typology, the slope angle

and the basin shape factor as key elements for instability (Fig. 5.20). These

morphometric data were directly derived from a digital elevation model and five

different susceptibility classes were identified based on different values for the

utilised classes.

Chapter 5 ________________________________________________________________________________________________________________________________________

112

Figure 5.19 Mobility function for the Pizzo d’Alvano massif

(Sirangelo & Braca, 2004)

Figure 5.20 A geological model for the Pizzo d’Alvano massif

(Pareschi et al., 2000)

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113

Di Crescenzo & Santo (2005) analysed flow-like mass movements

occurred on May 1998 and other similar events in the Campania region by

means of aerial photo interpretation, geological survey and geomorphological

and morphometric analyses at a 1:2,000 scale. Concerning the predisposing

factors, the Authors observed that: i) the most of investigated landslides

occurred above or below roads/tracks and rocky cliffs, ii) no clear correlations

seem to exist between unstable areas and both distribution of plants and slope

exposure, iii) high-altitude ephemeral springs may have played an important

role even though not addressed in their contribution. From the morphometric

analysis, the Authors concluded that the slope angles for sliding zones mainly

concentrate between 26° and 30° while the apical angle – the angle of the upper

vertex – of the most landslides ranges from 15° to 29°. On the other hand, the

Authors observed that the landslides are mainly triggered in proximity to

divides because of subsurface circulation of water in the uppermost portions of

the carbonate massifs; moreover, slope angle and crown zone and relief energy

seem to not directly influence the aperture of the apical angle of source areas

(Fig. 5.21).

Figure 5.21 Main morphological and morphometric parameters

of flow-like mass movements (Di Crescenzo & Santo, 2005)

Chapter 5 ________________________________________________________________________________________________________________________________________

114

Guadagno et al. (2005) suggested that all the flow-like mass movements

events occurred on May 1998 were characterised by triangular shaped source

areas. In particular, these last were mainly related to the presence of natural

scarps or artificial cuts inside pyroclastic covers according to morphological

schemes that takes into account essentially local in-situ conditions (Fig. 5.22).

On the other hand, some morphometric analyses were proposed referring

to the landslides geometrical characteristics. At this regard, the Authors found

that the apex angles of source areas is poorly correlated to the slope angles

while more strictly related to the height of the scarps. In this sense, the

obtained results highlighted the role of the energy of the unstable masses on

the evolution stage of the landslide.

Figure 5.22 Settings for the main source area types for 1998 landslides

(Guadagno et al., 2005)

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115

Brancaccio et al. (1999), in the framework of the activities of the

O.U.2.41, recognised and characterised three main settings inside the Pizzo

d’Alvano massif respectively constituted by the summital paleosurfaces

landscape, the border slopes (where steepest elements results in

correspondence of the flanks or head of gullies, as well as at the toe of morpho-

lithological frames) and piedmont zones (Fig. 5.23). The Authors observed that

the most of landslides of May 1998 occurred inside the border slopes and often

at the boundary with the summital paleo-surfaces landscape; moreover, they

connected such evidences to some aspects of the upper portions of the

bordering slopes where: i) the slope angles are often the maximum ones, ii) the

product of rainfall intensity times slope angles reach the maximum values, iii)

the pyroclastic covers exhibit relevant thicknesses and iv) the presence of

mountain ways results in lateral anthropogenic cuts and concentration of sub-

superficial flows. Moreover, several landslides occurred along the flanks of the

gullies where first-failure phenomena were related to high values or

discontinuities of the slope angles or possibly induced by lateral enlargement of

landslides or vibrations caused by previous instability phenomena. According to

the Authors flooding-induced landslides seem to be unrealistic inside the

channels. Finally, the Authors distinguish among downwards and upwards-

propagating landslides. The first ones exhibit triangular topographical shapes

while more stumpy shapes characterise the downwards-propagating landslides.

Guida (2003) stressed the relevant role played by zero order basins (zobs)

as location and material source of shallow slope failures and resultant “debris

flows” (Fig. 5.24). According to the Author, many processes take place inside

the ZOBs controlling production, distribution, intermediate deposition,

redistribution and successive mobilisation of slope material. As a consequence,

time evolution of ZOB is controlled by cycles of successive air fall deposition,

downslope and convergent material transfer, colluvial deposit infilling and

thickening, material mobilisation as flowslide-debris flow and colluvial refilling.

In particular, zobs cycle times in carbonate landscape of the Campania region

are considered by the Author shorter than those reconstructed in other

temperate climate regions.

Chapter 5 ________________________________________________________________________________________________________________________________________

116

Figure 5.23 Some geomorphological schemes for Pizzo d’Alvano massif:

1) summital paleosurfaces landscape, 2) border slopes,

3) piedmont zones, 4) plains (Brancaccio et al., 1999)

Figure 5.24 Predicted time series of stored/mobilised sediment and related

process in ZOB (a) and in a typical first/second-order channel (b) of the Pizzo

d’Alvano massif from 4000 years b.p. (Guida, 2003)

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117

The hydrogegological factors were deepened by several Authors such as,

for instance, De Vita & Piscopo (2002) who underlined that in some areas,

close to the trigger areas of the landslides of May 1998, strong variations (up to

three-order size) in hydraulic conductivity can be found in the first few metres

below the ground surface due to: i) heterogeneities inside the pyroclastic

deposits; ii) presence of a fractured carbonate bedrock; iii) variation in

fracturing degree of the upper carbonate bedrock (Fig. 5.25). According to the

Authors, the hydrogeological investigations allowed to assess that local

hydrogeological conditions might affect slope stability of pyroclastic covers

during heavy rainfall or after recharge period of superficial aquifers.

Figure 5.25 Hydrogeological factors for the Pizzo d’Alvano massif

(De Vita & Piscopo, 2002)

Similar contributions were provided by Cascini et al. (2000) who placed

the occurred phenomena in a unitary framework on the basis of geological,

geomorphological and hydrogeological studies. Among the various factors

pointed out as responsible for triggering, the Authors stressed the role played

by rainfall infiltration and by water outlets from bedrock. At this regard, the

Chapter 5 ________________________________________________________________________________________________________________________________________

118

presence of a rather low water outlet rainfall intensity equal to twice those

recorded at the toe of the massif seem to be fundamental factors for slope

instability conditions. The obtained results confirmed the findings of an hydro-

geomorphological model set up for some hillslopes inside the study area (Fig.

5.26). These results provided a significant insight about the triggering stage,

highlighting the need of an interdisciplinary approach, already adopted by the

O.U. 2.38 (1998).

Upper catchment (Z.O.B.)

Upper slope

Lower slope

Recent

Ancient fan

Lower morphological frame

Upper morphological frame

Talus Talus

LowerFilled

fan

catchmentmain channel

Recentfan

Bedrock

Water outlet

Runoff

OUTLET

Cascini et al. (2000)

Figure 5.26 Hydro-geomorphological model for the Pizzo d’Alvano massif

(modified form Cascini et al., 2000)

Other Authors utilised physical-based models among those discussed in

Section 4.2. For instance, Fiorillo & Wilson (2001) correlated the rainfall of

major storms recorded in recent decades and “debris-flow” occurrence by

means of a two-step procedure (Fig. 5.27). In the first one, soil moisture levels

were modelled, on a daily scale, through the hydrological balance between

precipitation and evapotranspiration. The second step was related to the

accumulation of surplus moisture from intense rainfall, leading to the

development of positive pore pressures, analysed through the model set up by

Wilson & Wiezoreck (1995). In combination with hourly rainfall records, this

model was used to compare hydrological effects of different storms. The

critical amount of rainfall was computed through the time occurrence of debris-

flow and rainfall intensity-duration thresholds were outlined, with values lower

than those proposed by other Authors.

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119

Figure 5.27 Physical-based models for rainfall threshold in the Campania region

(Fiorillo & Wilson, 2004)

Chirico et al. (2002) assumed, for the slopes facing the town of Sarno, a

transient physically based model to estimate the phreatic surface depths inside

the cover and an infinite slope model for computing the factor of safety. The

performed analyses highlighted that such an approach allows to reproduce only

a little amount of the observed unstable areas.

For the same slopes, Frattini et al. (2004) coupled an infinite slope model

with two different transient hydrological models taking into account, to some

extent, soil unsaturated conditions. According to the Authors, in fact, both

vertical and lateral fluxes are considered primary factors for the attainment of

instability conditions. In particular, a quasi-dynamic model was used to simulate

the effects of lateral flow while a diffusion model mainly addressed the vertical

response to rainfall. The results obtained with the two models are quite

different, probably highlighting that both the processes are relevant for slope

stability conditions. Moreover, it must be observed that the hydrogeological

aspects referred, for instance, by Cascini et al. (2000) and DeVita & Piscopo

(2002) are not taken into account (Fig. 5.28).

Chapter 5 ________________________________________________________________________________________________________________________________________

120

Figure 5.28 Findings of some physically-based models (Frattini et al., 2004)

Finally, for the events occurred on May 1998 along the hillslopes of Pizzo

d’Alvano massif, geotechnical analyses were also performed by some Authors

referring to their triggering stage. Among the most relevant contributions,

Cascini et al. (2003), performed seepage and slope stability analyses based on a

detailed stratigraphical reconstruction and stressed the importance of bedrock

outlets as key triggering factor for some observed failures (Fig. 5.29a).

Moreover, the Authors outlined the role of negative pore water pressures and

soil cover trasmissivity as relevant aspects towards their modelling.

Similar analyses for simplified stratigraphical schemes were proposed by

Crosta & Dal Negro (2003) that considered the presence of geometrical and

stratigraphical discontinuities within the pyroclastic covers (Fig. 5.29b). In

particular, the role of pumice layers is emphasized as well as the role of

transient features of pore water pressure regime induced by critical rainfall of

May 1998.

Guadagno et al. (2003) performed geotechnical analyses through a finite

difference code assuming different steady-state saturated flow conditions

involving either pumice layers or also ashy layers (Fig. 5.29c). In particular, the

Authors, for slope instability of the pyroclastic covers, emphasized the role of

natural or anthropogenic geometrical discontinuities that notably increase the

simulated displacements for upslope portions of the cover up to failure.

The case study ________________________________________________________________________________________________________________________________________

121

A

B

C

D

E

F

G

H

I

L

MN

O

0 25 50 100 125 150 175 200 250 275 300 325 35022575

725

700

675

650

625

600

550

525

500

575

OUTLET

Lenght (m)

z (m

a.

s. l

.)

zone

1

zone

2

zone

3

zone

4

zone

5

zone

6

zone

7

zone

8

zone

9

zone

10

zone

11

zone

12

zone

13

zone

14

zone

15

zone

16

zone

17

zone

18

750750

750

700

650

600

550

800

750

700

650

600

550

500

450450

A B C D E F G H I L,M,N,O

Upper ashy soils (Class B)

Pumice soils

Lower ashy soils (Class A)

0

1 m

2 m

R

R A

R A

A R

A R

A R

A

A

R

R

A

A

RA

RARA

R

ARA

RA

RA

A

R

AR

AR

Source areas

R

A R

0 25m 50m 100m

A

A

Seismic section

Pit

Slope section

Basin limit

c)

a) b)

Figure 5.29 Some schemes for geotechnical analyses performed by:

a) Cascini et al. (2003), b) Guadagno et al. (2003), c) Crosta & Dal Negro (2003)

With reference to the above contributions, it must be stressed that distinct

classifications were proposed for the analysed flow-like mass movements (Tab.

5.30), according to both the utilised approach and performed modelling (Sect.

3.3). For instance, some terms clearly testify a main focusing on the triggering

stage (e.g. landslide, debris slide, soil slip), other ones are more related to the

interpretation of post-failure stage (e.g. landslide-flowslide, flowslide) and,

finally, some other terms are used to mainly address the propagation stage (e.g.

Chapter 5 ________________________________________________________________________________________________________________________________________

122

debris avalanche, debris flow, flowslide-debris flow). As a matter of fact, a

shared classification of these phenomena is not available even though it is yet

an urgent need since an adequate landslides classification could improve the

choice of the most suitable methods and reference schemes (Sect. 3.3).

Landslide Del Prete et al. (1998), Cascini et al. (2000), Calcaterra & Santo (2004), Frattini et al. (2004)

Landslide-flowslide Olivares & Damiano (2004)

Debris slide Calcaterra et al. (2004)

Debris avalanche Revellino et al. (2004)

Debris flow

Guadagno et al. (1999), Pareschi et al. (2000), Fiorillo et al. (2001), Pareschi et al. (2002), Budetta (2002), D’Ambrosio et al. (2003), Aleotti et al. (2003), Zanchetta et al. (2004), Fiorillo & Wilson (2004), Revellino et al. (2004)

Multiple debris flow Calcaterra et al. (2000)

Mudflow Martino & Papa (2003), Sirangelo & Braca (2004)

Flowslide-debris flow Guida (2003)

Flowslide Cascini & Sorbino (2003), Cascini et al. (2003), Olivares et al. (2003), Musso & Olivares (2003), Picarelli et al. (2004), Bilotta et al. (2005)

Soil slip Montrasio & Valentino (2004)

Figure 5.30 Different classifications for May 1998 flow-like mass movements

The case study ________________________________________________________________________________________________________________________________________

123

5.2.3 Remarks on used approaches

Referring to the above contributions, it can be observed that despite a

wide literature, the scientific debate is yet open with reference to both the

triggering factors and mechanisms. This is essentially due to the used sector-

based approaches that tried to explain the occurred phenomena on the basis of

rainfall characteristics, geometrical features of source area, geomorphological

aspects and hydrogeological issues.

On the contrary, all the above factors influenced the triggering

mechanisms occurred phenomena that were strictly related to the in-situ

conditions at site and massif scale as well as to the soil behaviour at REV scale.

In particular, the study area represents a geo-environmental system composed

by two strongly interacting sub-systems (Fig. 5.31). The first one is constituted

by the pyroclastic cover affected by infiltration, mass wasting processes and

eventually slope instabilities.

summit

slopes

alluvial plane

a)

basal water table

seasonal spring

temporary springs

c)A SIMPLIFIED SCHEME

b)

PYROCLASTIC COVER

LIMESTONE BEDROCK

perennial springs

Figure 5.31 A scheme for the study area: the geo-environmental system

Chapter 5 ________________________________________________________________________________________________________________________________________

124

The second system is represented by the carbonate bedrock that strongly

influenced the morphology of pyroclastic covers, that are also affected by the

sub-superficial groundwater inside the bedrock.

Consequently, to address the flow-like mass movements occurred in this

complex geo-environmental system, a multidisciplinary approach is necessary

(Fig. 5.32). In particular, the used approach combines geological,

geomorphological and hydrogeological models at massif and site scales,

geotechnical engineering modelling at site scale and geomechanical issues,

related to soil mechanical behaviour, at REV scale. This procedure, as main

advantage, allows to join the potentialities of the available models as well as to

use the available data set at different scales. Moreover, the integrated use of

different approaches allows the comparison and the mutual validation of the

obtained results, that must be considered a fundamental step towards a shared

assessment of triggering factor and mechanisms.

GEOMECHANICAL

MODEL

Massif scale

“Geology”

Site scale

“Geotechnical Engineering”

REV scale

“Geomechanics”

GEOLOGICAL

MODEL

Figure 5.32 Followed multidisciplinary approach combining

geology, geotechnical engineering and geomechanics

Particularly, the triggering mechanisms are assessed through the analysis

of the main geological, geomporphological and hydrogeological features for

both the pyroclastic covers and carbonate bedrock. Then, in-situ conditions are

The case study ________________________________________________________________________________________________________________________________________

125

referred to set up detailed computational schemes for the geomechnical

analyses, that are performed with the aid of both traditional and advanced

methods.

127

6 LANDSLIDES CHARACTERIZATION IN THE STUDY AREA

“Unfortunately, soils are made by nature and not by man, and the

products of nature are always complex… As soon as we pass from

steel and concrete to earth, the omnipotence of theory ceases to exist.

Natural soil is never uniform. Its properties change from point to

point while our knowledge of its properties are limited to those few

spots at which the samples have been collected.”

Karl Terzaghi

Landslide characterization is generally aimed to outline information

related to the causes, features and effects of such phenomena. In the following,

the attention will be focused on the predisposition, triggering, aggravating and

revealing factors as introduced by Leroueil (2004). Particularly, the

predisposition factors refer to the present situation and determine the slope

response following the occurrence of a triggering factor. The triggering factors

are those responsible of slope instability phenomena (Sect. 4.3.2). On the other

hand, aggravating produce only a significant modification of stability

conditions, while revealing factors provide evidence of slope movements but

generally do not participate in the process.

Landslides characterization is here performed for the case study discussed

in the previous chapter (Sect. 5). In particular, based on the activities developed

just after the May 1998 events in the sample area (Cascini, 2004) and referring

to the subsequent studies (Cascini et al., 2006a, b), the in situ conditions, and

soil mechanical properties are hereafter summarized and utilised in order to

achieve insights on the triggering mechanisms for the occurred phenomena.

Chapter 6 ________________________________________________________________________________________________________________________________________

128

6.1 GEOLOGICAL ASPECTS

6.1.1 General features

The Pizzo d’Alvano massif (Fig. 6.1, 6.2) is a carbonate ridge built-up on a

limestone, dolomitic-limestone and, subordinately, marly-limestone

lithostratigraphic succession. The succession, several hundred metres thick and

Lower to Upper Cretaceous aged, is referred to the “Alburno-Cervati Unit”

(D’Argenio et al. 1973). The carbonate relief ridge mainly stretching from the

Campania Coastal Plain to the West form small monoclinal structures, generally

N, E and NE dipping.

%[

#Y

%[

%[

#Y

#

#

$

$

$Episcopio

Sarno

Quindici

Siano

Pizzo d'Alvano

B41

B40

B39

B38B37

B36

B35

B4

B5 B6

B7

B3

B2

B1B1bis

B8

B9

B10B11

B12

B14

B13

B14bis

B15

B16

B17

B18 B19 B20

B21

B22 B23

B24B25

B26

B34B27

B28

B29

B33

B31

B32

B42

B43

B44

B45

Lavorate

2

1

S

N

EW

600 0 600 1200 1800 m

Figure 6.1 Pizzo d’Alvano massif: basin involved (1) and not involved (2)

into the May 1998 events (Cascini et al., 2004)

Landslides characterization in the study area ________________________________________________________________________________________________________________________________________

129

An high-angle normal fault system pull down the morphostructure toward

the “nocerino-sarnese” plain, undercutting the original hemi-anticlinal structure,

with NW-SE axis strike and NE dip. Therefore, a down-slope attitude of the

bedrock characterises the north-east facing mountain side, near the Quindici

town, while a cross-slope to down-slope one along the Sarno south-west

mountain front. Figure 6.2 shows the massif structural network as major border

faults and inner secondary faults, defining, in turn, the main sub-structures.

0 1 2km

1

2

3

4

5

6

7

Pizzo d’Alvano

Sarno

Siano

Bracigliano

Quindici

0 1 2km

B

B

A

A

Figure 6.2 1 Simplified geological map of Pizzo d’Alvano massif (1:25000):

1) dolomized limestones, 2) microcrystalline limestone 3) calcarenites and

calcilutites, 4) calcarenites, 5) calcarenites and calcilutites, 6) dolomized

limestones, 7) calcarenites and calcilutites (modified after Cascini et al., 2006)

Chapter 6 ________________________________________________________________________________________________________________________________________

130

Inside the bedrock different lithologies can be distinguished: from the

base, dolomitized limestone, calcarenite, calcilutate, marls, fossiliferous layer

(Fig. 6.2). The origin and properties of these layers are quite different and they

have strong consequences from an hydro-geological point of view, as discussed

in the following.

Referring to geomorphological features, Pizzo d’Alvano range represents a

NW-SE oriented morpho-structure, characterised by gently sloping smooth

summit landscape with ridges, plains and endoreic basins (Fig. 6.3).

Surrounding the summit, relatively steep (generally >30°) mountain slopes,

characterised by a deeply incised and rectilinear valleys and ravines, are located.

The summit plains represent the remnants of an ancient morphological

karstic surface, modelled during several morphogenic stages from Lower

Pliocene to Upper Pliocene-Lower Pleistocene.

The mountain slope represent the final landforms of prolonged poliphase

modelling events: i) fault scarp creation, ii) slope replacement evolution and iii),

the subsequent linear dissection (Cinque et al. 1988). The profile’s regularity of

mountain slopes is affected by structural (minor faulting, jointing) and litho-

stratigraphic elements, as limestone beds more resistant to dissolution. The

mountain slopes are linked to the lowland by gently piedmont alluvial fan of

various ages and shapes. The fans are frequently anastomised, juxtaposed and

incised. The erosional and depositional landform of the alluvial fans highlights a

systematic repetition - still in progress - of depositional events, superimposed

on older coarse deposits. The parent material of these fan deposits results from

both air-fall pyroclastic soils and reworked materials, coming from the colluvial

hollow located in the uppermost mountain slope and/or entrenched along the

main and secondary channels.


131

km210

Pizzo d’Alvano

Sarno

Siano

Bracigliano

Quindici

0 1 2km

1

2

3

4

Figure 6.3 Simplified geomorphological map of Pizzo d’Alvano massif (1:25000):

1) colluvial deposits in karst areas, 2) zero order basins, 3) ancient landslides

zones, 4) alluvial fans (modified after Cascini et al., 2006).

