Geological Sciences and Geotechnologies UNIMIB - University of Milano ... · Geological Sciences and Geotechnologies UNIMIB - University of Milano-Bicocca Milano, Italy Czech Geological

Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012

Federico Agliardi

Geological Sciences and Geotechnologies

UNIMIB - University of Milano-BicoccaMilano, Italy

Czech Geological Survey - Geological Survey of Austria - UNIMIB

Educational Project Geological Field Trip and Workshop

Koefels – Suedtirol - Matrei, 17-20 June 2012

Slow, deep-seatedrock slope deformation

(DSGSD)


Deep-Seated Gravitational Slope Deformations

giant, non-catastrophic, long-lasting landslides

relatively low displacement rates ( tens mm/a) - large cumulative displacements

involve entire high-relief alpine slopes (toe to ridge)

morpho-structural evidence (middle-upper sectors)

toe bulging / enhanced rock fracturing (middle-lower sectors)

secondary, often catastrophic landslides (middle-lower sectors)


Sackung (Zischinsky, 1966; Sagging, Hutchinson, 1988;

Rock flow, Varnes, 1978)

morpho-structural evidence (upper slopes)

toe bulging («talzuschub»)

«creep» in rock mass or along structures

strain localization in a basal shear zone

(«gleitung»)

Hutchinson, 1988

Lateral spread (Varnes, 1978; Hutchinson, 1988)

Lateral movements: rigid slabs over ductile rocks

(liquefaction, softening)

Type A: no strain localization

Type B: spread over a localized shear zoneZaruba, 1964 Type B

Jahn, 1964 Type A

Types of DSGSD


Lithology and stratigraphy s.l.:

rock mass strength

sharp changes in “rheology” s.l.

geometry of weak layers

volume and degree of freedom of potentially

unstable slopes

Typical situations:

Foliated metamorphics or thinly bedded sedimentary

sackung (Alps, Apennines)

Fractured granitoids

sackung (Alpi)

Thickly bedded sequence of rock with strong

rheological contrasts

lateral spread (Apennines)Nemcok (1982)

Litho-stratigraphic control


Cu

mu

lati

ve d

isp

lace

men

t

time

Regressivesystem

Dynamic stability

Progressive system

COLLAPSE

Transitionalsystem

Regressive stage

Progressive stage

Failureinitiation

Broadbent e Zavodni (1982)

progressive vs. regressive evolution

possible, partial catastrophic evolution

Paleo-landslide

Alluvial terrace

Valfurva

Frodolfo river

Ruinon active landslide

Cavallaro valley rock glacier

Cavall

aro va

lley

Saline Ridge

Confinale cirque rock glacier

Saline Peak

Confinale Lake

1400 m a.s.l.

3000 m a.s.l.

Con

final

e va

lley

1 km

1

2

3

4

75

6

Ruinon rockslide(Valfurva, SO)

GB-InSAR displacements(Ellegi-LISALab)

Agliardi et al., 2001

Engineering significance

Sackung


damage to engineering structures (surface and underground)

rock mass strength degradation

M. Piazzo tunnel (LC)(SS36 – FFSS Lecco-Colico) Mt. Varadega (SO)

Ambrosi e Crosta (2006)

Gepatsch dam (Tirol)

Zangerl et al., 2010

Engineering significance (2)


Long ridges

DSGSD occurrence: topographic settings



Concave or laterally-confined slopes



Slopes bounded by tributary valleys



Convex slopes



Slopes crossing multiple catchments


Gravitational morpho-structures

morphological expressions of

gravitational structures

local evidence of global

DSGSD kinematics

individuals vs. associations

counterscarps

counterscarpsscarps

gravitational half-graben

Mt. Watles (Val Venosta)



Gravitational vs. tectonic

Morphological convergence (rock strength, maturity…)

less persistent / more scattered

swarms of multiple features, larger individual offsets

less continuous across ridges / drainage

usually limited to specific slope sectors

associated / fading to catastrophic slope instability

Tectonic: post-glacial fault

scarps

double-crested ridge

counterscarps

Gravitational: re-activation

control (at least passive) of structure on DSGSD

Upper Valtellina

Tectonic- frequency (N= 3574) -

Tectonic- frequency (N= 3574) -

Gravitational- frequency (N= 697) -



field mapping – kinematic interpretation

trenching (stratigraphy, sedimentology, structural

analysis, dating)

Agliardi et al. (2009)

Morpho-structures: characterization


Chigira, 1992 (Slope Tectonics)

shear zones: asymmetric, fabric-dependent

folds: flexural slip, intense fracturing in axial zones

gravitational shear zones

gravitational folds

Structures induced by DSGSD


1686

1758

Evidence of basal shear zones (Desio, 1962)

Pian Palù reservoir (Peio, TN) Beauregard reservoir (Valgrisanche, AO)

Evidence from deep site investigation data


Mechanisms

Dramatic variability – LARGE DATASETS NEEDED !!!