From an hydro-geological point of view, the Pizzo d’Alvano massif

constitutes a sub-structure of the larger Hydro-geological Structure of Monte

Pizzone - Pizzo d'Alvano (Civita et al. 1970; Civita et al. 1973; Celico & De

Riso 1978; Celico 1978).

In particular, the hydro-geological series is composed (Fig. 6.4) of the

following hydrogeological units (Civita et al. 1970):

basal dolomitic complex showing a medium permeability due to

highly fracturing and low karstification;

dolomitic-limestone complex with high-medium secondary

permeability;

Chapter 6 ________________________________________________________________________________________________________________________________________

132

limestone-marly complex: including the well known "green marl with

Orbitolinae level", "the only impervious bedding aquifer" (Civita et al.

1970), variable in thickness and continuity;

upper-limestone complex with high to very high permeability for

fractured and deep karstified beds and banks, constituting the

percolating upper zone in the karst hydro-geological structure;

tuffaceous and pyroclastic superficial complex, with medium to low

permeability, constituting a surrounding aquifer for basal water table

along the piedmont.

The hydro-geological structure is characterized by a high degree of

average production, a moderate frequency of underground down-flow and a

low degree of recharge.

The analyses performed by O.U. 4.21 N (1998a) - based only on known

spring discharges affected by permanent regimen - highlighted, in the upper

part of the Pizzo d’Alvano relief, a distinct minor groundwater flow system.

This last is characterised also by the presence of perched water tables due to: i)

the heterogeneity and anisotropy of the fractured rock; ii) the spatial variability

of the karstic system; iii) the presence of less permeable lithostratigraphic

horizons; iv) the presence of structural lineaments representing local aquifers

(Civita et al., 1970). In this sense, a paramount role is played by the

heterogeneity of carbonate bedrock (Fig. 6.4)

LITHOLOGY HYDROGEOLOGY

UPPER LIMESTONE

DOLOMIZED LIMESTONE

DOLOMITIC-MARN

LIMESTONE

EPHEMERAL SPRINGS

- fractures

- karsic conducts

- hydro-wedgeMEDIUM PERMEABILITY

- FRACTURATION

MEDIUM PERMEABILITY

SPRINGS

(for permeability limit)

BASAL SPRINGS

(for permeability threshold)

HIGH PERMEABILITY

- FRACTURATION

- KARSISM

Figure 6.4 Stratigraphical and hydrogeological elements within the limestone

bedrock (modified after Cascini et al., 2006).


133

The investigation carried out by the Territorial Survey (Cascini et al., 2006)

allowed to identify and located many of upland springs, including the ones

characterised by temporary flow rate. Figure 6.5 shows the main

hydrogeological sub-structures and the nested wedge-like hydro-structures,

which influence the upper groundwater paths in the limestone aquifer. These

hydro-structures allow the percolation toward converging preferential flow

paths to their lower apex where a minor karts network is thus developed and

associated to perennial or temporary springs.

Figure 6.5 Structural and hydro-structural conditions for Pizzo d’Alvano massif:

a) schematic geological section, b) scheme of an hydrowedge, c) summital

portion of an hydrowedge (modified after Cascini et al., 2006)

Significant relationships were observed between nested “hydrowedge”

structures and the spring flow rate during the Territorial Survey's activities

(Cascini et al., 2006). Perennial flow rates are associated to: i) first order "wedge

hydro-structures"; ii) seasonal egresses are related to second order wedges; iii)

occasional egresses relate to third order wedges. An hydro-structural map of

both permanent and temporary springs is provided in Fig. 6.6.

Chapter 6 ________________________________________________________________________________________________________________________________________

134

Spring connected to the upper

perched water tableFlow direction of the

perched water tableMain structural elements

Flow direction of the

temporary springs

Secondary structural elements

Deep karstic field filled by

pyroclastic deposits

Spring connected to the

intermediate perched water table

Spring connected to the deep

groundwater level

0 1 2 km

Figure 6.6 Pizzo d’Alvano massif: hydro-structural map

(modified after Cascini et al., 2006)

The high plains and the mountain slopes are diffusely covered by

pyroclastics soils, deposited from the explosive phases of the Somma-Vesuvius

volcanic complex activity (Sect. 2.1, 5.1.1), both as primary air-fall deposits and

successively re-worked deposits (Fig. 6.7).

In particular, according to Cascini et al (in prep.), the primary deposition

phenomena were deeply influenced by the limestone bedrock morphology

(Sect. 2.1). In particular, the clinostratified primary deposits, a few metres thick,

are characterised by almost continuous pumice and ashy layers.


135

The secondary deposits – reworked by sheet wash, mass-wasting and

fluvial processes – are mainly found as debris at the piedmont, colluvium on

the hillslopes hollow and alluvium in the upland tectonic-karstic depressions.

The total thickness of the pyroclastic cover in these landforms ranges between

about 1 and 20 metres. Similar deposits are also located in the upper portions

of the slopes where both the natural processes (diffused or localised erosion

processes, landsliding and sediment transport phenomena) and/or the

anthropogenic activities were able to heavily upset the cover setting so

modifying the topography, the stratigraphic conditions and the superficial

drainage network.

For both primary and re-worked soils, pedogenetic processes modified

their original mineralogy, forming paleosols according to mechanisms discussed

in Sect. 2.2. As a consequence, extremely variable configurations, with reference

to both thicknesses and stratigraphical settings, can be observed.

A

B

C

D

E

Fkm210

Pizzo d’Alvano

Sarno

Siano

Bracigliano

Quindici

0 1 2km

Figure 6.7 Pyroclastic cover thickness map: a) h=0 m, b) h<0.5 m,

c) 0.5m<h<1m, d) 1m<h<2m, d) 2m<h<5m, e) h<5m

(modified after Cascini et al., 2006)

Chapter 6 ________________________________________________________________________________________________________________________________________

136

6.1.2 Hillslope models

On the basis of the above information, further analyses are here carried

out in order to define a geomorphological model mainly for Sarno, Quindici

and Bracigliano mountain fronts, while other hillslope sectors seem

characterised by intermediate condition among the previous ones (Fig. 6.8)

(Cascini et al., in prep). For each of them, typical sections can be traced, along

both the ridges and the stream profiles, regardless of whether sub-aerial or

buried. Moreover, the morphometric, morphologic and morphoevolutive

features can be schematically arranged into homogeneous basic elements.

In all these models, the highest slope segment coincides with the summit

ridges where the natural geomorphic processes are negligible due to the

generally low slope angles. The only possible geomorphic processes are those

induced by pedogenetic processes and man-made activities such as agricultural

and forestry practice. The length of this segment can vary from few tens to

hundreds of meters while the longitudinal slope angles are not exceeding 3°.

The second segment is represented by a short shoulder located between

the summit upland and the uppermost downslope knick-line. This segment is

affected by sheet erosion and rill-gully erosion resulting only on barren slopes.

In the Sarno model, the shoulder slope segment generally terminates on the

free face "upper lithological scarps" (so called “pestella alta”). The length of this

segment varies from few meters to tens of meters and profile slope angles

among 5° and 10°.

The third segment notably differs for the three quoted models. In the

Sarno model, downslope the upper lithological scarp, an upper backslope

segment is characterised by a superficial regular and rectilinear profile with

average slope angles of about 30-32°. Sub-superficial transversal profile shows a

well developed paleo-drainage network, incised in the carbonate bedrock.

Actually, this paleo-drainage is modelled as colluvial swales, so called "Zero

Order Basin" (ZOB), previously covered by successive air-fall pyroclastic

deposits and gradually filled with colluvial mass wasting from upstream slope

and noses. In the Quindici model this segment is substituted by the upper part

of the slopes along the valleysides, and it is characterised by a pluricorsual

paleo-drainage over a slightly incised bedrock buried by a complex system of


137

multiple zobs (Guida, 2003). Finally, in the Bracigliano model, this segment is

represented by, at least, three orders of litho-morphological scarps (with heights

of tens of meters) along both the stream and ridge profiles. The multiple

branchs of drainage system are deeply incised in the bedrock and segmented in

various orders of scarps. Only in the upper portions of this segment

discontinuous and limited zobs are present.

Figure 6.8 Geological hillslope models: A) Sarno model, B) Quindici model,

C) Bracigliano model, 1) summit, 2) nose, 3) inner gorge, 4) talus-debris slope,

5) alluvial fan (a. ancient, b. recent), 6) main channel, 7) transient channel,

8) head valley, 9) zob, 10) litho-morphological scarp.

The upper backslope segment ends with subvertical “lower lithological

scarp” (up to 30m high), in the Sarno model. Conversely, for the Quindici

model, the scarp is reduced or absent and for the Bracigliano model, it

represents the first stage of a process leading to the formation of a slope with

multiple litho-morphological scarps. Finally, down-stream slope unit is

composed by: i) secondary ravines with main channel filled with debris and

alluvial materials, ii) nose interfluve, iii) triangular facet elements that are almost

completely barren.

Chapter 6 ________________________________________________________________________________________________________________________________________

138

The main channel, located downslope the lower lithological scarp, is

substantially different in the three models. For the Sarno model, it is shorter,

more incise and flatter. In the Quindici model it is quite wide, with flatter

flanks. Finally, in the case of Bracigliano model, intermediate lengths and more

incise inner gorge can be recognised.

In conclusion, the present morphological setting result from tectonic,

volcanic and denudational events, which included the following phases:

neotectonic fault-scarp creation;

mountain slope modelling by slope replacement modelling

incision of gullies and minor drainage network (bedrock hollows) of

subsequent type along structural (minor faults and joints)

discontinuity;

covering of the above landforms by air fall deposits in repetitive

phases (falling);

progressive filling of the resultant Zob’s by colluvium from mass-

wasting processes (filling); (colluvial hollows)

progressive filling of main channel from alluvial material by mass

transport and mass movement processes;

cyclical phases of discharging and re-filling, by shallow landslide

activity, fall covering and colluvial/alluvial refilling of Zobs, as well as

main channel.

In all the above models, the pyroclastic material gradually transfers from

shoulder to the upper backslope segment, through permanent mass diffusive

movements (diffuse mass wasting or temporary and local soil slip or landslides)

from the upper edges of the litho-morphological frame. During extreme events,

such as that occurred in May 1998, this mass transfer phenomena can be

significantly rapid and generalised. At this regard, Guida (2003) discussed the

role of zobs in the occurrence and recurrence of the huge flow-like mass

movements in pyroclastic soils, within the carbonate landscape.


139

6.2 TRIGGERING MECHANISMS

Moving within the framework discussed in the previous section, the main

morphological settings of the Pizzo d’Alvano relief, at massif scale, can be

outlined. These last essentially consist in open slopes, concavities and gullies.

The open slopes are mainly concentrated in the hillslopes facing the town of

Sarno; the wider gullies are located at the Quindici slopes, while concavities are

widespread all over the massif (Fig. 6.9).

The recognition of these different settings, at massif scale, is a

fundamental step towards a preliminary zoning of the areas susceptible to

different triggering mechanisms and, therefore, to distinct post-failure and

propagation stages for the occurring flow-like mass movements.

Pizzo d’Alvano

Sarno

Siano

Bracigliano

Quindici

0 1 2km

Figure 6.9 Simplified “morphological map”: 1) concavities,

2) open slopes, 3) gullies

Chapter 6 ________________________________________________________________________________________________________________________________________

140

The next step consists into the analysis of the source areas of the

instability phenomena occurred in May 1998. At this regard, an extreme

variability of the source areas can be easily recognised (Fig. 6.10) whose shapes

consisted, for instance, in elongated, triangular, coalescent, graped and linear

landforms. Relevant differences can be detected also with reference to the

dimensions of source areas coming from 100 m2 up to 20’000 m2. Moreover,

the unstable portions of covers were characterised by different thicknesses, and

the intensities of the occurred phenomena were extremely different in terms of

mobilised volumes and run-out distances.

Figure 6.10 Different source areas detected in-situ with elongated (a),

pseudo-triangular (b), anastomised (c), graped (d), linear (e, f) shapes.

Following the methodological approach discussed in Section 5.2.3, the

attention can be then focused, at site scale, to the main natural control factors,

capable to influence the slope stability conditions. Here, they have been

grouped into four main classes, namely, the bedrock-cover contact, the

stratigraphical setting, hydro-geological setting, sub-superficial groundwater


141

circulation (Fig. 6.111). As far as the bedrock-cover contact, it can be rectilinear

or slightly ondulated (BC1), with a buried (BC2) or visible (BC3) break of

slope. The stratigraphical setting can include different configurations of recent

soils over older pedogenitized soils with distinct cover-bedrock contact and

interface bewteen the layers (CV1, CV2) or completely remoulded material with

a regular cover-bedrock contact (CV3). The bedrock hydro-geological setting

can be characterised by the presence of hydrowedge (HG1), carsified levels

(HG2) or effimer diffuse egresses form bedrock (HG3). As far as the sub-

superficial groundwater circulation, rectilinear (GW1), convergent (GW2) and

divergent (GW3) configurations can be recognised. Finally, both gully, rill and

sheet superficial erosional processes (SF1, SF2, SF3) occur along the analysed

hillslopes.

All these elements have been mapped all over the massif, resulting in the

improvement of the available data set concerning all the source areas of the

phenomena occurred on May 1998.

1 2 3

BEDROCK - COVER CONTACT

BC

COVER SETTING

CV

HYDRO-GEOLOGICAL SETTING

HG

GROUNDWATER SETTING

GW

SUPERFICIAL WATER SETTING

SF

Figure 6.11 Relevant factors for triggering mechanisms

Chapter 6 ________________________________________________________________________________________________________________________________________

142

Moving within the above framework, and combining the relevant factors

detected at massif and site scale, six typical triggering mechanisms can be

identified and mapped (Cascini et al., in press). These last, respectively named

M1, M2, M3, M4, M5, M6 (Fig. 6.12) are characterised by different intensity in

terms of mobilised volume and, at the same time, not casually distributed on

the massif (Fig. 6.13).

Concave-convergent

bedrock-cover hollow

Open slope with

morpho-lithological scarpWide headvalley side

with tracks

Close headwater

with multiple channels

Convex slope

with incisionConvex-rectilinear-concave

footslope

Water inflow

M1 M2 M3

M4 M5 M6

Figure 6.12 Triggering mechanisms for the study area

The triggering mechanism M1 occurs inside colluvial hollows associated

to zero order basins (ZOBs) affected by convergent sub-superficial,

groundwater circulation inside pyroclastic covers and water supplies from

carbonate bedrock. The triggering mechanism M1 is characterised by “spoon-

shaped” source areas with a crown zone not exceeding ten meters, a central

part of some ten meters and a subsequent thinning towards the transport zone.

In the triggering zone, the transversal and longitudinal profiles result in a

setting that get deeper downwards in the pyroclastic covers.

The triggering mechanism M2 is characterised by a triangular shaped

landform with an upper crown (few meters wide) and flanks progressively

downslope enlarging. This mechanisms is observed in some upper open slopes

and it is associated to free or buried scarps based on the following in-situ

evidences: i) some instability phenomena occurr at the base of natural scarps; ii)

some slides (Cruden & Varnes, 1996; App. A) occurr at the top of natural

scarps not producing any further instability at the base of the scarp; iii) some


143

slides occur at the top of natural scarps and induced slope instability

phenomena at the base of the scarps. These in-situ evidences highlight the

possible relevant role of both water supplies at the base of the scarps and

impact of unstable masses on still in place soils. In all these cases, the slip

surfaces are located at the interface among different pyroclastic soil layers or at

the bottom of the cover. The same mechanism is detected also along the

triangular facets at the lowest portions of the hillslopes.

The triggering mechanism M3 produces complex source areas that, due to

the collected in-situ observations, are related to coalescent and/or laterally

enlarging local instabilities, greatly conditioned by man-made tracks. These last

induce heavy modifications to the pyroclastic covers and concentrate superficial

waters towards the bends of trackways or in singular points along the hillslopes.

The lateral enlargement is deeply influenced by the presence of a particular

morphostratigraphical setting of the cover, that is a multiple zobs configuration.

This mechanism is also favoured by parallel gullying and diffuse bedrock outlets

coming from carbonate franapoggio bedrock.

The triggering mechanism M4 is characterised by “graped” source areas

located at the head of gully and corresponding to multiple landslides. These last

are essentially related to heavy superficial water supplies and involve only a

portion of pyroclastic cover, with “V” shaped transversal profiles. These

phenomena correspond to a particular evolution of the vallive headwaters in a

general process of retrogression of the “transient channel” (Hock & Godlet,

1960).

The triggering mechanism M5 occurs along the open slope with a convex

longitudinal profile where, due to natural or anthropogenic factors, deep gullies

are present inside the pyroclastic cover. The observed source areas are

characterized by a parallel rectilinear landform elongated on the maximum

slope directions with a concave transversal profile.

Finally, the triggering mechanism M6 occurs at the base of convex-

concave hillslope or along the flanks of the inner-gorges (summital or along the

hillslopes), in correspondence of slope angle breaks (natural or produced by

anthropogenic activities). Independently from the location, it occurs in

correspondence of slope break and it involves limited volumes of the

Chapter 6 ________________________________________________________________________________________________________________________________________

144

pyroclastic cover with “short spoon-shaped” source areas affecting limited

depths of pyroclastic covers.

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Pizzo d’Alvano

Sarno

Siano

Bracigliano

Quindici

0 1 2km

M1

M2

M3

M4

M5

M6

Figure 6.13 Distribution of triggering mechanisms inside Pizzo d’Alvano massif

The variety of the above triggering mechanisms clearly outlines the

complexity of the occurred phenomena and, in the meantime, the necessity to

address both local factors and geomorphological conditions, at site and massif

scales. In fact, the recognised triggering mechanisms span a wide range of

geomorphological settings, local conditions and involved processes, not

allowing the use of a unique model or scheme to address them. In this sense,

similar analyses cannot be found among the available contributions referring to

these events. In some cases, a unique triggering mechanism is proposed to

explain the whole cluster of occurred phenomena. On the other hand, other

studies outlined some differences, only focusing on local factors without any

reference to the geomorphological contexts where they took place. Conversely,

in literature, some Authors discuss triggering mechanisms analogous to those

recognised (e.g. M1, M2, M3, M4) for the Pizzo d’Alvano massif.


145

With reference to the triggering mechanism M1 (Fig. 6.14), Johnson &

Sitar (1990), on the basis of in-situ measurements, addressed the groundwater

inside the bedrock as responsible of some instability phenomena observed on

the San Francisco Bay area in California. Moreover, Montgomery et al. (1997)

discussed the important role that spatial variation in near-surface bedrock flow

can play in piezometric response, runoff generation, and shallow landsliding in

mountain environments. According to the Authors, flow through shallow

bedrock exerts a significant control on pore pressure development in the

overlying colluvium. These phenomena are very important in the case of steep

catchments with highly conductive soils overlying relatively massive bedrock

and they surely complicate the hazard assessment for shallow landslides.

Anderson & Sitar (1997) studied experimentally the pathways of subsurface

flow in a small, steep catchment in the Oregon Coast Range. Here, bromide

point injections into saturated materials showed rapid flow in bedrock to the

catchment outlet. Bedrock flow returned to the colluvium, sustaining shallow

subsurface flow there. Finally, Lacerda (2004) showed the relevance of

superficial groundwater flow inside the bedrock on the pore water pressure

regimen.

a) b)

Figure 6.14 Some examples of triggering mechanisms induced by bedrock

outlets according to: a) Johnson & Sitar (1990) and b) Montgomery et al. (1997).

As far as the triggering mechanism M2, groundwater egresses coming

from the bedrock located at the upper portions of superficial soil covers – and

observed also some days after a huge event – were recognised by Santo & Di

Crescenzo (1999) and Budetta & de Riso (2004) as a key triggering factor in

Southern Italy (Fig. 6.15). On the other hand, the impact of unstable masses on

Chapter 6 ________________________________________________________________________________________________________________________________________

146

still in place soils was considered by Sassa et al. (1997) as the main triggering

mechanism for some events occurred in Japan (Fig. 6.16). Moreover, Costa &

Williams (1984), in a USGS debris flow video, showed mass liquefaction of

torrent deposits by impact due to a moving mass from a steeper slope. Similar

explanations were proposed by Guadagno et al. (1999) with reference to the

most of instability phenomena occurred in the Campania region, referring to

the contribution of Hutchinson and Bhandari (1971). These last Authors

recalled examples of scarps from which debris was discharged by sliding or

falling to load more gently inclined soil masses. At this regard, the Authors

underline that this mechanism is also likely to be of importance in submarine

mass movements.

a)

b)

Figure 6.15 Examples of triggering mechanisms induced by upper outlets:

a) Guadagno et al. (2005), b) Budetta & de Riso (2004)

Triangular shaped source areas were also recorded in snow avalanches and

on Mars (Fig. 6.17). For instance, Jamieson & Stethem (2002) discussed some

snow avalanches occurred in Canada. At this regard, Pielmeier et al. (2003)


147

Figure 6.16 Examples of triggering mechanisms induced by impact

of unstable soil masses (Sassa et al., 1997)

a) b)

Figure 6.17 Examples of “avalanche” phenomena for (a) snow (Jamieson &

Stethem, 2002), and (b) for volcanic ashes on Mars (Gerstell et al., 2004)

Chapter 6 ________________________________________________________________________________________________________________________________________

148

stressed the importance of stratigraphical settings for snow deposits that are

usually constituted by several layers with different features mainly related to the

climate and in-situ conditions during the deposition stages. On the other hand,

Gerstell et al. (2004) recognised on high resolution images, triangular shaped

avalanche phenomena occurred on Mars in a zone where volcanic activity and

the presence of ashy deposits have been documented by previous

contributions. In particular, these avalanches were some meters thick and in

some cases, at the head of the scars, impact craters were recognised.

With reference to the triggering mechanism M3, strictly related to the

presence of mountain roads and tracks, several contributions focus the

attention on several aspects (Fig. 6.18). For instance, Luce (2001) stressed that

erosion from mountain roads is an important contribution to the sediment

budget of many forested basins, particularly over short time scales. Similar

contributions on unpaved mountain roads were proposed by Croke (2001) with

reference to a forested catchment in south-eastern Australia and by McDonald

(2001) for a study area in the US Virgin Islands. At this regard, for a small

headwater catchment in Malaysia, Siedle et al. (2004) estimated the effects

induced by mountain roads by measuring the dimensions of all significant rills

and gullies along trackways. Finally, with reference to a sample area in Thailand,

Ziegler et al. (2004) observed that unpaved roads have the same importance of

agricultural lands in sediment production inside the stream network, despite

occupying a lower fraction of the total surface area in the basin. At this regard,

the nearly impervious nature of road surfaces may cause runoff generation. In

some circumstances, much greater volumes of runoff can be generated by cut

slopes, which may play an important role in severe erosion processes.

Moreover, lateral redistribution of runoff generated by roads can greatly affect

slope stability of neighbouring areas.


149

Figure 6.18 Some effects of mountain roads during storms: a) preferential

corridors for water and sediment, b) run-off phenomena from an upslope track,

c) network of gullies and rills, d) road runoff (Siedle et al., 2004)

As far as the triggering mechanism M4, it must be observed that some

Authors address similar phenomena occurring at the head of gullies (Fig. 6.19).

For instance, Vandaele et al. (1996), with reference to Western Europe, pointed

out the importance of ephemeral gully erosion in natural drainage ways,

induced by overland flow concentration (Fig. 6.20a). In particular, the initiation

of such process is related to the extension of drainage area, slope gradient,

Chapter 6 ________________________________________________________________________________________________________________________________________

150

presence of vegetation and magnitude of rainfall. A similar contribution was

provided by Tucker et al. (1998) that stressed the relation between the hillslope

processes and the catchment morphology and drainage density. Finally, for

Southeast Brazil, Futai et al. (2004) discussed a gully advancing mechanism in

unsaturated soils due to subsequent shallow landslides (Fig. 6.20b). In this case,

geotechnical analysis showed that erosional processes and gully evolution are

essentially related to a reduction of negative pore water pressures induced by

rainfall.

Figure 6.19 Some examples of gullies evolution through erosion and

landslide: a) Vandaele et al. (1996), b) Futai el al. (2004).


151

6.3 GEOTECHNICAL DATA SET

The geotechnical validation for the above triggering mechanisms is the

next step within the used approach and it can be pursued only through

adequate geotechnical analyses of the triggering stage, that allow to assess the

relevant triggering and predisposition factors. To this aim, accurate geotechnical

information are necessary on both environmental conditions and soil

properties. At this regard, the available data set is hereafter discussed, mainly

based on the in-situ and laboratory investigations carried out by the O.U. 2.38

(1998) and the Territorial Survey (Cascini et al., 2006).

6.3.1 In-situ conditions

In-situ investigations were performed all over the Pizzo d’Alvano massif

but, due to the relevant extension of the study area (60 km2), they mainly

addressed some basins recognised as representative of those involved by May

1998 flow-like mass movements (Fig. 6.20, 6.21). In-situ investigations were

carried out inside the summital plains, along the hillslopes and in piedmont

areas. Referring to the hillslopes where the source areas are located, mainly

topographical surveys, stratigraphical investigations and soils suction

measurements were performed.

Figure 6.20 An example of in-situ investigations for the hillslopes

facing the town of Bracigliano (Cascini, 2004)

Chapter 6 ________________________________________________________________________________________________________________________________________

152

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km210

Pizzo d’Alvano

Sarno

Siano

Bracigliano

Quindici

0 1 2km

Figure 6.21 Location of the in-situ investigations inside the study area

The topographic surveys highlighted that the ratio between volumes and

areas approaches values lower than 2,0m (volumes/areas = 1,2 ÷ 1,7), thus

underlying the small thickness of pyroclastic soils involved in first-time failure.

On the other hand, large volumes were eroded along the gullies and by minor

slides along the flanks of the gullies. Finally, an example of the aerial photo

survey, used for the definition of a detailed digital terrain model (DTM) and for

the analysis of the triggering mechanisms, is provided in Figure 6.22.

With reference to the stratigraphic conditions of the pyroclastic covers,

the in-situ investigations evidenced the extreme heterogeneity of the pyroclastic

cover stratigraphy, that did not always follow the chronology of the explosive

phases of the Somma – Vesuvius volcano. This is basically due to both the

slope varying orientations toward the volcanic complex and the cyclically

occurring colluvial, erosional and transport phenomena (Sect. 2.2.). However,

in spite of the role played by the local conditions on the stratigraphic sections,

some macroscopic differences between the pyroclastic covers mantling the hill

areas of the five affected towns (Fig. 6.23) (Cascini & Sorbino, 2003).


153

a) b)

Figure 6.22 An example of different topographical reconstruction for a

basin facing the town of Sarno: a) DTM (1:5000), b) elevation

contour lines (1:2000) (modified after Cascini, 2004)

Figure 6.23 Typical stratigraphical profiles (Cascini & Sorbino, 2003)

The soil suction measurements were performed all over the massif (Fig.

6.24) since November 1999, using “Quick-Draw portable tensiometers” and

“Jetfill” in-place tensiometers (Cascini & Sorbino 2004). Suction data were

collected at depths ranging from 0.2 m to 1.60 m from ground surface, along

verticals mainly located inside the upper portion of the slopes. Figures 6.24a

and 6.24b show all the data collected in the period November 1999 – May

2002, together with the daily rainfall recorded by rain-gauges installed in August

1998 at the top of the Pizzo d’Alvano massif. As discussed by Cascini &

Sorbino (2002, 2004) suction data, when analysed with reference to the monthly

Chapter 6 ________________________________________________________________________________________________________________________________________

154

Measurments sites

Measurments sites

0

10

20

30

40

50

60

70

80

Nov-9

9

Jan

-00

Mar-

00

May-0

0

Ju

l-00

Sep

-00

Nov-0

0

Jan

-01

Mar-

01

May-0

1

Ju

l-01

Sep

-01

Nov-0

1

Jan

-02

Mar-

02

May-0

2

da

ily

ra

infa

ll (

mm

)

0

10

20

30

40

50

60

70

Nov-9

9

Jan

-00

Mar-

00

May-0

0

Ju

l-00

Sep

-00

Nov-0

0

Jan

-01

Mar-

01

May-0

1

Ju

l-01

Sep

-01

Nov-0

1

Jan

-02

Mar-

02

May-0

2

suct

ion

(k

Pa

)

0

5

10

15

20

25

30

35

40

45

50

Mar-

00

May-0

0

Ju

l-00

Sep

-00

Nov-0

0

Jan

-01

Mar-

01

May-0

1

Ju

l-01

Sep

-01

Nov-0

1

Jan

-02

Mar-

02

May-0

2

suct

ion

(k

Pa

)

from 0.2 to 0.4m from 0.4 to 0.6mfrom 0.6 to 0.8m from 0.8 to 1.0m

0

5

10

15

20

25

30

35

40

45

50

Ma

r-0

0

Ma

y-0

0

Ju

l-0

0

Sep

-00

No

v-0

0

Ja

n-0

1

Ma

r-0

1

Ma

y-0

1

Ju

l-0

1

Sep

-01

No

v-0

1

Ja

n-0

2

Ma

r-0

2

Ma

y-0

2

suct

ion

(k

Pa

)

from 1.0 to 1.2m from 1.2 to 1.4mfrom 1.4 to 1.6m from 1.6 to 1.8m

a)

b)

d)

c)

Figure 6.24 In-situ soil suction measurements (Cascini & Sorbino, 2003)


155

average suction values, evidence the same trend (Figs. 6.24c, 6.24d) regardless

of the measurements site. In particular, at depths not exceeding 1.0 m from the

ground surface, suction data are characterized by a time trend strictly related to

rainfall regime, with maximum values attained in summer (35 kPa) and

minimum values reached in winter (5 kPa). At higher depths, suction presents

an analogous time trend but show two drying periods with maximum values

attained in summer (37 kPa) and at the end of autumn (30 kPa).

Chapter 6 ________________________________________________________________________________________________________________________________________

156

6.3.2 Soil mechanical properties

Laboratory investigations were aimed to characterise both the physical and

mechanical properties of the pyroclastic soils (Fig. 6.25) (Cascini et al., 2000;

Bilotta & Foresta, 2002; Sorbino & Foresta, 2002; Bilotta et al., 2005), that are

characterised by a strong variability, as discussed in Section 2.2. Anyway, three

main soil classes were recognised consisting in a pumice and two main ashy soil

classes (Tab. 6.26, Fig. 6.27). Particularly, the shallow ashy soil (class B) is

generally coarser than the deep one (class A), as confirmed by hydraulic and

shear strength properties in saturated and unsaturated conditions.

ASH “B”

PUMICE

1cm ASH “A”

Figure 6.25 Pyroclastic soils from the sample area

Physical

properties Hydraulic

conductivity Strength Collapse

Volumetric deformation

Stiffness

A * * * * * *

B * * *

Pumice *

Table 6.26 Available data set

With reference to the hydraulic properties of ashy soils in saturated conditions,

hydraulic conductivity ranges between 5.0×10-6 m/s and 4.8×10-5 m/s, while

the pumice soils are characterised by values between 1.0×10-5 and 1.0×10-2 m/s

(Fig. 6.28). With reference to the unsaturated conditions of the ashy soils, the

Suction Controlled Oedometer, the Volumetric Pressure Plate Extractor and

the Richard Pressure Plate provided the water characteristic curves shown in

Figure 6.28. For pumice soils, volumetric water content and the hydraulic


157

conductivity curves for pumice soils were obtained by using empirical

relationships (Bilotta et al., 2005).

Figure 6.27 Grain distribution size (Bilotta et al., 2005)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.1 1 10 100 1000

matric suction (kPa)

Vo

lum

etri

c w

ater

co

nte

nt q

w

pumice soil

upper ashy soil

lower ashy soil

1.E-13

1.E-11

1.E-09

1.E-07

1.E-05

1.E-03

1.E-01

1.E+01

0.1 1 10 100 1000matric suction (kPa)

Hydra

uli

c co

nduct

ivit

y k

w (m

/s)

1.0E-1

1.0E-3

1.0E-5

1.0E-7

1.0E-9

1.0E-11

1.0E-13

1.0E-15

pumice soil

upper ashy soil

lower ashy soil

class "B"

class "A"

Figure 6.28 Soil water characteristic curves

(Bilotta et al., 2005; Sorbino & Foresta, 2002)

The saturated shear strength envelopes for ashy soils reveal effective

friction angles ranging from 32° to 35° for class A soil, and from 36° to 41° for

the class B soil (Fig. 6.29, 6.30). Moreover, these soils, when reworked, show a

Chapter 6 ________________________________________________________________________________________________________________________________________

158

relevant increase in the friction angle and a decrease in the porosity. Referring

to effective cohesion, class B soil shows values approaching to zero, while for

class A soil values ranging from zero to 5 kPa were provided by the laboratory

tests. Higher values were obtained by Calcaterra et al. (2004) and Crosta & Dal

Negro (2003) for analogous soils located inside the Campania region, as well as

for those belonging to the Pizzo d’Alvano massif.

specimen n° v - ua (kPa) specimen n° v - ua (kPa)

TASAI1 I1 54 TATP2_1 13 79TASAI3 I3 20 TATP2_3 15 49TASAI4 I4 104 TATP2_4 16 104TASAI13 I13 79 TATP2_5 17 39TASAI16 I16 9 TATP2_6 18 59TASAI18 I18 39 TATP2_10 44 9

TATP2_11 45 20TATP2_12 46 29

class "A" class "B"

na = 0.71 to 0.73; wn = 72 to 75% na = 0.64 to 0.67; wn = 38 to 43%

I1

I3

I4

I13

I16

I18

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7horizontal displacement dh

(mm)

shea

r st

ress

tf (k

Pa)

I1

I3

I4 I13

I16

I18

-0.9-0.7-0.5-0.3-0.10.10.30.50.70.91.11.31.5

0 1 2 3 4 5 6 7horizontal displacement h

(mm)

ver

tica

l d

isp

lace

men

t v

(m

m)

13

15

16

18

44

4546

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7horizontal displacement dh

(mm)

shea

r st

ress

tf (k

Pa)

13

15

16

1718

44

45

46

-0.9-0.7-0.5-0.3-0.10.10.30.50.70.91.11.31.5

0 1 2 3 4 5 6 7horizontal displacement h

(mm)

ver

tica

l d

isp

lace

men

t v

(m

m)

S = 0.71 S = 0.76

Figure 6.29 Results of some direct shear tests on unsaturated

pyroclastic soils (Bilotta et al., 2005)

The unsaturated shear strength is characterised by a non-linear envelope

in respect to suction (Fig. 6.30). The increase of the shear strength with suction

was found for the class A soils strongly dependent from the suction values

range detected in-situ, while the class B ashy soils do not show any significant

dependence for suction values exceeding 30kPa. Finally, for both soil classes,

the angle of shearing resistance in respect to suction was found to be ranging

between 20° and 30° for the vertical net stresses acting in situ (Bilotta et al.,

2005).


159

0

10

20

30

40

50

60

70

80

90

0 20 40 60 80 100 120

net vertical stress v - ua (kPa)

shea

r st

ren

gth

tf (k

Pa)

38 ÷ 5050 ÷ 6070 ÷ 8585 ÷ 100100

Srf (%)

a)

to

to

to

to

0

10

20

30

40

50

60

70

80

90

0 20 40 60 80 100 120

net vertical stress v - ua (kPa)

shea

r st

rength

tf (k

Pa)

31 ÷ 50

50 ÷ 60

60 ÷ 70

70 ÷ 85

85 ÷ 90

95 ÷ 100

100

Srf (%)

b)

to

to

to

to

to

to

Figure 6.30 Peak shear strength versus net vertical stress for different saturation

degree (Srf): a) soils class “A”, b) soils class “B” (Bilotta et al., 2005)

Moreover, the experimental programme carried out by Bilotta et al. (2005)

allowed a good definition of the unsaturated shear strength envelope, for a

group of class ‘A’ soils. The results obtained reveal a non-linear trend for this

envelope, its dependence on the net normal stress applied and a constant value

of the fb angle (equal to the effective friction angle f’), in the range of matric

suctions not exceeding the soil air entry values. Finally, the contribution of

matric suction to shear strength is significant for the entire range of in situ

suction values so far collected (0 - 65 kPa) (Fig. 6.31)

Chapter 6 ________________________________________________________________________________________________________________________________________

160

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90 100 110

matric suction (kPa)

shea

r st

ren

gth

tf (k

Pa)

100 kPa 80 kPa50 kPa 40 kPa20 kPa 10 kPa

net vertical stress

qualitative trend

Figure 6.31 Shear strength with suction (Bilotta et al., 2005)

For the same class ‘A’ soil, suction controlled triaxial tests were performed

by Bilotta et al. (2005; in press) in order to evaluate the stress-strain behaviour

at different matric suctions. The triaxial isotropic tests have shown, in the

preconsolidation field, an anisotropic behaviour of the material that

progressively reduces for high values of mean net stress. Moreover, the triaxial

shear tests highlighted that the material exhibits a contractive behaviour for all

values of the applied suction (s = 10, 20, 50 kPa) with an isotropic

consolidation net stress of 30 kPa and 50kPa; instead, for an isotropic

consolidation net stress of 10 kPa, the soil has a dilatant behaviour which

increases with suction. For instance, in Figure 6.32, the measured values of

deviatoric stress and volumetric strain are plotted against vertical strain, for

suction values of 20 and 50 kPa, under constant values of net minor principal

stress respectively equal to 10, 30 and 50 kPa. In agreement with the

experimental results of the direct shear tests, carried out at similar level of net

stress, a slightly strain-softening behaviour was revealed by tests performed at a

low stress level while no strain-softening behaviour or a reduction of volume

specimens upon shearing was observed for the tests at higher stress levels.

From these tests and in particular from the (q-a) graphs of the shear stage, the

values of E50 modulus were obtained. This quantity represents the secant

modulus at 50% of the failure deviatoric stress (qf); the variation of E50 with

suction is represented in Figure 6.33. As it can be observed the E50 modulus

increase with suction, similarly to the values for the oedometer modulus,

although the latter are slightly higher.


161

0

20

40

60

80

100

120

140

160

180

200

Dev

iato

ric

Str

ess

(kP

a)

a)

3 - ua = 50 kPa

3 - ua = 10 kPa

3 - ua = 30 kPa

suction = 10 kPa

suction = 50 kPa

1

6

5

4

3

2

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20

Axial strain y (%)

volu

met

ric

stra

in

v (

%)

b)

1

6

5

4

3

2

Figure 6.32 Suction controlled triaxial tests at different net vertical stress

and suction (Bilotta et al., 2005)

0

1

2

3

4

5

6

7

0 20 40 60s = ua-uw [kPa]

Ee

d

[M

Pa

]

b)

0

1

2

3

4

5

6

7

0 20 40 60s = ua-uw [kPa]

E5

0

[MP

a]

a)

Legend

E50 = as2 + bs + E

050

a=-0,0011; b=0,14; r2=0,99

Eed = as2 + bs + E

0ed

a=-0,0018; b=0,18; r2=0,97

c)

1

2

3

4

q

qf

qf/2

(p -ua)pcon pf/2

K0

Kf

• •

• ••

Figure 6.33 Suction effect on the stiffness: (a) triaxial modulus E50;

(b) oedometer modulus Eed (Bilotta et al., in press)

163

7 GEOMECHANICAL MODELLING IN THE STUDY AREA

“The purpose of these models is not to give a mirror image of reality,

not to include all its elements in their exact sizes and proportions, but

rather to single out and make available for intensive investigation

those elements which are decisive. We abstract from non-essentials,

we blot out the unimportant to get an unobstructed view of the

important, we magnify in order to improve the range and accuracy of

our observation. A model is, and must be, unrealistic in the sense in

which the word is most commonly used. Nevertheless, and in a

sense, paradoxically, if it is a good model it provides the key to

understanding reality.”

Baran and Sweezy, 1968

The available data set on in-situ conditions and soil mechanical properties

provides all the necessary elements for the modelling of the triggering stage, to

be considered an important step in the understanding of the occurred flow-like

mass movements. Among the available methods, geotechnical and

geomechanical modelling seem, at the present, the most appropriate

approaches as they allow to take into account several relevant factors and the

environmental conditions recognised through the landslide characterization.

Modelling is here performed following the framework described in

Section 5.2.3 and the obtained results are compared with those coming from

other models. However, due to the number of detected instability phenomena,

the attention is focused on three main triggering mechanisms that resulted in

flow-like mass movements particularly intense, with reference to both the

mobilised volumes and run-out distances.

Chapter 7 ________________________________________________________________________________________________________________________________________

164

7.1 INSIGHTS TOWARDS MODELLING

The analysed triggering mechanisms are those respectively named M1,

M2, M3. The adopted procedure is shown in Fig. 7.1 while the theoretical

framework of the used methods is addressed in the App. D.

Particularly, using an uncoupled approach, the finite element code

SEEP/W (Geoslope, 2004) was adopted for groundwater modelling both in

saturated and unsaturated conditions of the involved soils (App. D). Referring

to the computed pore water pressures, the triggering stage was addressed

through both limit equilibrium and stress-strain analyses.

Limit equilibrium analyses were performed by means of the SLOPE/W

code (Geoslope, 2004), using the methods proposed by Morgenstern & Price

(1965) and Janbu (1954). In some cases, the shape and location of critical slip

surfaces were found using the optimization procedure proposed by Khran

(2004) to minimize the safety factor values (App. D).

Uncoupled stress-stain analyses, aimed to provide further insights on the

triggering stage and failure conditions, were developed through the SIGMA/W

code (Geoslope, 2004) (App. D). In such a case an elastic-perfectly plastic soil

behaviour was assumed, since a constitutive model, able to reproduce all the

features of the observed mechanical response, was not yet available (Sect. 4.3).

Understanding and modelling

of triggering mechanisms

LIMIT EQUILIBRIUM METHOD

SLOPE/W (Geo-Slope, 2005)

FINITE ELEMENT METHOD

(UNCOUPLED)

SIGMA/W (Geo-Slope, 2005)


(HYDRO-MECH. COUPLING)

GeHoMADRID (Pastor, 1999)

GROUNDWATER MODELLING

SEEP/W (Geo-Slope, 2005)

Figure 7.1 Methods utilised for the geomechanical modelling

Geomechanical modelling in the study area ________________________________________________________________________________________________________________________________________

165

Referring to the hydro-mechanical coupled approach, both 2D and 3D

stress-strain analyses were performed by means of the GeHoMADRID code

(Pastor et al., 1999) that allows to take into account both static and dynamic

conditions. This enhanced tool was utilised to validate the results coming from

the above quoted methods and to highlight the relevant processes and factors

not considered in the other modelling (App. D).