No unique

mechanisms !

Wide range of:

geo- settings

morpho- settings

geometry

kinematics

maturity / activity

Relationships with:

stratigraphy

structural patterns

individual structures



Orogen-scale DSGSD inventory: Alps

“blind” GoogleEarth mapping, single expert

mapped area: 103.000 km2 (I, F, CH, A)

DSGSD: 5472 km2 (about 5%)

individual areas: 0.2 108 km2

different settings / units

904

individual

DSGSD

Not rare

phenomena!

Crosta et al., 2008Agliardi et al., 2012


DSGSD settings

Local relief

> 1000 m

(mean: 1800 m)

Slope

71% DSGSD:

rock uplift rate

> 1 mm/a

LGM surface

elevation:

1700-3000 m

Tectonically active settings heavily overprinted by glaciations

SRTM (90m) kernel radius = 5 km

Kahle et al. (1997) Florineth (1997); Kelly (2004)


Spatial scales of DSGSD

Frequency density

Power-law exponent (-b):

904 DSGSD: 2.48

791 large landslides: 2.49

Joint dataset: 2.49

smooth transition from mapped “large landslide” to DSGSD

definition depends on morpho-structural expression (“typical” DSGSD A > 10 km2))

DSGSD: maximum involvement of geological structure

“Typical”DSGSD

Largelandslides

+DSGSD

rollover6 km2



Ambrosi and Crosta (2006)

Geological structure and DSGSD

Rock type

Structural controls

local scale: fabric, fractures,

master features

regional scale: major structures,

nappe boundaries

(passive, active)


Geological map 1:500.000

Homogeneous dataset over a large area: about 82.500 km2 , 725 DSGSD

Rock-type control

anisotropic, moderately strong rocks

sustain high relief without failing

catastrophically

high DSGSD density: metapelites (13%)

orthogneisses (6.5%), flysch s.l. (4%)

carbonates and granitoids poorly affected

Gra

nit

oid

sM

etab

asit

es

Vo

lcan

ics

Ort

ho

gn

eiss

Met

apel

ite

Fly

sch

s.l.

Car

bo

nat

es


Cima di Mandriole (Trentino, Italian Central Alps) (Agliardi et al., 2012; in press)

Finite Difference modelling: deglaciation

kinematic freedom

occurrence and geometry of

morpho-structures

“barrier” effects

Persistent steep fractures:

Tectonic lineaments

- length (N=135) -

DSGSD morphostructures- length (N=115) -

Slope-scale structural controls: steep fractures


Nappe boundaries, thrusts:

stratigraphy, fault rocks

shear strain localisation

Mt. Watles (Val Venosta, Italian Central-Eastern Alps) (Agliardi et al., 2009)

Finite Difference modelling:deglaciation


Rockslidedeposits

Schlinig Fault

2500

2000

1500

1000

Mt. Watles(2555 m a.s.l.)m a.s.l.

Pfa ffen Lakes

BurgusioAdigeRiver

Glo

renza

Fau lt ?

Pa ragne iss with ort ogneiss and amphibolite layers (Oetzt al Nappe)

Pa ragne iss and phyllites (Sesvenna-Camp o Nappe )

Midd le Triassic carbonate s (S-charl Nappe)

0 1 km

Zerzer Valley

Alluvialdeposi ts

500

?

?

Glacio-fludeposit

NW

Paragneiss/phyllite

Paragneiss

carbonate w/ evaporites

Regional, low-angle fault (Schlinig Fault)

Slope-scale structural controls: low-angle faults


Passo Vallaccia (Vizze Valley, Eastern Alps)

Slope-scale structural controls: folds

(Massironi et al., 2010)

Folded multi-layer:

stress-strain distributions

rock mass strength

degradation

detachment layers


In-situ stress: topographic + tectonic + stress anisotropy

extent / distribution of plastic strain (Savage e Varnes, 1987)

topographic stress amplification (Miller e Dunne, 1996)

1

35°

K = 0.3 (pure gravity, extensional regime)

K = 1.0 (compression, strike-slip)

K = 2.0 (compression)

Brown and Hoek (1978)McGarr and Gay (1978)

K = h/v

Ambrosi and Crosta (2011)

Role of in situ stress


In areas of moderate magnitude seismicity (e.g. Apennines)

seismic ground shaking (Radbruch-Hall, 1978; Dramis, 1983; Sorriso-Valvo, 1988)

co-seismic deformation observed by satellite Radar interferometry (DInSAR) for

earthquakes with M > 6 (e.g. Umbria 1997; L’Aquila, 2009)

mainly re-activation of existing DSGSD

Chini et al (2009)

Role of active tectonics


In areas with low magnitude seismicity (e.g. Alps)

seismic ground shaking effects: difficult to demonstrate in alpine areas (M < 5-5.5;

rare permament ground deformations, rare sediments recording co-seismic effects,

e.g. liquefaction; Keefer, 1996)

Central Alps: mechanical consistency between the kinematics of faults re-activated

by DSGSD, EQ moment tensor solutions, and regional stress regime)

DSGSD clustered in areas with

inferred active faults

Possible mechanisms:

contribution to in-situ stress

rock damage and rock mass

strength degradation

Agliardi et al (2009)

Role of active tectonics (2)


Area ~ 20.600 km2 - 354 DGPV (1967 km2)

Rock uplift dataset: NRP20 (Kähle et al., 1997) - Swiss Alps and surroundings

DSGSD: 71% in areas with recent uplift > 1 mm/yr

minimum local relief in DSGSD areas ~ 1000 m (often >1500-2000 m)

tectonically active areas: local relief large enough to induce stress concentrations

large enough to activate DSGSD in moderately strong rock masses

0,1 0,3 0,5 0,7 0,9 1,1 1,3 1,5 1,7

Present day rate of uplift (mm/yr)

0

50

100

150

200

250

300

350

400

Cum

ulative frequency(#)

N = 354Mean = 1,1281StdDv = 0,2125

0,1 0,3 0,5 0,7 0,9 1,1 1,3 1,5 1,7

Present day rate of uplift (mm/yr)

0

50

100

150

200

250

300

350

400

Cum

ulative frequency(#)

N = 354Mean = 1,1281StdDv = 0,2125

Mean local relief (m)

Fre

qu

en

cy (

#)

800 1200 1600 2000 2400 2800 32000

10

20

30

40

50

60Granitoids-MetabasitesMetapelites-ParagneissesOrtogneissesFlysch s.l.VolcanicsCarbonates

N = 728Mean = 1915,7StdDv = 331,3

Recent and present-day uplift

DSGSD frequency vs. uplift Local relief map DSGSD frequency vs. local relief


Role of active tectonics (3)


Harbor, 1995

90 ka

50 ka

100 ka

0 ka

tempo

Evoluzione del profilo

Glaciation: valley deepening / widening, steepening, valley cross-profile

LGM Initial deglaciation End of deglaciation

Tensile damage:Finite Element modelling


Deglaciation: debuttressing, tensile damage, joint opening, groundwater regime

DGPV triggering: Finite Difference model

no interfaceinterface

Counterscarps

Shear bands

Scarp


Role of glaciation / deglaciation


Changes in reservoir water level

reservoir filling stages

seasonal low-stand levels: rapid

drawdown, seepage

Example: Hochmais-Atemkopf (A)

DSGSD partial re-activatio in the

initial reservoir filling stage

later: highest displacement rates

(surface monitoring) correlate with

seasonal reservoir level minima

Zangerl et al (2010)

Role of groundwater changes (2)


Campo-Ortler nappe (Austroalpine)

Ortler sedimentary rocks

(limestones, dolomites)

------ Zebrù Line -------

Campo metamophics

(metapelite, gneiss,gabbro,

metabasites, marble)

complex tectono-metamorphic evolution

isoclinally-folded, transposed foliation + alpine overprint (D3)

major tectonic features: Zebrù Line, overthrusts

recent regional fracturing – complex fault associations

Zebrù line

Regional structural sketch (after GK500, Swisstopo)