The analyses were performed utilising the parameters listed in Table 7.2

which summarises the available data set for the Pizzo d’Alvano pyroclastic

covers as well as further data provided by literature for analogous soils.

Soil properties Class B ashy soil

Pumice soil

Class A ashy soil

References

Dry unit weight d (kN/m3) 7.30 6.20 9.10

Bilotta et al. (2005) Cascini et al. (2003)

Calcaterra et al. (2004) Crosta et al. (2003) Pellegrino (1967)

Sorbino & Foresta (2002)

Saturated unit weight sat (kN/m3) 13.1 13.1 15.7

Porosity n (-) 0.58 0.69 0.66

Saturated hydraulic conductivity

ksat (m/s) 10-5 10-4 10-6

Effective cohesion c’ (kPa) 0 5 0 5 15

Friction angle ’ (°) 36 41 37 32 35

Rate of increase in shear strength due to suction

b (°) 20 20 20

Earth pressure coefficient at rest

k0 (-) 0.41 - 0.47

Bilotta et al. (in press) Bilotta (pers. comm)

Foresta (pers. comm.) Sorbino (pers. comm.)

Poisson’s ratio n (-) 0.26 - 0.30

Young’s modulus E (kPa) 5000 7000 - 1000 3000

Dilation angle (°) 10 20 - 10 20

Table 7.2 Parameters utilised for the geomechanical modelling

In order to properly address the stress-strain analyses, some preliminary

calculations were aimed to evaluate the initial stress-strain conditions following

the air-fall deposition processes. The obtained results - by adopting the

parameters of Table 7.2 - show relevant differences among the three

environmental settings where the triggering mechanisms M1, M2 and M3

respectively took place (Fig. 7.3). For instance, the open slope scheme is

characterised by null shear stresses along planes parallel to the ground surface

Chapter 7 ________________________________________________________________________________________________________________________________________

166

xx (kPa)

yy (kPa)

zz (kPa)

xx (kPa)

yy (kPa)

zz (kPa)

a)

z

y

x

z

y

x

b)

Figure 7.3 Simulated in-situ stress conditions: a) open slope, b) concavity


167

and constant lateral normal stresses (yy) (Fig. 7.3a). Conversely, the maximum

values of lateral normal stress (yy) and shear stress are simulated in the central

zone of a concavity, and they rapidly decrease towards the lateral boundaries

(Fig. 7.3b). Finally, the flank of gully is analogous to an open slope, in its upper

portion, while the normal and shear stresses are not constant at its lower

portion, as they are deeply influenced by the lower boundary.

7.2 M1: A BEDROCK OUTLET INDUCED TRIGGERING

MECHANISM

The triggering mechanism named M1, as discussed in Section 6.2, took

place inside the ZOBs and it was strictly connected to the presence of

temporary bedrock outlets (Fig. 7.4). This mechanism was deepened with

reference to the schemes reported in Figure 7.5 that were utilised to set up

computational schemes at both site and massif scale. The hypotheses and

procedures for modelling are discussed in the following sections.

a) b)a)

bedrock outlet

bedrock outlet


from the basins n. 1 (a), 14 (b) facing the town of Sarno

Chapter 7 ________________________________________________________________________________________________________________________________________

168

Figure 7.5 Reference schemes for the triggering mechanism “M1”

7.2.1 Site-scale analyses by limit equilibrium method

Due to the variability of the in-situ conditions, the analyses at site scale

were performed with reference to a sample basin that allows an accurate

stratigraphical reconstruction on the basis of the available data set (Sect. 6.3).

Particularly, referring to the slope section of Figure 7.6, pore water

pressures were preliminarily analysed, through the SEEP/W code, for the

period January 1, 1998 - May 3, 1998 in order to obtain a reasonable initial

condition at the beginning of the critical storm. Referring to January 1, 1998,

uniform pore water pressure distributions were assumed as initial conditions,

with values respectively equal to 5, 10, 15 and 20 kPa, all over the pyroclastic

cover. Then, the 4-5 May 1998 meteoric event was taken into account together

with the presence of the outlet from the bedrock (Fig. 7.7)

At ground surface, the boundary condition was represented by daily

rainfall during the analysed time period; evapotranspiration phenomena were

not taken into account according to Sorbino (2005), that evidenced the

negligible effects of such phenomena in the modelling of suction regime during

the winter and spring. At the contact between the pyroclastic cover and the

limestone bedrock, two different boundary conditions were considered: in the

first case, the bedrock was assumed impervious; in the second one, this


169

impervious condition was replaced by a flux condition only in correspondence

of the bedrock outlet zone.

The obtained results highlight that during the 4 - 5 May 1998 pore water

pressure reached their highest values in the lowest part of the slope, while in

the central one, they were mainly related to the presence of the bedrock outlet.

However, in each portion of the slope, the stratigraphical setting plays a

relevant role as highlighted in Figure 7.8.

A

B

C

D

E

F

G

H

I

L

MN

O

0 25 50 100 125 150 175 200 250 275 300 325 35022575

725

700

675

650

625

600

550

525

500

575

OUTLET

Lenght (m)

z (m

a. s.

l.)

zone

1

zone

2

zone

3

zone

4

zone

5

zone

6

zone

7

zone

8

zone

9

zone

10

zone

11

zone

12

zone

13

zone

14

zone

15

zone

16

zone

17

zone

18

750750

750

700

650

600

550

800

750

700

650

600

550

500

450450

A B C D E F G H I L,M,N,O

Upper ashy soils (Class B)

Pumice soils

Lower ashy soils (Class A)

0

1 m

2 m

R

R A

R A

A R

A R

A R

A

A

R

R

A

A

RA

RARA

R

ARA

RA

RA

A

R

AR

AR

Source areas

R

A R

0 25m 50m 100m

A

A

Seismic section

Pit

Slope section

Basin limit

May 1998

Figure 7.6 Tuostolo sample basin: a) plan view, b) stratigraphical reconstruction

0E+00

1E-04

2E-04

3E-04

4E-04

5E-04

12

/09/9

8

12

/10/9

8

11

/11/9

8

11

/12/9

8

10

/01/9

9

09

/02/9

9

11

/03/9

9

10

/04/9

9

10

/05/9

9

09

/06/9

9

Flo

wra

te

(m3/s

)

00E+00

1E-04

2E-04

3E-04

4E-04

5E-04

12

/09/9

8

12

/10/9

8

11

/11/9

8

11

/12/9

8

10

/01/9

9

09

/02/9

9

11

/03/9

9

10

/04/9

9

10

/05/9

9

09

/06/9

9

Flo

wra

te

(m3/s

)

0

t

hmq

y

hk

yx

hk

xwwyx

2

bwaan )tgu(u')tgu(σc'τ lim

BEDROCK OUTLET

Duration: 48 – 72 hours

Flow rate: 4.1710-6 m3/s – 1.6710-5 m3/s

RAINFALL

1th January – 3th May 1998

4 – 5 May 1998 (heavy storm)

MEASURED BEDROCK OUTLET

STABILITY ANALYSIS

Janbu’s method

GROUNDWATER ANALYSIS

010203040

5060708090

01

/01

/98

01

/02

/98

01

/03

/98

01

/04

/98

01

/05

/98

01

/06

/98

pio

gg

ia g

iorn

ali

era

(m

m)

1

0100200300400

500600700800900

pio

gg

ia c

um

ula

ta (

mm

) 1

Dai

lyra

infa

ll(m

m)

Cum

ula

ted

rain

fall

(mm

)

120 mm on May 4-5, 1998

010203040

5060708090

01

/01

/98

01

/02

/98

01

/03

/98

01

/04

/98

01

/05

/98

01

/06

/98

pio

gg

ia g

iorn

ali

era

(m

m)

1

0100200300400

500600700800900

pio

gg

ia c

um

ula

ta (

mm

) 1

Dai

lyra

infa

ll(m

m)

Cum

ula

ted

rain

fall

(mm

)

120 mm on May 4-5, 1998

MEASURED RAINFALL

Figure 7.7 In-situ measured rainfall and bedrock outlet flows assumed as

hydraulic boundary conditions

Chapter 7 ________________________________________________________________________________________________________________________________________

170

Figure 7.8 Simulated pore water pressure regime inside

the slope section of Figure 7.6.

Using the previously computed pore pressures values, the stability

conditions of the pyroclastic cover were then evaluated via Janbu’s limit

equilibrium method. To this aim, the shear strength was modelled, in both

saturated and unsaturated conditions, by means of the extended Mohr-

Coulomb failure criterion proposed by Fredlund et al. (1978). For the analysed

slope sections, the soil layers were assumed homogenous, isotropic and

characterised by physical and mechanical properties ranging among those listed

in Table 7.2. In particular, slope safety factors were simulated at each time step

and the stratigraphical section was updated removing the unstable portions of

the hillslope.

While referring to Cascini et al. (2003) for a detailed description of the

adopted procedures, it must be noted that, only when considering the presence

of bedrock outlets, the obtained results are in good agreement with

eyewitnesses and in-situ evidences collected after the event (Cascini et al.,


171

2000). In such a case, the performed analyses are capable to reproduce the

extent of triggered areas, matching the time period of failures (Fig. 7.9).

The same geotechnical analyses allows to outline the key factors for slope

instability conditions that are represented by: cover thickness and bedrock

profile; bedrock outlets; suction regime; and shear strength properties of ashy

layers. Particularly, the geometrical profile of the bedrock and the cover

thickness distribution along the slope section can generate unfavourable pore

pressure conditions in some zones of the ZOB, so inducing local instability

phenomena – mostly located above the lower ashy layers (class A soil) – that

develop upslope according to multiple slides (Hutchinson, 1988; App. A).

Similar findings were obtained by other stability analyses performed for

pyroclastic covers located in analogous environmental settings (Dell’Osso,

2000).

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Time (hours)

Mo

bil

ised

are

aa (

m2 )

_

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Time (hours)

Mo

bil

ized

are

a (

m2

) 1

Total mobilized area=18500m2

Period of flowslide occurrence

5kPa

10kPa 15kPa 20kPa

May 5, 1998 May 4, 1998

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Time (hours)

Mo

bil

ized

are

a (

m2

) 1



5kPa

10kPa 15kPa 20kPa

May 5, 1998 May 4, 1998

a)

initial suctions (5 – 20 kPa)

510

1520

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Time (hours)

Mobiliz

ed a

rea

(m2)

?

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48

Time (hours)

Mo

bil

ized

are

a (

m2

) 1



5kPa

10kPa 15kPa 20kPa

May 5, 1998 May 4, 1998

b)

outlet starting from (3 – 4 May)

3

4

3

4

Mo

bil

ised

area

(m

2)

Figure 7.9 Simulated unstable areas for different cases, considering (a)

and not considering (b) the presence of the bedrock outlet

7.2.2 Massif-scale analyses by limit equilibrium method

At massif scale, the effects of rainfall and bedrock outlets were analysed

by using the typical stratigraphical settings provided by in-situ investigations

(Sect. 6.3.1) and infinite slope schemes, characterised by slope angles ranging

between 30° and 35°, as obtained from the available DTM for the source areas

(Fig. 7.10). Other local factors – such as slope gradients, cover depth variations

Chapter 7 ________________________________________________________________________________________________________________________________________

172

and changes in layer arrangements – were instead disregarded in order to better

outline the instability conditions induced by only rainfall and bedrock outlets.

The used approach, the initial and boundary conditions, physical and

mechanical soil properties of the pyroclastic soil layers and boundary conditions

were assumed equal to those referred in the previous site scale analyses. In

particular, with reference to the boundary conditions at the contact between the

pyroclastic cover and the limestone bedrock, four different cases were analysed.

In the first case, an impervious bedrock was assumed; in the second one,

according to the assumptions of some Authors (Olivares et al., 2003; Calcaterra

et al., 2004), the limestone bedrock was considered as a draining layer; in the

third and fourth cases, the impervious and the draining boundary condition was

locally replaced by a flux condition acting only during the period May 3 – May,

5 1998 (Cascini et al., 2003). As far as the flux values, they were assumed

ranging from 4.17×10-6 m3/s to 1.67×10-5 m3/s, which are among the lowest

flow rates recorded for the bedrock outlets during spring (Cascini et al., 2000).

Class B ashy soils

Pumice soils

Class A ashy soils

Scheme 3

0.40

1.68

0.362.06

0.36

1.120.47

1.68

1.23

1.682.82

1.680.30

0.82

0.471.23

1.76

1.680.30

0.40

1.680.300.820.47

0.470.360.40

1.680.30

2.52

4.5

m

Scheme 2

Scheme 8Scheme 7

Scheme 6Scheme 5Scheme 4

Scheme 1

4.5

m

4.5

m4

.5 m

4.5

m

4.5

m4

.5 m

4.5

m

Figure 7.10 Computational schemes at massif scale

(numbers specify the depths in metres)


173

The performed analyses clearly show that rainfall is capable to determine

complex saturated-unsaturated flow patterns in relation to the assumed

stratigraphical and hydraulic boundary conditions. Figure 7.11 shows the

simulated pore pressure vertical distributions, respectively before and after the

storm of 4 - 5 May, 1998, without considering the flux condition at the bottom

of the cover, for schemes 1, 2 and 8 of Figure 7, that provide the maximum

differences among the computed pore water pressures. Independently from the

scheme and the assumed boundary condition, the highest values of pore

pressures, before and after the storm, are found at the bottom of the

pyroclastic cover (Fig. 7.11). Particularly, when a draining bedrock is assumed,

negative pore pressures characterise the whole pyroclastic cover, also after the

storm, except for the bottom where a nil value is attained according to the

assumed boundary condition. Conversely, considering the bedrock as an

impervious layer, positive pore pressures develop at depths higher than 2.0 m

(for scheme 2) and 3.5 m (for scheme 1).

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-16 -8 0 8 16

pore pressure (kPa)

dep

th (

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-16 -8 0 8 16

pore pressure (kPa)

dep

th (

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-16 -8 0 8 16

pore pressure (kPa)

dep

th (

m)

1 2 8

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-16 -8 0 8 16

pore pressure (kPa)d

epth

(m

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-16 -8 0 8 16

pore pressure (kPa)

dep

th (

m)

1 2 8

c) a) d) b)

impervious imperviouspermeable permeable

Figure 7.11 Pore water pressure induced by rainfall for some schemes (1, 2, 8)

among those depicted in figure 7.10.

The role played by bedrock outlets is highlighted in Figure 7.12. In

particular, distinct initial conditions can be recognised for a point at the same

depth (2.80 m), with reference to the assumed schemes. Moreover, flux of 0.5

l/min, operating for 48 hours, in conjunction with the May 1998 rainfall event,

is capable to produce pore pressure increments equal to 15 kPa, 2 kPa, 5 kPa

respectively for the scheme 1 (ashy soils belonging to class B), scheme 2 (class

A and B ashy soil layers) and scheme 8 (class A and B soil layers with the

presence of pumice layers).

Chapter 7 ________________________________________________________________________________________________________________________________________

174

-10

-5

0

5

10

15

20

0 4 8 12 16 20 24 28 32 36 40 44 48

time (hours)

po

re p

ress

ure

(k

Pa)

_ considering outlets

without outlets

1

2

8

Figure 7.12 Pore water pressure regime induced by rainfall and bedrock outlet

for some schemes (1, 2, 8) among those depicted in figure 7.10.

On the basis of the calculated pore water pressure regime, the slope

stability conditions were then evaluated, through the limit equilibrium method,

with reference to planes parallel to the ground surface. The ratio of the shear

strength to the shear stress, acting at different depths, evidences that instability

conditions do not occur whatever are the shear strength properties of the soil

layers ranging among those listed in Table 7.2. Analogous results are also

obtained with reference to the most severe pore pressure regime induced by the

assumption of an impervious bedrock (Fig. 7.13), providing for the class A ashy

soil layer shear strength properties equal to the highest values furnished by

literature (Tab. 7.2). Figure 7.13 also shows that the lowest ratio of shear

strength to the mobilised shear stress is attained at the bottom or inside the

pyroclastic cover, according to different stratigraphic conditions as well as to

the shear strength properties assumed for the soil layers.

On the contrary, introducing bedrock outlets, local instability phenomena

can be systematically simulated for any of the shear strength properties listed in

Table 7.2, in agreement with both the site scale analyses (Fig. 7.14) and in-situ

evidence (Sect. 6.2). In particular, whenever only ashy soils belonging to class B

are present, the maximum depths of the sliding surfaces are systematically

located at the contact between the limestone bedrock and the pyroclastic cover.

If the pyroclastic cover is constituted by both ash and pumice soils (schemes 2,

8), slip surfaces develop inside the upper ashy soil layer (class B) or reach the


175

bottom of the cover, respectively when assuming for the class A ashy soil the

highest or the lowest shear strength properties among those listed in Table 7.2.

The safety factors computed for some typical slip surfaces are shown in Figure

7.15, and they are closely correlated to pore water pressures at different depths

in relation to the stratigraphical setting.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

1.0 1.4 1.8 2.2

tres / tmob

dep

th (

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

1.0 1.4 1.8 2.2

tres / tmob

dep

th (

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

1.0 1.4 1.8 2.2

tres / tmob

dep

th (

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

1.0 1.4 1.8 2.2

tres / tmob

dep

th (

m)

Fig. 20

f’ c’(kPa) fb

Class B 36° 0 20°

Pumice 37° 0 20°

Class A 32° 5 20°

f’ c’(kPa) fb

Class B 41° 0 20°

Pumice 37° 0 20°

Class A 35° 15 20°

c) a) d) b)

impervious imperviouspermeable permeable

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-16 -8 0 8 16

pore pressure (kPa)

dep

th (

m)

1 2 8

Figure 7.13 Ratio of the shear strength to the shear stress acting on

planes parallel to the ground surface

5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H

Superfici di scorrimento

calcolate

0 5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0

a)

5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0

5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0

c)

b)

Class B ashy soils

Pumice soils

Class A ashy soils

Outlet

H = 4.5 m, = 35°

Computed slip surfaces

Class B ashy soils

Pumice soils

Class A ashy soils

Outlet

H = 4.5 m, = 35°

Computed slip surfaces

FS = 1

5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0

Figure 7.14 Slope instability scenarios simulated through

limit equilibrium method

Chapter 7 ________________________________________________________________________________________________________________________________________

176

Figure 7.15 Slope safety factor reduction versus pore water pressure increase

In conclusion, massif scale analyses seem able to well reproduce the

triggering mechanisms M1, referring to the key factors producing instability

conditions. However, the simulated first-failure phenomena can certainly

produce further instability phenomena, as shown in the previous site scale

analyses (Sect. 7.2.1). Particularly, site-scale analyses highlight that both the

upslope and downslope instability zones must be necessarily addressed taking

into account the local in-situ conditions.


177

7.2.3 Massif-scale analyses by finite element method

To validate the obtained results and to achieve further insights on the

triggering mechanism M1, stress-strain analyses were performed using both an

uncoupled and coupled approach.

The uncoupled approach was referred to the previously described

schemes and mechanical properties as well as to the above computed pore

water pressures.

The obtained results show that the only rainfall at the ground surface do

not induce any instability condition. For instance, for the scheme of

homogeneous slope (scheme 1 of Fig. 7.10), the simulated displacements

present a quite regular time trend up to values equal to about 0.003 m (Fig.

7.16). On the contrary, pore water pressures induced by rainfall and bedrock

outlets determine the arising of displacements higher than the previous ones,

with maximum values concentrated at the base of the cover (Fig. 7.16). In

particular, after 18 hours, relevant displacement gradients clearly outline failure

conditions.

10

15

20

25

30

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48time (hours)

eff

ecti

ve v

ert

ical

stre

ss (

kP

a)

_

analisi uncoupled degli effetti indotti dalla pioggia

schema 110kPa_0.0075_4M98_1lmin_1

Head File: 10kPa_0.0075_ANNUA_1

0.000

0.004

0.008

0.012

0.016

0.020

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48time (hours)

dis

pla

cem

ent

(m)

?

distanze [m]

0 20 40 60 80 100 120 140 160 180 200 220

qu

ote

[m

]

80

100

120

140

160

180

200

220

240

260

distanze [m]

0 20 40 60 80 100 120 140 160 180 200 220

qu

ote

[m

]

80

100

120

140

160

180

200

220

240

260

P

-10 – 0

0 – 10

10 – 20

pore water

pressure (kPa)

0 – 0.005

0.005 – 0.010

0.010 – 0.015

displacement

contours (m)

P

Point P

Without outlet

With outlet

Point P

With outlet

Without outlet

Figure 7.16 Deformations and effective vertical stress variations induced

by a bedrock outlet for the scheme 1 of Figure 7.10

Chapter 7 ________________________________________________________________________________________________________________________________________

178

Following the hydro-mechanical coupled approach, pore water pressure

values were firstly computed assuming only rainfall as boundary condition. The

obtained results are very close to those computed by the uncoupled approach,

mainly due to low induced strains that, moreover, do not localise in any portion

of the slope. In agreement with the previous results, any failure condition

cannot be simulated confirming that only rainfall is not capable to induce

failure conditions in any of the analysed cases.

On the contrary, considering both rainfall and bedrock outlets, relevant

differences arise for pore water pressures, in respect with the previously

computed ones (Fig. 7.17). In particular, hydro-mechanical coupling produces

higher values evolving in time with higher gradients than those simulated with

an uncoupled approach. As a matter of fact, these results highlight that failure

can occur after shorter time periods than those provided by the uncoupled

approach. Conversely, simulated instability scenarios are similar to those

obtained with both limit equilibrium analyses and uncoupled stress-strain

analyses. In particular, the largest displacements provided by the stress-strain

analyses well match the unstable masses simulated through limit equilibrium

analyses (Fig. 7.17).

Pore water

pressure (kPa)Plastic strains (-)

Point P

Point P

Figure 7.17 Pore water pressure and plastic strains induced by a bedrock outlet

Finally, 3D finite element analyses were performed (Fig. 7.18) to achieve a

slope instability scenario capable to take into account other features of the

environmental setting (i.e. morphological concavities) where the triggering

mechanism M1 occurred. The obtained results clearly highlight that, due to

both the initial in-situ stress conditions (Sect. 7.1) and the induced pore water

pressures, plastic strains mainly develop in the central portion of the concavity,

in correspondence of the bedrock outlet and at the base of the cover.


179

z

y

x

z

y

x

Figure 7.18 3D analysis of plastic strains induced by a bedrock outlet

7.2.4 Remarks on modelling

The performed modelling allows some general considerations on the

investigated triggering mechanism M1.

First of all, the bedrock outlets represent a key factor for slope instability,

in agreement with the geomorphological models set up by Cascini et al. (2000).

An example of this last model is provided in Figure 7.19 that refers to a sample

basin where failures occurred only where bedrock outlets were present.

A further confirmation is provided by other models that, instead, do not take

into account the bedrock outlets. For instance, the available physically based

model, which assume an impervious boundary condition at the bottom of the

cover, systematically provide a misleading overestimation or underestimations

of the unstable areas (Chirico et al., 2000; Frattini et al., 2004) as discussed in

Sect. 5.2.2.

Similar findings were also outlined applying, for the basin of Figure 7.6,

other physically-based models, such as that implemented by Dietrich &

Montgomery (1998) in a GIS-based system (Sect. 4.2). In this case, the

unreliability of the obtained results is related to the drastic overestimation of

the source areas, when considering values for the physical and mechanical

Chapter 7 ________________________________________________________________________________________________________________________________________

180

properties of the pyroclastic soils among those reported in Table 7.2. In

particular, with reference to the ZOB A of Figure 7.20, it is not possible to

reproduce stable conditions during the May 1998 rainfall event, also assuming

the maximum values for physical and mechanical properties of the pyroclastic

soils. These findings can be surely explained in relation to the hypothesis of

saturated steady-state flow conditions as well as for the impossibility of taking

into account local boundary conditions.

Upper catchment (Z.O.B.)

Upper slope

Lower slope

Recent

Ancient fan

Lower morphological frame

Upper morphological frame

Talus Talus

LowerFilled

fan

catchmentmain channel

Recentfan

Bedrock

Water outlet

Runoff

OUTLET

Cascini et al. (2000)

Figure 7.19 Findings of the hydro-geomorphological model

set up by Cascini et al. (2000)

1

2

3

a) b)

A

BC

D

A

BC

D

Figure 7.20 Instability scenarios outlined for the basin N. 3, with

SHALSTAB code: (1) ZOBs, (2) observed and (3) simulated source areas


181

In agreement with the in-situ evidence (Sect. 6), the analyses discussed in

the previous Section also outline that the location of the slip surfaces is strictly

related to the stratigraphical setting of the pyroclastic cover. This finding is

extremely important for the evaluation of the potential unstable masses whose

magnitude can strongly influence the propagation stage and the run-out

distances.

5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0 5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0

5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0

5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0

FS = 1

5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0

Figure 7.21 Possible slope instability scenarios

related to different stratigraphical settings

Another consideration coming from all the performed analyses is

represented by the possibility that three different methods simulate the same

relevant features of the analysed phenomena such as the slope instability

scenarios and time occurrence of the slope instability conditions (Fig. 7.22).

analisi uncoupled degli effetti indotti dalla pioggia

schema 110kPa_0.0075_4M98_1lmin_1

Head File: 10kPa_0.0075_ANNUA_1

P

Displacement contours

LIMIT EQUILBRIUM METHODFINITE ELEMENT METHOD

(UNCOUPLED)


(HYDRO-MECH. COUPLING)

5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0 5m 10m

Schema 3

Schema 2Schema 1

Cinerite (Classe B)

Pomici

Cinerite (Classe A)

Immissioni

H

H = 4.5 m, = 35°

H

H


calcolate

0

P P

Critical slip surface Plastic strains

Figure 7.22 Slope instability scenarios outlined with distinct methods: a) limit

equilibrium analysis; b) uncoupled FEM analysis; c) coupled FEM analysis

Moreover, hydro-mechanical coupled finite element analyses evidence that

pore water pressures may be higher than those usually computed with the

Chapter 7 ________________________________________________________________________________________________________________________________________

182

traditional uncoupled approaches (Fig. 7.23). In this sense, such analyses shall

be improved when advanced constitutive will be available, in order to simulate

both the triggering and the post-failure stages of the instability phenomena.

-15

-10

-5

0

5

10

15

20

25

30

0 20 40 60 80time (hours)

pore

wate

r pre

ssure

(kP

a)

?

coupl

uncoupl

Point Peffect of H-M coupling

tf

-15

-10

-5

0

5

10

15

20

25

30

0 20 40 60 80time (hours)

pore

wate

r pre

ssure

(kP

a)

?

coupl

uncoupl

-15

-10

-5

0

5

10

15

20

25

30

0 20 40 60 80time (hours)

pore

wate

r pre

ssure

(kP

a)

?

coupl

uncoupl

Point Peffect of H-M coupling

tf

-15

-10

-5

0

5

10

15

20

25

30

0 20 40 60 80time (hours)

pore

wate

r pre

ssure

(kP

a)

?

coupl

uncoupl

EFFECTIVE

STRESS

Figure 7.23 Comparison of computed pore water and vertical effective stress

outlined with (a) uncoupled and (b) hydro-mechanical coupled approaches

At the present, the obtained results seem quite significant especially

referring to those provided by physical models such as the centrifuge tests

performed by Take et al. (2004) (Fig. 7.24a). Particularly, these Authors outlined

the importance of groundwater flow patterns inducing slope instability

conditions and subsequent static liquefaction phenomena, independently by the

relative density of the involved soils. Therefore, the modelling of pore water

pressure regime by means of the most enhanced methods must be encouraged

in order to adequately assess the role of static liquefaction that seems to occur

independently by the soil arrangement and as a consequence rather than a

triggering factor, as already argued by Eckersley (1990) on the basis of flume

tests (Fig. 7.24b).


183

loose dense

triggering post-failure

a) b)

Figure 7.24 Flow-like mass movements analysed through physical models:

a) centrifuge tests (Take et al., 2004), b) flume tests (Eckersley, 1990)

In conclusion, the triggering mechanism M1 is related by different models

to the same factors, that are rainfall, bedrock outlets, pore water pressure

regime. However the selection of the most appropriate model is strictly

connected to the pursued aims. Particularly, the analysis of slope stability

conditions can be performed with traditional limit equilibrium methods.

Viceversa, an accurate understanding of the mechanisms that rule the triggering

and post-failure stages must be necessarily addressed by means of advanced

tools capable to better simulate the soil mechanical behaviour both in saturated

and unsaturated conditions.

Chapter 7 ________________________________________________________________________________________________________________________________________

184

7.3 M2: TRIGGERING MECHANISMS FOR TRIANGULAR

SHAPED LANDSLIDES

The triggering mechanism named M2 took place in the upper portion of

open slopes (Sect. 6.2) and was characterised by pseudo-triangular shaped

source areas and post-failure movements evolving according to a snow

avalanche style (Fig. 7.25). In some cases, first-failure phenomena occurred at

the toe of the bedrock scarps, where the presence of a karstic conduct was

recognised, without any failure in the upper border of the limestone frame

(M2a) (Fig. 7.26a). In other cases, a close correspondence was outlined between

the upper instability zones and the source areas at the base of morphological

scarps (M2b) (Fig. 7.26b); also in such a case, the presence a water inflow was

individuated in correspondence of the morphological scarps. Finally, some

slope instability phenomena occurred at the edge of natural scarps not evolving

in flow-like mass movements (Fig. 7.26c).

All these phenomena were analysed taking or not into account the effects

of the upper outlets on the instability conditions of the source area located at

the base of the morphological scarps (Fig. 7.27).

Figure 7.25 In-situ evidences for triggering mechanism “M2”: examples from

the basin n. 3 (a) and basin n. 9 (b) of Pizzo d’Alvano massif


185

a)

c)b)

Figure 7.26 Some relevant in-situ evidences: a) karstic conduct (Guadagno,

2005); b) impact induced landslides; c) impact not causing failure


Chapter 7 ________________________________________________________________________________________________________________________________________

186

7.3.1 Upper outlets induced phenomena

The analyses of the mechanism M2a, performed using the approach

discussed in Sect. 5.2.3, were essentially aimed to investigate the role of the

upper outlets during rainfall storms. These last, in fact, are generally

characterised by fluxes up to 10-4 m/s and act over periods shorter than 24

hours (Guida, pers comm.). In particular, the analyses were addressed to

evaluate the effects caused by the water inflow (Fig. 7.28a) or by the filling of

fractures (Fig. 7.28b) that are quite frequent in the upper portion of the

pyroclastic covers.

a) b)

Figure 7.28 Computational schemes for the triggering mechanism “M2”:

a) water inflow; b) hydrostatic pore water pressure

Referring to an uncoupled approach, seepage analyses were developed for

different stratigraphical settings, referring to the same soil mechanical

properties introduced in Section 7.2 and assuming, as initial condition, the pore

water pressures simulated at the end of the period January 1, 1998 – May 3,

1998. With reference to the hydraulic boundary conditions, i) the previously

discussed rainfall values were imposed at the ground surface; ii) the contact

between the pyroclastic cover and the bedrock was considered as an impervious

boundary, iii) at the upper lateral boundary of the cover, a water inflow or

hydrostatic pore water pressure distributions were introduced in the analysed

schemes. The modelling was carried out for time periods not longer than 48

hours.

The obtained results show a wide influence of the stratigraphical setting

on the effects induced by the upper outlets. Particularly, the pore water

pressure increase up to values equal to 30 kPa in 48 hours, for the scheme


187

containing only ashy B soil, while simulated pore water pressure reach values

equal to 40 kPa, in the case of both ashy A and B soils (Fig. 7.29).

Similarly, the hydrostatic condition induces relevant variations of pore

water pressure (Fig. 7.30) in relation to the stratigraphical setting (Fig. 7.30).

For instance, referring to fractures over depths equal to 1.8 m and 4.5 m, the

presence of the A ashy soil increases the simulated values while pumice layers

have the main effect to involve larger portion of the cover respect to the

previous ones.

Figure 7.29 Pore water pressure increase due to upper water inflows

for different stratigraphical settings.

Figure 7.30 Pore water pressure increase due to fracture filling phenomena for

different stratigraphical settings: a) deep fracture, b) superficial fracture

Chapter 7 ________________________________________________________________________________________________________________________________________

188

Adopting the simulated pore water pressures, limit equilibrium analyses

were then performed, referring to both planar and curvilinear slip surfaces. For

this last case that have the minimum safety factors, only a water inflow values

greater than 3×10-5 m/s induces the slope instability after periods of 20 hours.

In a similar way, hydrostatic pore pressure distributions (related to fracture

filling) are capable to induce slope instability for action periods longer than 20

hours (Fig. 7.31). The simulated unstable masses have different magnitude and

their safety factor change with time, depending on the hydraulic boundary

conditions. In particular, instability conditions are simulated for time action

periods ranging from 17 to 27 hours.

1) 2) 3)

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

0 4 8 12 16 20 24 28 32 36 40 44 48

time (hours)

safe

ty f

acto

r

?

1

2

3

ashy B

pumice

ashy A

Figure 7.31 Slope stability conditions computed via Limit Equilibrium Method:

a) water inflow, b) fracture filling

Referring to the previous scenarios, stress-strain analyses reveal that upper

outlets or filled fractures produce both volumetric and deviatoric strains in the

highest portions of pyroclastic covers (Fig. 7.32). In particular, the water inflow

cause relevant deformations at the bottom or inside the pyroclastic cover. For

instance, referring to the homogeneous slope scheme (scheme 1), the simulated

displacement field is characterised by a time trend well correlated to the pore

water pressure increase inside the pyroclastic cover (Fig. 7.32)


189

a) b)

0.005

0.01 0.015

0.020

0.025

0.005

0.01

Figure 7.32 Effects of upper outlet (a) and fracture filling (b) simulated

through uncoupled stress-strain analyses

Finally, the hydro-mechanical coupled approach highlights relevant

differences in the computed pore water pressure values. Particularly, both the

time occurrence and pore water pressure at failure significantly differ from

those computed with the uncoupled approach. Conversely, simulated unstable

masses are similar to the previous ones.

In conclusion, independently from the used approach, all the performed

analyses clearly highlight the upper outlets as a relevant instability factor.

Moreover, the simulated unstable masses can certainly produce further

instability phenomena in the downslope portions of the cover. Anyway, these

aspects call for a further deepening as they strictly depend from local features

that can be analysed only through site scale analyses. Moreover, these further

failures certainly claim for adequate evaluations on post-failure phenomena that

cannot be addressed without adequate constitutive models that are not available

at the present.

Chapter 7 ________________________________________________________________________________________________________________________________________

190

7.3.2 Impact-induced phenomena

Referring to the triggering mechanism M2b, the impact phenomenon can

be ideally divided into two main stages consisting in load application and its

removal. To address similar phenomena Sassa et al. (2002) proposed theoretical

evaluations and Wang et al. (2003) referred to the results of laboratory tests; the

results obtained by some geomechanical analyses are hereafter discussed.

In particular, the impact forces were firstly estimated, in relation to the

scarps heights that generally range from few meters up to 20 m, following the

procedure proposed by Wang et al. (2003. Starting from this evaluation, impact

loading applied stresses were assumed in the range between 5 kN/m and 30

kN/m. Due to both the small volume and the limited heights of scarps, the

loading stresses were assumed not larger than 30 kN/m.

In order to adequately evaluate the role of the loading impact, several

initial conditions were considered taking account of the slope angle, the season

of the year and the presence of the upper outlets. Moreover, with reference to

the impact scenarios both “drained loading”, and “undrained loading” were

considered (Fig. 7.33).

DRAINED UNDRAINED DYNAMIC

???

Figure 7.33 Schemes for loading impact over still in place soils

The analyses concerning the “drained impact” scenario were carried out,

assuming constant over the time the initial pore water pressure condition.

Moreover, the role of the initial pore water pressure condition was analysed

assuming different uniform distributions, with values ranging between 5 kPa

and 60 kPa, that are the extreme values among those recorded during the in-

situ investigations (Sect. 6.3).


191

The obtained results clearly highlight that the applied vertical forces are

not capable to induce failure conditions since the safety factors remain greater

than one for any value of the loading forces (Fig. 7.34). In particular, referring

to the slip surfaces with the minimum safety factors, the applied forces increase

both the driving and resistance forces. Anyway, due to geometric configuration

of both pyroclastic cover and potential slip surfaces, the resistance forces arise

more than the driving ones with the consequent increase of the safety factors.

For instance, Figure 7.34 shows the slope safety factors computed on May 3,

1998 referring to pore water pressures computed from the analysis of the

period 1 January – 3 May. As it can be seen, after the load removal, the safety

factors along these slip surfaces achieve their initial values.

Referring to the same pore water pressure conditions, stress-strain

analyses outline that the highest deformations concentrate in zones of the slope

that well match the critical slip surfaces (Fmin) computed with the limit

equilibrium analyses. Anyway, inside these zones, the displacement field is not

characterised by any asymptotic time trend and failure conditions are not

achieved for any initial pore water pressure distribution (Fig. 7.35).

1.00

1.50

2.00

2.50

0 10 20 30 40

applied pressure (kPa)

safe

ty f

acto

r

?

1

2

3

1)

2)

3)1.00

1.50

2.00

2.50

0 10 20 30 40applied pressure (kPa)

Facto

r o

f sa

fety

_?

1

2

3

Figure 7.34 Drained loading versus safety factor

Chapter 7 ________________________________________________________________________________________________________________________________________

192

0.000

0.002

0.004

0.006

0.008

0 2 4 6 8 10time (hours)

dis

pla

cem

ents

(m

) _

P

Figure 7.35 Typical displacements simulated through stress-strain analyses

for drained loading

Taking into account that the slope failures cannot be justified referring to

drained impact phenomena, “undrained impact” scenario was then considered

with the aid of both uncoupled and coupled approaches (Sect. 5.2.3, App. D).

Adopting an uncoupled approach, pore water pressures were firstly

computed relating their increments to the total stress variations through the

pore pressure coefficients and (Henkel, 1960; App. D). Considering that

saturated conditions can be reasonably referred for suction values up to 5 kPa ÷

10 kPa, the coefficient was assumed as ranging between 0.33 and 1, while the

coefficient was assumed equal to one.

On the basis of these assumptions and referring to the case of

homogeneous slope scheme – that was always characterised by the maximum

slope safety factors in correspondence of drained impact – the slope safety

factor remains greater than one (F=1.53) (Fig. 7.36a), just after the load

application; viceversa, instability conditions are attained (F=0.98) after the load

removal (Fig. 7.36b). In this analysis, and values were assumed respectively

equal to 0.5 and 1, and the initial pore water pressures were those computed on

May 3, 1998.

Similar scenarios can be drawn referring to an initial suction value equal to

5kPa. At this regard, it’s worthwhile to observe that the initial suction values are

strictly related to the period of the year and, especially in the upper portions of


193

the pyroclastic covers, they are affected also by the eventual presence of the

above upper outlets.

F=1.531 F=0.989

Figure 7.36 Typical slope stability scenarios due to an undrained impact:

a) before, b) just after load application, c) after rapid load removal

Adopting the previously computed pore water pressures, uncoupled

stress-strain analyses highlight that, especially under the loading zone, deviatoric

stresses increase while isotropic stresses significantly decrease due to the build

up of the pore water pressure (Fig. 7.37). In particular, the portions with lowest

mean effective stresses are strictly related to the soil deformability, the applied

load forces and the initial pore water pressure conditions. Of course, static

liquefaction phenomena, that may occur in such covers, cannot be simulated by

to the simple used constitutive model.

Mean effective pressure (p’) after loading

0

5

10

15

20

25

30

0

5

10

15

20

25

30

0

5

10

15

20

25

30

20kPa as load,

pore pressure before storm

20kPa as load,

pore pressure after storm

50kPa as load,

pore pressure before storm

Figure 7.37 Effects of an undrained loading impact over mean effective stresses

computed through an uncoupled approach

Chapter 7 ________________________________________________________________________________________________________________________________________

194

The same homogeneous slope was analysed by the hydro-mechanical

coupled approach. The results obtained with 2D analyses well match the

previously simulated instability scenarios with reference to both pore water

pressure and plastic strains (Fig. 7.38). On the other hand, 3D analyses show

that the applied vertical forces strongly affect the stress conditions; in

particular, shear stress (yz) arises mainly along a line that is inclined respect to

the x-direction. Moreover, plastic strains show a tendency to enlarge downslope

(Fig. 7.39).

Pore water

pressure (kPa) Plastic strains (-)

Figure 7.38 Pore water pressures and plastic strains computed through an

hydro-mechanical coupled approach

yz (kPa) Plastic strains (-)

z

y

x

z

y

x

Figure 7.39 3D effects induced by an undrained impact loading

over an open slope

At this regard, it can be noted that the triggering mechanisms, induced by

upper outlets and/or impact loading, are both characterised by the presence of

an unstable mass at the upper portions of open slopes. For these last, the stress


195

field is constant along planes parallel to the ground surface where stress values

depend from the depth and soil mechanical properties (Fig. 40). Such planes are

here referred to simulate the action of the unstable masses on the downslope

still stable zones (Fig. 7.40). In the performed analyses, stresses lower than 20

kN/m are applied at the top of this plane referring, for sake of simplicity, to

drained conditions.

The obtained results show that, due to the applied load, deviatoric stresses

increase and shear strains concentrate along directions related to the stress field

values (Fig. 7.40). Particularly, small forces produce yielding conditions over

large portions mainly due to the initial and low stress levels along the y-

direction (Fig. 7.41). From the performed analyses, it results that initial high

lateral stress values y reduce the effects of applied load, while high s stresses

strongly contribute to the achievement of the yielding conditions.

Figure 7.40 Stress and strains evolution, in a plane parallel to the ground surface,

due to load applied at the upper limit of an open slope

Chapter 7 ________________________________________________________________________________________________________________________________________

196

Figure 7.41 Instability scenarios for different initial

lateral stress y – increasing from case (a) to (c) –

In conclusion, the obtained results clearly evidence how the presence of a

loading in the upper portion of open slopes can lead to the formation of

triangular shape of source area which other Authors relate only to the

occurrence of intense erosion-avalanche phenomena (Guadagno et al., 2005;

Revellino et al., 2005).

Of course, these different interpretations of the same phenomenon calls

for an adequate deepening of the triggering mechanism, that is a crucial point

for the correct assessment and characterisation of the landslide source areas


197


The performed analyses highlight that both upper outlets and impact of

unstable masses can induce slope instability phenomena along open slopes. In

particular, the simulated instability phenomena, related to upper outlets, occur

after time period longer than 16 hours (Fig. 7.42), in good agreement with

those outlined by Calcaterra et al. (2004), that simulated pore water pressure for

similar pyroclastic covers in the Sorrento Peninsula (Sect. 5.1.3). Specifically,

the Authors related the portion of cover affected by this boundary condition

(ranging from 2 m to 12 m) to time action periods comprised between 6 and 24

hours.

Figure 7.42 Geotechnical analyses for pore water pressures induced by upper

outlets in some pyroclastic covers (Calcaterra et al., 2004)

For the impact loading phenomena, the performed analyses show that the

“drained impact” scenario do not justify slope instability conditions (Fig. 7.34).

Chapter 7 ________________________________________________________________________________________________________________________________________

198

Conversely, undrained loading can lead to failure due to the pore water pressure

increase, whose magnitude is related to the soil mechanical properties, the

stratigraphical setting and the initial conditions (Fig. 7.37). At this regard, it’s

worthwhile to note that “undrained loading” is frequently addressed in the

available literature as the most probable triggering cause for the occurrence of

the triggering mechanism M2, although geomechanical analyses are not

provided to support this statement.

Anyway, the previous analyses do not take into account the high velocity

of loading that could be an important factor for this triggering mechanism. In

fact, loading in dynamic conditions could induce oscillations in effective vertical

stresses and the waves amplitude could be a paramount factor eventually

leading to yield conditions for loading forces lower than those applied in the

static conditions.

Finally, with reference to the downslope enlargement of the source areas it

can be observed that for any assumed depth from the ground surface, the initial

stresses s are related to the slope angle while the initial stresses y are related

to lateral confinement conditions. In this sense, the obtained results address

open slopes with high slope angle and low lateral confinement as particularly

susceptible to landsliding. Probably the deepening of these conditions can

better address the analyses performed by means of geological approaches that

are aimed to find correlations among some geometrical characteristics of the

source areas (Fig. 7.43) (Guadagno et al., 2005; Di Crescenzo & Santo, 2005).

Figure 7.43 Correlation between scarps and source areas

(modified after Guadagno et al., 2005)


199

In conclusion, the triggering mechanism M2 can be related to the upper

outlets and/or impact loading phenomena. However, the conditions leading to

failure and the downward evolution of the source areas represent two

fundamental issue to be taken into account for an appropriate evaluation of

potential unstable masses.

Chapter 7 ________________________________________________________________________________________________________________________________________

200

7.4 M3: A TRACK INDUCED TRIGGERING MECHANISM

The triggering mechanism named M3 took place essentially along the

flanks of gullies where cuts and trackways produced heavy modifications of

slopes topographic setting as well as variations of superficial and groundwater

circulation (Fig. 7.44).


from the basins n. 35 (a), 36 (b), 38 (c) facing the town of Quindici

This mechanism was recognised by many Authors (Sect. 6.2) that outlined

the role of geometrical discontinuities for slope instability conditions of

pyroclastic covers and some remarks are hereafter outlined (Fig. 7.45).

In particular, along the hillslopes, natural scarps have heights often about

tens of meters while anthropogenic scarps are 1-2 m high. Referring to

hydraulic boundary conditions, upper outlets were sometimes recognised at the

base of natural scarps while, during storms, concentrated superficial water flow

frequently occurred at the bend zones or in specific points along the trackways.

Moreover, at the base of a natural scarps, the stress conditions are mainly

related to air-fall deposition and colluvial processes, while more complex stress

histories are the consequence of trackway air-fall deposition and subsequent

man-made modifications determine more complex stress histories. Finally,


201

a) b)

Figure 7.45 Differences between natural scarps (a) and artificial cuts (b)

2

1

1

Figure 7.46 Complex environmental setting in the basin n. 35 facing

the town of Quindici: 1) man-made elements, 2) bedrock scarp

natural and artificial cuts often combine in complex environmental settings

strongly influencing both the triggering and propagation stages of flow-like

mass movements (Fig. 7.46).

On the basis of these considerations, the natural and artificial

morphological scarps were separately addressed taking into account their

different features. Particularly, the following section focuses the attention on

the anthropogenic morphological discontinuities affecting the flunks of gullies,

assuming the reference schemes reported in Figure 7.47, to outline

computational schemes at massif scale.

Chapter 7 ________________________________________________________________________________________________________________________________________

202


7.4.1 Insights towards massif-scale analyses

The analysis of the triggering mechanism M3 must necessarily start from

the evaluation of the rainfall run-off and infiltration phenomena that can

involve mountain roads and that are essentially related to rainfall intensity,

topographical settings, soil properties and in-situ soil conditions. When

referring to possible run-off phenomena, the evaluation of the most affected

zones can be outlined by using the so-called “contributing area”. To this aim,

the hillslope can be divided in cells of finite dimensions, recognizing for each of

them, the catchment area (i.e. contributing area) (Fig. 7.48).

b)a)

Figure 7.48 Contributing area index: a) definition (Pack et al., 1998), b) an

example for an hillslope landscape (Tucker et al., 1998)


203

To this aim, some typical situations are hereafter discussed and a

qualitative assessment of the most affected areas is proposed. Particularly, the

attention is focused on the case of a steep mountain road with slope angle equal

to 14° over a 30° hillslope, with one or more bends (Fig. 7.49). The performed

analyses highlight that different scenarios can arise in relation to the

topographical settings of hillslope and trackway. For the first case, the presence

of a trackway bend strongly increases the contributing area index in the

neighbourhood of a bend (zone A in Fig. 7.49a). On the contrary, two or three

bends produce more complex scenarios because several zones with high

contributing areas are simulated (zones B and C in Fig. 7.49b). These effects are

even more evident for gentler trackways that heavily modify superficial water

patterns.

a)

b)

Figure 7.49 Qualitative evaluation of different mountain roads arrangements

through the evaluation of the topographic index

Certainly, these analyses furnish only a qualitative insight for the most

landslide-prone zones because the conditions eventually leading to run-off

Chapter 7 ________________________________________________________________________________________________________________________________________

204

phenomena must be evaluated and groundwater phenomena must be

addressed. Moreover, the achievable results are strictly related to the quality of

the available digital elevation model for the topographical surface. Nevertheless,

such preliminary evaluations allow to set up some useful computational

schemes for geomechanical analyses at massif scale.

At this regard, it must be observed that the trackways are preferential ways

for superficial water movement and they certainly represent the zones more

prone to eventual run-off phenomena and ponding conditions, as discussed by

Ziegler et al. (2004) with reference to both paved mountain roads and

trackways (Fig. 7.50).

Figure 7.50 Superficial and groundwater circulation related

to the presence of a mountain road (Ziegler et al., 2004)

7.4.2 Massif-scale analyses

Based on the previous insights, massif scale analyses were performed,

referring to the computational schemes that reproduce the in-situ conditions

respectively in correspondence of a trackway bend and far from it (Fig. 7.51).

These analyses were essentially aimed to evaluate the role of trackways, during


205

rainfall storms, when they impose severe hydraulic boundary conditions over

large portions of the hillslope. On the basis of this consideration, a ponding

local condition was assumed in correspondence of trackway bend, for action

periods shorter than 30 hours (Fig. 7.52).

Class B ashy soils

Pumice soils

Class A ashy soils

Scheme 3

0.40

1.68

0.362.06

0.36

1.120.47

1.68

1.23

1.682.82

1.680.30

0.82

0.471.23

1.76

1.680.30

0.40

1.680.300.820.47

0.470.360.40

1.680.30

2.52

4.5

m

Scheme 2

Scheme 8Scheme 7


Scheme 1

4.5

m

4.5

m4.5

m

4.5

m

4.5

m4.5

m

4.5

m

Class B ashy soils

Pumice soils

Class A ashy soils

Scheme 3

0.40

1.68

0.362.06

0.36

1.120.47

1.68

1.23

1.682.82

1.680.30

0.82

0.471.23

1.76

1.680.30

0.40

1.680.300.820.47

0.470.360.40

1.680.30

2.52

4.5

m

Scheme 2

Scheme 8Scheme 7


Scheme 1

4.5

m

4.5

m4.5

m

4.5

m

4.5

m4.5

m

4.5

m

2)1) 3)

Figure 7.51 Considered stratigraphical schemes

ponding

ponding

a) b)

Figure 7.52 Computational schemes for zones (a) corresponding

and (b) contiguous to the bends of trackway

Using an uncoupled approach, pore water pressure were analysed

referring, once again, to different stratigraphical settings and with reference to

the 4-5 May 1998 event (Fig. 7.53). In particular, rainfall values were assumed

equal to those utilised for the previous analyses (Sect. 7.2, 7.3) and a ponding

condition was adopted for a 5 m length, downslope the trackway bend.

The obtained results highlight that the same boundary conditions applied

at the ground surface produce very different scenarios in relation to the

stratigraphical settings (Fig. 7.53). In particular, when only ashy B soils are

present, a little pore water pressure increase is simulated; conversely, when ashy

Chapter 7 ________________________________________________________________________________________________________________________________________

206

A soils are present, strong variations are observed. Finally, for slopes with

continuous pumice soil layers, the affected portion is even more larger than the

previous ones.

uw > 0

uw < 0

uw > 0

uw < 0

uw < 0

1 2 3

pondingpondingponding

Figure 7.53 Pore water pressure induced by rainfall and a local ponding

condition over different stratigraphical schemes

Referring to these pore water pressure regime, limit equilibrium analyses

provide different slope stability scenarios. For instance, considering a local

ponding condition in the scheme 1, the computed slope safety factors are

always greater than one, also in correspondence of track bends (Fig. 7.54).

Conversely, in the schemes 2 and 3, slope instability phenomena involve large

unstable also not considering any geometrical discontinuity (Fig. 7.55).

V V

1)

FS=1.10

2)

FS=0.89

ashy B

ashy A

Figure 7.54 Limit equilibrium analyses for the mechanism “M3” in

correspondence the bands of trackways


207

1

2

VV

ashy B

pumice

ashy A

1)

FS=1.25

2)

FS=0.883)

FS=0.97

Figure 7.55 Limit equilibrium analyses for the mechanism “M3” for zones

contiguous to the bends of trackways

Adopting the previous pore water pressure and with the aid of uncoupled

and coupled approaches, stress-strain analyses provide the displacements values

reported in Figure 7.56, clearly showing that rainfall and a local ponding

condition induce displacements that after 12 hours are characterised by a quite

high gradients leading to failure conditions after about 24 hours (Fig. 7.57).

after 10 hours after 20 hours ore after 24 hours

P

0.0005 – 0.002

0.002 – 0.0035

0.0035 – 0.005

displacement

contours (m)

Figure 7.56 Displacement field obtained through uncoupled FEM analyses

Chapter 7 ________________________________________________________________________________________________________________________________________

208

0.000

0.001

0.002

0.003

0.004

0.005

0 3 6 9 12 15 18 21 24 27time (hours)

dis

pla

cem

ents

(m

) _

only rainfall M3

only rainfall

with ponding

Point P

Figure 7.57 Displacement versus time for the unstable mass

Referring to the in-situ evidence, the obtained results highlight the strict

correspondence between trackway bends and local slope unstable areas.

Moreover, starting from these results and adopting the procedure described in

Cascini et al. (2003) multiple slope instability phenomena can be surely

achieved. Conversely, lateral enlargement of the source areas, that systematically

characterised such triggering mechanism, calls for the application of more

advanced numerical models not available at the present.


The performed analyses highlight that topographic modifications, related

to tracks construction, have a strong influence on superficial and sub-superficial

water circulation producing severe hydraulic boundary conditions, especially, in

correspondence of their bends. As a consequence, intricate source areas can

develop, through the occurrence of both multiple landslides and/or lateral

enlargement of first-mass movements.

With reference to the local effects of trackways, it must be stressed that

the stratigraphical setting plays a relevant role towards slope failures. In fact, the

limit equilibrium analyses show that slope instability conditions occur in


209

relation to the presence of ashy A soil layers (Fig. 7.54, 7.55). These findings are

also confirmed by stress-strain analyses that furnish also further insight on

deformation processes leading to failure after time periods similar to those

evaluated with the previous analyses.

In conclusion, the previous results seem to highlight the fundamental role

played by the hydraulic boundary conditions imposed at the bends of

trackways. On the other hand, other Authors stressed the primary role played

by topographical modifications on slope instability conditions. In particular,

Guadagno et al. (2003), assuming steady-state pore pressure regimes and, with

the aid of geotechnical finite differences analyses, related the failure to the

presence of man-made cuts (Fig. 7.58). Similar instability scenarios were also

outlined by Crosta & Dal Negro (2003), by means of transient seepage

modelling and limit equilibrium analyses.

Figure 7.58 Geotechnical analyses for slope instability inside upwards slope at

morphological scarps (Guadagno et al., 2003)

8 CONCLUDING REMARKS

The desire to know is natural to good men.

Leonardo da Vinci

8.1 THE LESSON FROM THE CASE STUDY

Investigations, studies and considerations performed in Southern Italy, at

massif and regional scales, allow some considerations on the features and

modelling of the analysed flow-like mass movements. Moreover, some

elements are pointed out, hopefully to be considered in improving the actual

flow-like mass movements classifications.

With reference to the study area, it must be primarily observed that a

variety of interpretations and classifications have been proposed for the

occurred phenomena (Sect. 5.2.2). This is essentially related to the frequently

utilised sector-based approaches, that focus on some features of the occurred

events. For instance, some Authors (Rossi, 1998; De Vita & Piscopo, 2002)

related the occurrence of the May 1998 events to the main hydrological factors

such as the cumulated and antecedent rainfalls. Other Authors addressed the

characters of the mountain basins (Pareschi et al., 2000), the morphometric

characteristics (Di Crescenzo, 2005; Guadagno, 2005) the geomorphological

features (Brancaccio et al.., 1999; Di Crescenzo & Santo, 2005) and the

hydrogegological factors (De Vita & Piscopo, 2002) as relevant elements for the

occurred phenomena. Moreover, some geotechnical studies, focusing on the

mechanical behaviour of pyroclastic soils (Olivares & Picarelli, 2003), outlined

Chapter 8 ________________________________________________________________________________________________________________________________________

212

the static liquefaction phenomena as the main explanation of the May 1998

events. All these approaches highlighted some factors (and/or conditions) that

certainly contributed to the occurrence of the above phenomena even though a

unique factor doesn’t allow a satisfactory interpretation of all the phenomena

occurred inside a context that is only apparently homogeneous. At this regard,

it must be observed that the above approaches do not rely on a preliminary

evaluation of the triggering mechanisms and, consequently, they consider the

overall first-failure phenomena of the May 1998 event as belonging to a unique

typology.

The proposed approach attempts to join the potentialities of geological,

geomorphological and hydro-geological studies, at local scale (1:5000), with

engineering models both at site (1:2000 and larger) and local scales, on the basis

of the available data set.

Moving within this framework and taking into account, at different scales,

only the geology, geomorphology and hydrogeology of the sample area, six

distinct triggering mechanisms were identified for the first-failure stage of these

phenomena, – respectively named M1, M2, M3, M4, M5, M6 – (Sect. 5.1.2).

Particularly, the mechanism M1 essentially occurred inside ZOBs affected by

water supplies coming from the bedrock towards the pyroclastic covers. The

mechanism M2 originated inside triangular shaped source areas and mainly

occurred in the upper portions of open slopes, in correspondence of bedrock

morphological discontinuities. The mechanism M3 produced complex shaped

landslides, as result of multiple or concatenated instability phenomena, strictly

influenced by anthropogenic activities. The mechanism M4 was related to the

gully advancing processes towards the head of basin, so originating grape

shaped landslides, strongly related to the site morphology. The mechanism M5

occurred along convex slopes resulting in elongated source areas. Finally, the

mechanism M6 developed at the toe of short convex hillslopes, in

correspondence to changes in slope angle and concavity, generally involving

small volumes. Referring to these mechanisms, it must be stressed that they

were characterised by different intensity in terms of mobilised volume and

travel distance and, at the same time, not casually distributed on the massif.

Discussion ________________________________________________________________________________________________________________________________________

213

Then, according to the followed procedure (Sect. 5.2.3), the validation of

the previous triggering mechanisms was pursued on the basis of both

geotechnical and geomechanical analyses, as discussed in the Section 7.

Particularly, for the triggering mechanism M1 (Sect. 7.2), it was possible to

get a satisfactory insight of the key instability factors, joining the findings of

both hydro-geomorphological, physically based and geotechnical models. At

this regard, the geotechnical analyses highlight the importance of the factors

controlling the mechanical soil behaviour during the triggering stage. On the

other hand, advanced geomechanical analyses allowed further simulations of

the in-situ experienced stress-strain processes to be compared to the direct

observations achieved in laboratory and small scale tests (centrifuge and flume

tests) (Olivares & Picarelli, 2003; Take et al., 2004). Moreover, for all the

previous mechanisms, distinct geotechnical analyses gave matching results for

the same phenomena, as discussed in Sections 7.2 – 7.4. Finally, the potentiality

of 3D geomechanical analyses was demonstrated by the good agreement among

the in-situ evidence, the geomorphological issues and findings of

geomechanical modelling (Sect. 7.2 – 7.4).

On the other hand, with reference to the triggering mechanism M2 and

M3 (Sect. 7.3, 7.4), the followed approach highlighted the need to utilise

(and/or set up) further advanced computational tools to take into account all

the aspects related to these mechanisms that result particularly difficult to be

modelled. Some relevant issues are, for instance, the dynamic loading in

unsaturated conditions in the case of M2, or the effects of instability conditions

upon surrounding areas, in the case of M3. These issues are to be surely added

to the need of further laboratory investigations eventually aimed to the setting

up of advanced constitutive models.

Notwithstanding the absence of the necessary tools for a complete

validation of all the recognised triggering mechanisms, the results of the

numerical analyses allow some significant considerations.

First of all, the static liquefaction phenomena seems a consequence rather

than a cause of the landslides’ triggering, in agreement with some laboratory

flume tests (e.g. Eckersley, 1990) and centrifuge tests (eg. Take et al., 2004). In

this sense the static liquefaction is to be considered a relevant post-failure

Chapter 8 ________________________________________________________________________________________________________________________________________

214

phenomena but not a common feature of all the triggering mechanisms (Sect.

7.2-7.4)

On the other hand, a key factor for the first-failure stage is certainly

represented by water supplies that are indirectly related or independent from

the rainfall storms. In the first case, a primary role is played by anthropogenic

structures (such as trackways and cuts) which convey relevant water volumes in

singular point or portion of the slope. In the second case, the water supplies are

instead connected to the sub-superficial water circulation inside the massif

(bedrock outlets related to idrowedges structures and/or karstic conducts).

These processes, however, are strictly related to the seasons of the year (O.U.

2.38, 1998; Cascini & Ferlisi, 2003). In fact, the run-off phenomena are more

frequent and important in the Autumn, due to the high soil suction values (and

consequently low permeability), and they are associated to the most intense

storms during the whole hydrological year. Conversely, the bedrock outlets

attain their maximum values in the Spring, about 3 - 4 months later the end of

wet season.

Therefore, it can be concluded that, during the hydrological year, distinct

triggering mechanisms can occur, strictly related to the storm rainfall patterns,

to the long-term sub-superficial groundwater inside the bedrock, short-term

infiltration within the pyroclastic covers and/or run-off phenomena

To confirm and generalise the previous discussion it is useful to consider

the whole territory of the Campania region covered by pyroclastic soils.

At this regard, it can be observed that pyroclastic covers lying on a

carbonate bedrock demonstrated, over the centuries, to be particularly prone to

flow-like mass movements (Sect. 5.1). In this environmental setting, landslide

phenomena are more frequent and, usually, they are generally characterised by

the highest intensities compared to those phenomena triggered in other

geological contexts of the Campania region (tuff and flysch).

Inside the environmental setting constituted by limestone massifs the

events with low intensities are often associated with a bad regulation of

superficial waters (Di Crescenzo & Santo, 2002; 2005) and/or to bedrock

outlets (Budetta & de Riso, 2004). In all these previous cases, the presence of

Discussion ________________________________________________________________________________________________________________________________________

215

modest water egress in surrounding bedrock fronts have been noted, also some

days later the landslide events (Sect. 5.1.3).

On the contrary, in the same environment, the most catastrophic events

are related to severe storms, as in the case of 1954 event in the Lattari mounts,

when extreme rainfall values (500 mm in 9 hours) were recorded (Sect. 5.1.3).

In these cases, the instability phenomena seem to be strictly related to intense

run-off and erosion phenomena.

From these evidences, as observed for the sample area, it appears that also

inside an “homogenous” environmental setting (“the carbonate bedrock

environment”), very different phenomena can occur (Sect. 5.1). They are

closely related to the small-scale massif features (order of mountain basins,

presence/absence of karstic plains), to the bedrock morphological characters

(sub-superficial water circulation), to the cover characteristics (thicknesses,

lithotypes), to the rainfall values (including also the antecedent rainfall values)

and the season of the year (soil suction values, rainfall patterns). Once again,

with reference to the triggering stage, it seems not possible to categorise

(associate or “collapse”) such a variety of phenomena within a unique typology.

In this sense, the only approaches, potentially capable to make these necessary

distinctions, seems to be the multidisciplinary procedures. As discussed for the

study area, such an integrated approach can allow a more shared interpretation

of the occurred phenomena and eventually their correct classifications towards

the assessment and mapping of susceptible areas.

From the previous discussions, some general remarks can be outlined

for the analysed flow-like mass movements.

Primarily, it must be observed that multidisciplinary studies must be

strongly encouraged for both back-analysis and forecasting of these

phenomena. In fact, sector-based approaches generally focus the attention only

on some aspects that surely contribute to the instability phenomena but, alone,

are not capable to explain them.

Inside such procedures, “state of the nature maps” have a paramount role

as discussed by Cascini et al., 2004. These maps enclose and summarise

accurate evaluations of both geological, geomorphologic, hydrogeological

Chapter 8 ________________________________________________________________________________________________________________________________________

216

elements, as illustrated for the sample area (Sect. 6.1). As a consequence, these

maps prove to be extremely useful towards the recognition of significant

triggering mechanisms as well as to set up the most adequate computational

schemes for the geotechnical and geomechanical analyses.

At this regard, it seems that the validation of the findings coming from the

geological sciences have to be pursued through geotechnical and geomechanical

analyses, based on both suction values and soil mechanical properties.

Certainly, the agreement between geological, geotechnical and geomechanical

insights could result in a shared interpretation of these phenomena (Sect. 5.2.2,

5.2.3).

With particular reference to the geotechnical and geomechanical analyses,

it must be observed that several methods are available and the choice among

them rely on both the features of the studied phenomena, the aims and target

of the performed analyses and the available data set. In this sense it seems that

the comparison of the results coming from the different methods can result in a

more satisfactory interpretation of the studied phenomena.

Another topic that requires considerable efforts concerns the available

classifications that, actually, do not take into account some relevant aspects

with reference to the triggering stage (Sect. 2.3). In fact, despite a not yet

completely internationally accepted terminology, some landslide classifications

(Varnes, 1978; Hutchinson, 1988; Cruden & Varnes, 1996) and, in particular,

the classification reviews proposed for the flow-like mass movement by Hungr

et al. (2001) and Hutchinson (2004) are capable to give an useful conceptual

framework mainly with reference to the post-failure and propagation stages. On

the contrary, they do not help in clearly recognising and distinguishing the

triggering mechanisms, environmental conditions and processes leading to

failure in the source area (Cascini et al., in press).

Discussion ________________________________________________________________________________________________________________________________________

217

8.2 FUTURE DEVELOPMENTS

The previous discussion pointed out some relevant topics to be deepened,

in order to improve both modelling and classifications of the analysed

phenomena.

Certainly, the possibility to collect extensive data set concerning the in-situ

conditions is a relevant issue, in order to perform analyses similar to those

discussed in the thesis. The set up of advanced (fast and low-cost) in-situ

investigation methodologies can result in a fundamental contribution, because

flow-like mass movements often involve large areas whose upper portions are

difficult to be reached (Cascini 2004). In this sense, promising techniques (for

instance, geophysical methods) seem capable to provide information on the

morphological layouts of bedrock and stratigraphy. Moreover, some insights

are also obtained on soil mechanical properties and, indirectly, on

hydrogeological aspects. On the other hand, techniques aimed to soil water

content measurement can highly improve the understanding of suction regime

fluctuations during the hydrological year, allowing significant temporal and

spatial assessment of the most susceptible areas.

In addition, mechanical characterisation of pyroclastic soils is a relevant

topic, due to their diffusion and peculiar mechanical features. At this regard,

further suction controlled laboratory tests could be performed in order to

better assess their mechanical behaviour towards the setting up of enhanced

constitutive models, capable to simulate their mechanical behaviour during the

triggering and post-failure stages of the analysed phenomena.

Referring to the first-failure stages of flow-like mass movements, the

proposed approach should be validated through its application to similar

geological contexts. Moreover, the setting up of advanced computational tools

can be fundamental in order to include some features of the these phenomena

in the performed analyses. In this sense, it appears that relevant efforts could be

oriented towards the implementation of hydro-mechanical coupled models for

dynamic loading in unsaturated conditions. These last, in different stages, are a

frequent feature of several flow-like mass movements. On the other hand, the

use of 3D geomechnical analyses should be strongly encouraged because they

can allow a significant improvement in the assessment of the susceptible areas

Chapter 8 ________________________________________________________________________________________________________________________________________

218

and potentially mobilised volumes, to be eventually compared with those

assessed through geological approaches.

Anyway, it must be observed that the analysis of triggering stage is only

the first step, towards the assessment of risk prone areas for whose detection,

studies addressing the propagation stage are certainly necessary. To this aim,

several models have been proposed, based on different approaches. Conversely,

it’s not yet available a unique model capable to translate the whole phenomena

from the source to the deposition areas, even though a theoretical framework

has been proposed by Pastor et al. (2003), capable to address both the

triggering and propagation stages respectively in a lagrangian and eulerian

reference system. Similar studies could be encouraged towards a complete

validation of still existing approaches and eventually for the setting up of low

time-consuming tools to be utilised over large areas.

Finally, on the basis of studies and models on both triggering and

propagation stages of flow-like mass movements, significant improvements

could be surely towards an adequate classification of these phenomena.

Certainly, geomechanical modelling of flow-like mass movements is a

relevant issue, as it can allow back-analyses and forecasting for these events

and, consequently, it can improve landslide risk assessment and mapping, aimed

to safeguard life and properties from such a relevant natural hazard.

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APPENDIX A

LANDSLIDE CLASSIFICATIONS

Some aspects of the available landslide classifications are presented in this

Appendix that is not exhaustive while it can be a useful support to the

discussions of chapters 3, 5, 6 about flow-like mass movements.

The first landslide classifications were based on landslide morphology

such as, for instance, those proposed by Skempton (19533) and Blong (1973).

Skempton (1953) suggested a primary division of landslides based on the D/L

ratio, where D is the maximum thickness of the landslide and L is the

maximum length in the direction of the maximum slope. Values of the ratio

decrease considering the flows, slides or slumps (rotational slides) ranging from

0.3 to 0.005. According to Crozier (1986), the D/L ratio is the most useful

single morphometric index in distinguish landslide process group.

Moving within this framework, Blong (1973) divided landslides into four

groups (slides, slumps, flows, falls) (Fig. A1). Slides are characterized by a slide

plane or planes essentially parallel to the existing ground surface. Much of the

material is rafted down as blocks of material. Slumps are characterized by

curvilinear (concave upward) shear plane and they are generally deep compared

with their length and the failed material remains almost intact. They are rare on

natural hillslopes while probably the most common mode of failure in man-

made excavations or fills. Flows are characterized by slide planes essentially

parallel to the existing ground surface but most of the material flows

downslope as viscous mass. Falls are mass failure that lose contact with ground

for at least part of the travel distance. At this regard it must be observed that

many slope failures begin as slide continue as flows. Within these classifications

it is not present any further distinction among the “flows”.

Appendix A ________________________________________________________________________________________________________________________________________

238

Figure A.1 Morphological landslide classification (Blong, 1973)

Varnes (1978) highlighted that landslides can be differentiated by the

kinds of materials and according the mode of movements and proposed a

classification system mainly based on these aspects (Fig. A2).

Within this framework, “flows” are categorised on the basis of the

percentage of coarse materials (grain diameter greater than 2 mm), that can be

higher (i.e. “debris”) or lower (i.e. “earth”) than 20%. This criterion, although

apparently simple, is not easy to apply at flow-like mass movements. For these

phenomena, in fact, unstable materials are certainly related to the soils triggered

inside the source areas that, in turn, are often capable to erode further materials

along the path depending on the topographic conditions, soil properties and

propagation modes. As a consequence, displaced materials frequently differ

from source materials.

Figure A.2 Slope movement types and processes (Varnes, 1978)

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239

On the other hand, Hutchinson (1988) presented a classification system

mainly focusing on morphology with some considerations on mechanisms,

material and rate of movement (Fig. A3). At this regard, the Author observed

that “landslides exhibit an initial failure stage followed by a run-out and a

central problem in classification is what weight to give to each of these two,

often contrasting stages”. In this classification, the mainly addressed landslides

are those with moderate run-out.

Based on landslides morphology, long run-out landslides are grouped as

“flow-like form landslides” with, however, some differences in their features

and mechanics. In this sense, according to the Author, “mudslide”

predominantly slide rather than flow, “flowslides” and “debris flows” slide and

flow and “sturzstroms” essentially flow.

Figure A.3 Geotechnical classification (Hutchinson, 1988)

Appendix A ________________________________________________________________________________________________________________________________________

240

Cruden & Varnes (1996) deepened the previous classification system

provided by Varnes (1978) focusing on landslide activity (e.g. state, distribution

and style) and other features such as landslide rate, water content, material and

type (Fig. A4, A5). According to the Authors, “the name of a landslide can

become more elaborate as more information about the movement becomes

available” and the possibility to form expert classification systems is certainly

related to the use of standards for the recognisance and evaluation of landslide

features.

This classification provides an useful multi-scale approach, first by time

and then by spatial location. Referring to the time scale, first mass movements

and subsequent slope instabilities are referred. To this aim, the addition of the

descriptor “complex” to the name indicates the sequence of movements in the

landslide. As it concerns the spatial scale, the suggested procedure addresses

firstly the whole landslide, continuing with the landslide parts up to the

involved materials. This framework is particularly useful for the “flows” whose

style and rate often change during the movement of the unstable masses. In the

landslide category “flow”, debris flow, earth flow and debris avalanche are

included and the Authors stressed that “there is a gradation from slides to flows

depending on water content, mobility, and evolution of the movement”.

Type of movement

Type of material

Engineering soils

Bedrock Predominantly coarse Predominantly fine

Fall Rock fall Debris fall Earth fall

Topple Rock topple Debris topple Earth topple

Slide Rock slide Debris slide Earth slide

Spread Rock spread Debris spread Earth spread

Flow Rock flow Debris flow Earth flow

Figure A.4 Landslide types and processes (Cruden & Varnes, 1996)

ACTIVITY


241

STATE DISTRIBUTION STYLE

Active Advancing Complex

Reactivated Retrogressive Composite

Suspended Widening Multiple

Inactive Enlarging Successive

Dorment Confined Single

Abandoned Diminishing

Stabilised Moving

Relict

DESCRIPTION OF FIRST MOVEMENT

RATE WATER CONTENT MATERIAL TYPE

Extremely rapid Dry Rock Fall

Very rapid Moist Soil Topple

Rapid Wet Earth Slide

Moderate Very wet Debris Spread

Slow Flow

Very slow

Extremely slow

DESCRIPTION OF SECOND MOVEMENT

RATE WATER CONTENT MATERIAL TYPE

Extremely rapid Dry Rock Fall

Very rapid Moist Soil Topple

Rapid Wet Earth Slide

Moderate Very wet Debris Spread

Slow Flow

Very slow

Extremely slow

Figure A.5 Glossary for forming names of landslides (Cruden & Varnes, 1996)

Leroueil et al (1998) stressed the importance of an accurate

“geotechnical characterization” of slope movements for their classification

(Fig. A6). First of all, the proposed procedure suggests that the stage of

movement must be referred since it is strictly mutual correlated to movement

style and soil mechanical properties (Fig. A6b). Then, predisposition and

triggering factors (Fig. A6a), as well as the involved materials (Fig. A6c), must

be assessed.

The above issues are certainly relevant for landslide modelling and could

be taken into account for their classification. At this regard, it must be noted

that landslide modelling requires detailed information eventually focusing on

Appendix A ________________________________________________________________________________________________________________________________________

242

peculiar features of both soils and in situ conditions. Conversely, landslide

classification tries to summarise in few terms general features and modes. In

this sense, it seems that landslide modelling and classification require

contrasting efforts. Anyway, adequate landslide classifications can allow

accurate modelling and, in the meantime, they can be improved on the basis of

the obtained results.

a)

c)

b)

Figure A.6 Slope movements: a) scheme for landslide characterization,

b) different stages, c) involved materials (Leroueil et al., 1998)


243

Hungr et al. (2001) reviewed landslides classification with special reference

to flow-like mass movements, discussing criteria and structures of the most

diffused classification systems. In particular, mowing within the framework

proposed by Varnes (1978), the Authors proposed a “typological, purpose

oriented” classification scheme, assuming numerous distinctive characteristics

for each group. In particular, ten classes were proposed based, on material type,

water content, presence of excess pore-pressure or liquefaction at the source of

the landslide, presence of a defined recurrent path (channel) and deposition are

(fan), velocity, and peak discharge of the event (Fig. A7).

Despite the several landslide classes, it must be observed that some

ambiguities still remains. In fact, some phenomena can be included in different

categories due to the change of landslide materials during the propagation.

Conversely, in some cases, more than one term could be necessary in order to

adequately describe the different morphological conditions, along the path, that

affect landslide style and the features of propagation areas.

Varnes (1978) Hutchinson (1988) Hungr et al. (2001)

“Flows” “Debris movement of flow-like form” “Landslides of the flow type”

Wet sand, silt flow Flow slide Sand, silt flow slide

Rapid earth flow Flow slide (clay) Clay flow slide

Loess flow Flow slide (loess) Loess flow slide

Dry sand flow - Dry sand flow

Earth flow Mudslide Earth flow

- Mudflow Mud flow

Debris avalanche Hillslope debris flow Debris avalanche

Debris flow Debris flow Debris flow

- Hyperconcentrated flow Debris flood

Rock avalanche Sturzstroms Rock avalanche

Figure A.7 Classification of flow-like mass movements (Hungr et al., 2001)

Finally, Hutchinson (2004) proposed a review of the main contributions in

the field of landslides classification, since “notwithstanding the recent progress

made by Hungr et al. (2001), problems still remain in the area”. As a matter of


Appendix A ________________________________________________________________________________________________________________________________________

244

fact, Hutchinson (2004), in order to clarify some not yet shared terms, set up a

simplified scheme based on few main classes, namely: debris flows,

flowslides, rock avalanches, mudslides (Fig. A8).

a) b)

c)

d)

Figure A.8 Classification of flow-like mass movements: a) debris flow,

b) flow slides, c) rock avalanches, d) mudslides (Hutchinson, 2004)


245

Debris flow are very to extremely rapid flows of predominantly saturated,

widely graded rock debris of low clay fraction and cohesion.

Flowslides are typically very rapid movements of sorted or unsorted

granular materials of low clay fraction, involving the liquefaction of a saturated

or near-saturated, often basal zone.

Rock avalanches are extremely rapid, massive, flow-like motions of

fragments rock from large rock slides or rock falls.

Mudslides are relatively slow-moving, shallow masses of softened

argillaceous debris which advance predominantly by sliding on discrete

boundary shear surfaces at or near residual strength.

This scheme presents relevant potentialities mainly related to the presence

of very different classes, that avoid any possibility to categorise the same

phenomena with different terms. In this sense, the use of wide classes can

certainly be very useful towards the landslide analysis, in order to select proper

methods for landslide characterisation and modelling.

On the other hand, this classification can provide some shortcoming

when more information become available from in-situ surveys and landslide

modelling. Moreover, the use of a unique descriptor seems to limit the

possibilities to adequately address “complex” phenomena and/or affecting

distinct environmental settings along the path.

APPENDIX B

VOLCANISM IN THE CAMPANIA REGION

This Section concerns with the main volcanic complexes located in the

Campania region (Southern Italy) and it must be considered a support to the

discussions developed in Chapters 2 and 5.

Volcanic activity in Campania region is closely related to the Plio-

Pleistocene tectonic phases that generated the splitting of the western border of

the Central Apennines and the creation of the structural depression of the

Campanian Plain. Due to this process, suitable conditions developed for

magma formation and rise that supplied the eruptive activity of volcanoes in

the Campania region. In particular, three main volcanic areas exist: the

Roccamonfina Volcano, the Phlegraean Volcanic Area (including Naples, the

Phlegraean Fields and the islands of Ischia and Procida), and the Somma-

Vesuvius volcanic complex (Fig. B1). For each of them, some aspects are here

summarised also with special reference to the originated soils.

12

3

Figure B.1 Main volcanic complexes in the Campania region (Southern Italy):

1) Vesuvius, 2) Phlegrean fields, 3) Roccamonfina

Appendix B ________________________________________________________________________________________________________________________________________

248

The Roccamonfina volcano lies to the north of Mount Massico, inside

the Garigliano depression, in the northwest area of Campania (Fig. B2). The

magmatism of this area is aged between 630,000 and 50,000 years ago. The

Roccamonfina volcano is a volcanic complex composed of a main strato-

volcano located in the Garigliano tectonic depression and of a series of

secondary vents. The strato-volcano has an elliptic summit caldera which was

generated by the gravitational collapse of part of the volcanic edifice (for

tectonic reasons), rather than by strong explosive eruptions.

The eruptive history of the Roccamonfina volcano can be divided into

three main eruptive periods.

During the first eruptive period (630,000 - 400,000 years), the magma

reached the surface creating a series of small vents scattered over an area of

approximately 1,000 km2. After this initial phase, the eruptive activity was

concentrated in the Garigliano depression and caused the formation of the

Roccamonfina strato-volcano, consisting of deposits of mainly effusive and

subordinately explosive eruptions. The effusive eruptions caused the formation

of lava flow and the explosive eruptions caused Strombolian and Subplinian

pyroclastic fallout deposits, as well as pyroclastic flow deposits. Some small

lateral vents appeared during this phase. The first eruptive period ended

approximately 400,000 years ago, with the gravitational collapse of the eastern

part of the strato-volcano. In total, it was calculated that 100-120 km3 of lava

and pyroclastic particles were expelled.

After a long period of quiescence, eruptive activity started again about

385,000 years ago with the explosive eruption of Tufo Leucitico Bruno (Brown

Leucitic Tuff), and continued with a series of eruptions, supplied by the vents

inside the caldera. These predominantly explosive eruptions originated the

complex pattern of White Trachytic Tuff of Cupa, Aulpi, S. Clemente and

Galluccio, between 327,000 and 230,000 years ago. During the second eruptive

period, 8.5-11 km3 of magma were expelled forming pyroclastic flows

(ignimbrites), but also surge and fallout deposits.

During the last eruptive period, explosive eruptions and the formation of

two lava domes inside the collapsed area, were recorded particularly along the

system of eruptive fractures oriented NE-SW. In addition, low regime effusive

and explosive activity was present along the eruptive fracture oriented N-S. The

Volcanism in the Campania region ________________________________________________________________________________________________________________________________________

249

total volume of magma expelled during this period is approximately 1 km3. The

Roccamonfina activity presumably ended about 50,000 years ago.

a) b)

Figure B.2 Roccamonfina volcanic complex

The Phlegraean Volcanic District includes the Phlegraean Fields, the

city of Naples, the volcanic islands of Ischia and Procida, and the north-west

section of the bay of Naples (Fig. B3).

The beginning of volcanic activity in the Phlegraean area is not precisely

known. Sequences of lava and pyroclasts of approximately 2 million years in

age have been found in bores between Villa Literno and Parete; the oldest

outcropping volcanic products date back around 60,000 years and consists

Appendix B ________________________________________________________________________________________________________________________________________

250

mainly of pyroclastic deposits and parts of lava domes (Fig. B4, B5, B6). The

rocks originated by volcanism older than 39,000 years are exposed only along

the escarpments bordering the Phlegraean Fields, and in a quarry situated on

the north-east side of the Quarto plain, where the pyroclastic deposits, of at

least ten different eruptions, are clearly exposed. Indeed, few of the eruptive

centres of these deposits are still visible today.

The first main eruptive period started 39,000 years ago. Due to the

volcanic activity, the Campanian Ignimbrite originated and buried a large part

of Campania under a thick blanket of tuffs. During the eruption a caldera was

formed which caused the subsidence of a vast area including Phlegraean Fields,

part of the city of Naples and part oh the bays of Naples and Pozzuoli. The

rocks erupted in the period between 39.000 and 15.000 years ago, are exposed

along the rim of the Campanian Ignimbrite caldera, within the city of Naples

and along the north-west and south-west sides of Posillipo hill. The eruptive

centres were situated inside the Campanian Ignimbrite caldera, both in the part

of the caldera currently emerged and the part of caldera at present submerged

in the gulf of Naples.

The second main eruptive period started since 15,000 years from present.

During this eruption, several tens of km3 of magma were emitted from a centre

situated in the Phlegraean Fields, and an area of approximately 1,000 km2 has

been covered by pyroclastic deposits. These deposits have been found in

Neapolitan-Phlegraean area and in the Campanian Plain as far as in the

Appennines. This eruption was accompanied by the formation of a caldera

which caused the subsidence of an area including the Phlegraean Fields and the

bay of Pozzuoli.

After this eruption, quiescence periods alternated with three periods of

intense activity that created numerous volcanic edifices, many of which are still

well preserved and visible in the Phlegraean Fields. For instance, after an period

approximately 3,000 years of quiescence, the most recent eruption occurred in

1538, originating Monte Nuovo caldera.


251

a)

b) c)

d)

Figure B.3 The Phlegraean Fields (Rosi et al., 1999; DeVita et al., 1999)

Appendix B ________________________________________________________________________________________________________________________________________

252

Figure B.4 Diffusion of soils originated by the Phlegraean Fields

(Rosi et al., 1999)

Figure B.5 Stratigraphic conditions of soils originated by the Phlegraean Fields

(DeVita et al., 1999)


253

Figure B.6 Soil originated by the Phlegraean Fields (Rosi et al., 1999)

The Somma-Vesuvius complex, is a medium sized volcano reaching a

height of 1,281 m above sea level at its highest point, located at the convergent

boundary where the African Plate is being subducted beneath the Eurasian

Plate (Fig. B7). It comprises the older volcano, the Somma, whose summit

collapsed, creating a caldera, and the younger volcano, Vesuvius, which rises

out inside of this caldera. According to geophysical investigations (Auger et al.,

2001) and the petrological data (Marianelli et al., 1999; Fulignati et al., 2000),

the magmatic system of Somma-Vesuvius is characterised by a serbatoio

profondo at a depth of about 10-20 km, where different mgmas are present. Its

lava is composed of viscous andesite (a dark gray volcanic rock). From this

reservoir, magma go up to a more superficial magmatic chamber at depths of 3-

5 km before plinian eruptions while 2 km deep before strombolianian activity.

In the magmatic chamber, magma deriving from different eruptions can mix

each other.

ancient Somma

volcano

Figure B.7 The Somma-Vesuvius complex

Appendix B ________________________________________________________________________________________________________________________________________

254

Monte Somma is 1149m high, separated from the main cone by the

valley of Atrio di Cavallo, which is some 5 km long (Fig. B8). The slopes of the

mountain are heavily scarred by lava flows but are heavily vegetated, with scrub

at higher altitudes and vineyards lower down.

Volcanic activity in the Somma-Vesuvius area goes back at least 400,000

years, as shown by the age of lavas sampled at a depth of 1,345 m. The history

of Somma-Vesuvius volcanic edifice began almost 25,000 years ago, with the

growth of Mount Somma due to predominantly effusive eruptions, and

subordinately low energy explosive eruptions. This activity lasted up until about

19,000 years ago and resulted in the formation of the volcanic edifice of Mt.

Somma; its estimated profile is shown in red in the image below (based on

Cioni et al., 1999). The northern part is partially preserved and forms Mt.

Somma. The first plinian eruption of Pomici di Base (18,300 years ago) marked

the begin of severe change in the shape of the Somma volcanic edifice, with the

formation of a caldera due to the collapse of its summit. After this event,

volcanic activity and later phases of collapse contributed to the formation of

the younger volcano, Vesuvius. The activity of this volcano, which had grown

inside the caldera of Somma, has been characterised by great variability both in

styles of eruptions and in chemical composition of the magmas emitted.

a) b)

Figure B.8 The Mount Somma: a) inner view, b) outside view

Mount Vesuvius was regarded by the Greeks and Romans as being sacred

to the hero and demigod Hercules, and the town of Herculaneum, built at its

base, was named after him (Fig. B9). The mountain itself is also named after

Hercules, albeit in a less direct manner. He was the son of the god Zeus and

Alcmene of Thebes. Zeus was also known as Ves (Υης) in his aspect as the god

of rains and dews. Hercules was thus alternatively known as Vesouuios

(Υησουυιος), the son of Ves. This name was corrupted into "Vesuvius."


255

According to other sources, Vesuvius came from the Oscan word fesf which

means "smoke."

The eruptive history of Vesuvius volcano was characterised by a wide

variety of eruptive behaviours that can generally be attributed to the alternating

of periods of open-conduit activity and longer periods of quiescence with

closed-conduit, followed by major Plinian or Subplinian eruptions (Fig. B9).

The open conduit-periods are characterised by persistent strombolian activity,

frequent lava effusions and sporadic, but even more dangerous, both effusive

and explosive eruptions. On occasion, the eruptions were so large that the

whole of southern Europe has been blanketed by ashes, as in 472 and 1631

when Vesuvian ashes fell on Constantinople (now known as Istanbul, over

1,000 miles away.

Going further into details, from 18,000 to 16,000 years ago, two great

plinian eruptions occurred: the eruption of Pomici di Base (18,300 years ago)

and the eruption of Pomici Verdoline (16,000 years ago). Lavas produced by

small effusive eruptions has been found between the deposits from these two

eruptions. As a consequence of the first Plinian eruption, the collapse of the

volcanic edifice of Somma began, with the formation of a caldera inside which

the new volcano Vesuvius then grew.

From 8,000 years to 79 A.D, three plinian eruptions (namely, the eruption

of Pomici di Mercato - 8,000 years ago -, the eruption of Pomici di Avellino -

3,800 years ago -, and the Pompei eruption - 79 A.D. -) alternated with at least

six subplinian eruptions, dated between the Pomici di Avellino and Pompei

eruptions, and preceded by long periods of inactivity.

From 79 to 1631 A.D, volcanic activity included at least two subplinian

eruptions: the Pollena eruption (472 A.D.) and the 1631 eruption. In addition

to these ones, several small, low-energy effusive and explosive eruptions of

medieval age caused lava flows on the southern and western flanks of the

volcano, and strombolian scoria deposits.

After the 1631 eruption, until 1944, on Vesuvius there was predominantly

open-conduit activity. In this period 18 strombolian cycles can be distinguished,

separated by brief periods of quiescence, lasted less than 7 years and

terminating in violent eruptions. Within each cycle frequent, predominantly

Appendix B ________________________________________________________________________________________________________________________________________

256

effusive eruptions took place. The 1906 eruption was the most violent one in

the 20th century while the most recent occurred in 1944 as both effusive and

explosive event and it marked the volcano's transition to a state of closed-

conduit activity.

>18000 years ago 18000 years ago 8000 years ago

3400 years ago 79 A.D. today

Figure B.9 Historical evolution of Vesuvius volcano due to its explosive activity

(Cioni et al., 1999)

APPENDIX C

MAY 1998 FLOW-LIKE MASS MOVEMENTS

This Section concerns the main features of the flow-like mass movements

occurred on 4-5 May 1998 inside the study area. In particular, based on the

activities developed just after the events (Cascini, 2004) and referring to the

subsequent studies (Cascini et al., 2006), some aspects briefly discussed in

Chapters 5-6 are here further addressed.

The critical rainfall of 4-5 May 1998 was recorded by several rain-gauges

located in the neighbouring area of Pizzo d’Alvano massif. On the basis of

these information, O.U.2.38 (1998a) and SIMN (1998a,b) performed

hydrological analyses highlighting that rainfall began on May 4, 1998 at 2.00

a.m. and continued until 8.00 a.m., with moderate intensity and cumulated

rainfall values around 15 - 20 mm in the plain at the base of Pizzo d’Alvano

massif. Inside the study area, cumulated rainfall values lower than 150 mm over

a 48-hour period were recorded with three principal rain centres located all

around the Pizzo d’Alvano massif (Fig. C1, C2).

Statistical analyses were also performed by O.U.2.38 (1998a) and SIMN

(1998a, b), referring to cumulated rainfall over different time periods (1 hour, 1

day, 2-240 days), in order to evaluate the return period of this storm. The

obtained results highlighted that, over an yearly scale, the storm was not

unusual with a return period lower than 30 years and cumulated rainfall over

different periods were lower than values so far recorded. On the contrary, with

reference to spring, different insights were outlined: a) rainfall recorded from

May 4 to May 6, 1998 was almost twice the average during May and exceeded

the highest ever recorded in 2-3 days during May, with a return period of about

100 years; b) consecutive 9 rainy days between March and August were

recorded only in two cases, in the period 1917 – 1998, however with values

lower than 158,4 mm (recorded between April 28 and May 6; c) about 60% of

the previous rainfall amount was concentrated in the last two days.

Appendix C ________________________________________________________________________________________________________________________________________

258

Figure C.1 The 4-5 May 1998 meteoric event (data from Meteosat)

As a matter of fact, the May 1998 storm was not an extreme event, on a

yearly scale, while due to occurrence of independent phenomena (rainfall in

previous months, 9 consecutive rainy days, relevant rainfall during the last two

days) it can be surely considered an extraordinary event for spring.

However, it must be stressed that the above rainfall values were measured

inside zones with altitudes lower than 300 m a.s.l., while the source areas of the

occurred flow-like mass movements were mainly located between 700 - 1100 m

a.s.l.. At this regard, some analyses performed by O.U.2.38 (1998a) and SIMN

(1998a, b) clearly showed that rainfall was heavier at higher altitudes but with

values lower than twice those recorded in the plain (SIMN, 1998a; U.O.2.38,

1998a). These insights were also confirmed by further analyses regarding

rainfall recorded at the toe and at the top of Pizzo d’Alvano massif, that

showed negligible differences on the cumulated rainfall over periods longer

than one month while possible relevant differences for each rainstorm up to

twice those recorded at the toe rain gauges.

The May 1998 event ________________________________________________________________________________________________________________________________________

259

Figure C.2 Measured rainfall values at different rain gauges

(Cascini et al., in press)

Due to the 4-5 May 1998 rainfall storm, over an area of almost 60 km2,

along the hillslopes of Pizzo d’Alvano massif, most basins (36 over a total of 47

basins) were affected by huge flow-like mass movements (Fig. C3, C4). These

phenomena essentially occurred at the upper portions of the basins and the

initial volume of unstable masses greatly increased as they travelled downslope,

mainly due to the erosion of the soil in the gullies below and, in some cases,

because of minor slides mobilised along the flanks of the gullies.

Appendix C ________________________________________________________________________________________________________________________________________

260

%[

#Y

%[

%[

#Y

#

#

$

$

$Episcopio

Sarno

Quindici

Siano

Pizzo d'Alvano

B41

B40

B39

B38B37

B36

B35

B4

B5 B6

B7

B3

B2

B1B1bis

B8

B9

B10B11

B12

B14

B13

B14bis

B15

B16

B17

B18 B19 B20

B21

B22 B23

B24B25

B26

B34B27

B28

B29

B33

B31

B32

B42

B43

B44

B45

Lavorate

2

1

S

N

EW

600 0 600 1200 1800 m

Figure C.3 Pizzo d’Alvano massif: basin involved (1) and not involved (2)

into the May 1998 events (Cascini et al., 2004)

As a whole, the occurred flow-like mass movements mobilised a 3,000,000

m3 soil volume, as reported by Cascini (2004) (Fig. C5). In particular, just after

the events, on the basis of a landslide inventory map and a cover thickness

map, at 1:25,000 scale, mobilised volumes were evaluated, multiplying the

involved soil thicknesses with the areas of source, transport and deposition

areas (tab. X). Later, based on detailed topographical surveys (Fig. C6), more

accurate evaluations were performed that substantially confirmed the previous

simplified estimations, with relative differences lower than 30-40%.

The May 1998 event ________________________________________________________________________________________________________________________________________

261

a)

b) c)

Figure C.4 Examples of flow-like mass movements

Basin Volumes (m3)

- aerial photos - Volumes (m3)

- topographical surveys - Differences

(%)

B3 121367 78820 +35

B4-B5 135492 91454 +32

B9 158370 187636 -16

Figure C.5 Different estimation of total mobilised volumes

h < 0.5m

h < 0.5 – 1.0m

h < 1.0 – 2.0m

h < 2.0 – 3.0m

h < 0.5m

h < 0.5 – 1.0m

h < 1.0 – 2.0m

Figure C.6 Examples of detailed evaluation of mobilised volumes

Appendix C ________________________________________________________________________________________________________________________________________

262

With reference to the source areas, it must be stressed that several

landforms were recognised strictly related to different triggering mechanisms, as

discussed in Sect. 6. On the other hand, transport zones were characterised by

several phenomena such as flows of triggered volumes, erosion of still in places

soils, subsequent minor landslides at the flanks of gullies or channels.

In-situ surveys and detailed analysis of buildings failure mechanisms,

allowed Faella and Nigro (2003b) to estimate the unstable mass velocities, that

were found up to 20 m/s (Fig. C7). In particular, despite the obvious

approximations and the low correlation of the regressions certainly due to local

factors (e.g. topographic conditions, presence of channels or ditches), it seems

that unstable masses reached velocities up to 20 m/s at the toe of the gullies

with a later decrease by around 1 m/s for every 100 m of run with 8% average

ground surface inclinations. For these phenomena, Revellino et al. (2004)

estimated unstable masses velocities, by measuring superelevation of the debris

surface in channel bends and they found values in agreement with those

proposed by Faella and Nigro (2003).

Figure C.7 Estimation of unstable masses velocity (Faella & Nigro, 2003)

The May 1998 event ________________________________________________________________________________________________________________________________________

263

Different run-out distances resulted from the above factors, raning from

few meters up to more than 2 km. At this regard, it must be noted that, in some

cases, over different situations.

Unfortunately, these flow-like mass movements caused 159 casualties in

the four threatened towns (Bracigliano, Quindici, Sarno, Siano) located at the

toe of Pizzo d’Alvano massif (Fig. C8). Most of victims were caused by the

events occurred in the Sarno zone, also some hours later the occurrence of the

first landslides. In the other towns, less victims were recorded because fewer

structures were exposed at flows impacts and also due to a rapid escape of local

inhabitants.

The most frequent damage typology was the impact of the materials

transported by the flowslides on walls perpendicular to the flow direction,

resulting in many masonry structures becoming seriously damaged. On the

contrary, structures with reinforced concrete frameworks were not seriously

weakened by damage to their brick outside walls (Fig. C9) and showed

problems only if they had been directly impacted by flowslides. As regards

indirectly hit structures, situated inside the expansion and deposition areas of

flowslide materials, basements and ground floors were often filled with

pyroclastic soils; this only rarely happened to upper floors where, instead,

frames and some partition walls were damaged.

Town Inhabitants Victims Buildings destroyed

Buildings heavily damaged

Buildings slightly damaged

Bracigliano 5105 6 2 7 2

Qundici 3023 11 19 154 46

Sarno 31509 137 126 195 66

Siano 9265 5 5 34 10

159 152 390 124

Figure C.8 Victims caused by flow-like mass movements







Appendix C ________________________________________________________________________________________________________________________________________

264

a)

b)

c)

Figure C.9 Examples of damages induced by flow-like mass movements

in (a) Sarno, (b) Bracigliano, (c) Siano.

APPENDIX D

USED GEOMECHANICAL METHODS

In this appendix, some aspects were addressed about the geomechanical

methods utilised in Sect. 7 to model the triggering stage of flow-like mass

movements occurred inside the study area (Sect. 5-6). This Section is obviously

not exhaustive on the topic while it must be considered a support to the

analyses and discussions reported in the thesis. In the following, the used

theoretical frameworks are briefly addressed while numerical issues are not

deepened.

Seepage analyses were performed by means of the code SEEP/W

(Geoslope, 2005) and they are essentially based on a saturated-unsaturated

approach that combine the Darcy’s law (Eq. 1) with the balance of water flow

(Eq. 2), assuming that total stresses are constant and that pore-air pressure

remains equal to atmospheric values during transient processes. As a

consequence, volumetric water content is only dependent by pore water

pressure (Eq. 3) and the following equation can be stated (Eq. 4).

(1)

(2)

(3)

(4)

where q is water inflow, H represents the total head, i represents the total head gradient and k is

the soil permeability.

Appendix D ________________________________________________________________________________________________________________________________________

266

In this equation a relevant role is played by the soil water characteristic

curve q(pw), permeability curve k (pw) and hydraulic boundary conditions Q

that are related to direct rainfall, bedrock outlets, concentrate flow rates at the

ground surface.

Limit equilibrium analyses were performed by means of the

SLOPE/W code (Geoslope, 2005) referring to the methods proposed by Janbu

(1954) and Morgenstern & Price (1965), that together with the other methods

of slices can be included in the General Limit Equilibrium Method (Fredlund &

Rahardjo, 1995).

In these analyses, a rigid-perfectly plastic behaviour was assumed and soil

shear strength and mobilized shear sm (Eq. 5) were related to soil suction

through the extended Mohr-Coulomb criterion (Fredlund, 1978). Moreover, for

any potentially unstable mass, the factor of safety F is defined as that factor by

which soil shear strength must be divided in order to bring this mass into a

state of limiting equilibrium along a specified slip surface. Failure conditions are

attained when the safety factor F reaches value lower than one.

In order to compute the factor of safety, the analysed soil mass is divided

into vertical slices and the following static equilibrium equations are solved: a)

from the summation of forces along the vertical direction, for each slice, the

normal force at the base of slice N is computed (Eq. 6); b) from the summation

of forces along the horizontal direction, for each slice, the interslice normal

force E is computed (Eq. 7); c) from the moment equilibrium equation, for all

the slice around a common point, the factor of safety FM can be computed; d)

from the equilibrium equation along the horizontal direction, for all the slices,

the factor of safety FF can be computed.

Based on these equations, the factor of safety can be computed according

the most commonly used methods of slices that differ for satisfied static

equilibrium equations and assumptions regarding the interslice forces.

(5)

Used geomechanical methods ________________________________________________________________________________________________________________________________________

267

(6)

(7)

(8)

(9)

where is the base length of each slice, is the angle between the tangent at the base of each

slice and the horizontal, X represents the vertical interslice shear force, E is the horizontal

interslice normal force, D represents an external load, N is the normal force at the base of each

slice, Sm is the mobilised shear force at the base of each slice and R is the moment arm associated

with the mobilised shear force Sm.

Uncoupled stress-strain analyses were performed by means of the

SIGMA/W code (Geoslope, 2005). The used mathematical approach provides

the incremental displacements {a} computed from the incremental applied

loads {F} (Eq. 10). In particular, nodal displacements {u, v} and strains {} are

related in the Eq. 11 through the strain matrix (B). Moreover, strains can be

related to stresses {}in the Eq. 12 by means of different constitutive equation.

(10)

(11)

(12)

where K is the stiffness matrix, a is the displacement, Fb is the body forces, Fs is the pressure

applied on surface boundary, Fn is the nodal forces, B is the strain-displacement matrix and C is

the constitutive matrix.

Appendix D ________________________________________________________________________________________________________________________________________

268

In the performed analyses, an elastic-perfectly plastic soil behaviour was

referred adopting the above strength parameters and deformability parameters

pointed out on the basis of the available data set. For these analyses, failure

scenarios are outlined referring to the development of displacements and plastic

strains. In particular, the non-convergence of the algorithm used to detect

stress and displacement fields capable to satisfy the Eq. 10-12 is referred as

representative of failure conditions.

Within this uncoupled approach, both drained and undrained conditions

were assumed for the performed analyses

In particular, for drained conditions, pore water pressure were evaluated

using the SEEP/W code, for both saturated and unsaturated soil conditions,

and then used as input data to compute stress and strain referring to the

effective stresses field.

For undrained conditions, pore water pressure generated due to a change

in total stress was estimated by the Eq.13 (Henkel, 1960) where and are

empirical pore water pressure parameters related to the Skempton’s A and B

pore pressure parameters (Eq. 14). This evaluation is based on the assumption

of nil volumetric change that is adequate only for saturated conditions. On the

contrary, for unsaturated conditions, if still used, this expression certainly

provides an overestimation of the induce pore water pressures.

u=dq + dp (13)

(14)

Coupled stress-strain analyses were carried out by GeHoMADRID

code (Pastor et al., 1999), based on the mathematical approach proposed by

Pastor et al. (1999).

Within such a mathematical model, for any infinitesimal soil element, the

soil volumetric variation v is related to the variations of its phases and net fluid

flow rate through it (Eq. 15). Moreover, the balance of linear momentum for

the fluid phase (Eq. 16) and for the mixture (Eq. 17) are stated, eventually taken

Used geomechanical methods ________________________________________________________________________________________________________________________________________

269

into account soil unsaturated conditions. At this regard, it must be stressed that

air pore pressure is assumed constant and equal to the atmospheric value while

both permeability and volumetric water content are considered dependent by

the pore water pressure.

(15)

(16)

(17)

where v is the volumetric strain, Sw is the degree of saturation, is the relative displacement of

the fluid phase, Cs is specific storage coefficient, Q* is a water flux, b is the body forces, kw is the

soil permeability, u is the nodal displacement, n is the porosity

As stated by Zienkiewicz (1980), in several case, relative accelerations

between fluid and solid phases can be disregarded and from the Eq. 16, it can

obtained the following Eq. 18.

These last combined with the previous equations allowed to eliminate the

relative displacements of the fluid phase , obtaining Eq. 19. The main

advantage of this formulation surely consist in the possibility to use only two

field variables, respectively, displacement u and pore water pressure pw.

(18)

(19)

(20)

(21)

Appendix D ________________________________________________________________________________________________________________________________________

270

To simulate the triggering stage of flow-like mass movements, in Sect. 7,

both uncoupled, hydro-mechanical coupled and uncoupled dynamic analyses

were performed.

For uncoupled analyses, pore water values were computed assuming nil

nodal displacements.

Coupled analyses were performed assuming as field variables both the

nodal displacements and pore water pressure values.

Finally uncoupled dynamic analyses were performed, keeping constant

pore water pressures, while taking into account the nodal accelerations in Eq.

18, essentially related to the imposed nodal boundary conditions.

UNIVERSITÀ DEGLI STUDIDI SALERNO

Dipartimento diIngegneria Civile

CORSO DIDOTTORATO DI RICERCA IN

Ingegneria Civile perl’Ambiente ed il Territorio

Correlatori_

Prof. ing. Manuel PASTORProf. Ing. Giuseppe SORBINO

_

GEO

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DELLIN

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Sab

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Relatore_

Prof. ing. Leonardo CASCINI

UNIVERSITÀ DEGLI STUDI DI SALERNOVia Ponte don Melillo84084 Fisciano (SA)Tel. 089 96 4029 Fax 96 4343www.unisa.it

Ciclo IV Nuova Serie - Coordinatore: prof. Ing. Rodolfo M.A. NAPOLI

GEOMECHANICAL MODELLING OF TRIGGERING MECHANISMS

FOR FLOW-LIKE MASS MOVEMENTS IN PYROCLASTIC SOILS

Sabatino Cuomo

_

Tesi di Dottorato

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