Regional scale: Upper Valtellina


Active tectonics

6

Engad

ine Line

Tonale Line

Giu

dica

rie L

ine

Pas

siri

a Fa

ult

Merano-M

auls Line

SWITZERLAND

AUSTRIA

ITALY

R esia Pass

Inn River

MERANO

INNSBRUCK

BOLZANO

46° 40'

10° 30 ' 11° 30'11° 00’10° 00 ’

2

6

8

7

12

13

1114

1516

11

4

9

10

51

47° 20'

3

46° 20'

10

10

5

7

17

8

2

2

1

44

4

Nor

th G

iudi

cari

e B

elt

Adamello

Adige River

47° 00'

6

- seismic record (ECOS)

- MT solutions (ETHZ, Harvard CMT)

- uplift data (Kahle et al., 1997)

- permanent GPS (Tesauro et al., 2005)

active tectonic processes


shallow earthquakes (< 15 km)

maximum recorded Mw: 4.9 (1999, N of Bormio)

uplift rate up to 1.4 mm/yr

high topographic relief (> 1500-2000 m)

N-S compression regime


striated fault planes at 43 location in Central Alps

slickenside geometry and relative chronology

stress inversion by iterative and direct methods (Angelier, 1984, 1990)

Recent faults in the Central Alps

SWITZERLAND

SWITZERLAND

N

1 0km5km01

2a

2b

2c

29

28

714

12-13

53

4

108-911

20-21

17-18

2325

24

15

16

22 19

2627

NW-SE COMPRESSION

PEJO

COGOLO

MART

EL V

ALLE

Y

VENOSTA VALLEY NATURNO

SENALES VALLEY

RESIA PASS

WEISSKOGEL

ROMBO PASS

JAUFEN P .

PE NNE S P.

PA

SSIR

IAV.

AUSTRIA

ITALY

SW

ITZE

RLA

ND

MALLES

1 30' 147

o oo

o2

o46 40 'MERANO

BOR MI O

10 Km

1

2a

2b

3

31

30

5

7

8-9

10

11

12-13

14

17-18

20-21

26

2324

25

2928

15 16

22

19

27

N - S COMPRESSION

4

PEJ O

COGOLO

PREMA DIO

BORM IO MAR

TEL V

ALLE

Y

VENOSTA VALLEY NATURNO

RE SIA PASS

WEISSKOG EL

ROMBO PAS S

JAUF EN PASS

PENNES PASS

PAS

SIR

IA V

AL

LEY

AUSTRIA

SWIT

ZER

LAN

D

MALLES

1 30' 147o

o

o

o2

oMERANO

ITALY

46 40'

(Agliardi et al., 2009)

1) NW-SE compression: E-W dx + N-S / NNE-SSW sx

strike-slip faults + reverse NE-SW

2) N-S compression: NNW-SSE dx + NNE-SSW sx

strike-slip faults + N-S dip-slip

Stage 2 consistent with present day seismicity

Bormio area

s1s3


Lineament mapping

Tectonic lineaments (3574)

faults, master joints

Gravitational features (697)

scarps, counterscarps, trenches

Aerial photos + fieldwork

Criteria:

- morpho-structural evidence

- relationships with topography

- relationships with drainage


Tectonic- frequency (N=3574) -

Tectonic- length (N=3574) -


Gravitational- length (N=697) -

Tectonic vs. gravitational morpho-structures

Statistical analysis of lineament trend

frequency (absolute + length-weighted)

control by recent tectonic features

on gravitational morpho-structures

topographic control

Tectonic lineaments:

- 2 main sets: NW-SE, NE-SW

- consistent with recent faults and

present-day regional stress

DSGSD features:

- clearly re-activate tectonic features

- NW-SE dominant

- larger dispersion


DSGSD: large-scale mapping

Max number ( 5°): 11

Max length (5° ): 151 32

136 DSSGD - LARG E SLIDES

Val Pola rock avalanche accumulation

Active slide source area

Large sl ide (dormant)

Area involved in DSSGD10 km50

Val Viola

Dosso Resaccio

M.SattaronM.Forcellina

Motta Grande

Foscagnopass

Bosco d el Conte

M.Grava

Corno di

P.Bor ron

DossoLe Pone

M.RinalpiS.Colombano

IsolacciaVald identr o

Cime di Plator

M.Trela Montedelle Scale

Dosso ilFiletto

M.Zandila

Val Pola 1997rock-avalanche

Morignone

Bormio

Val

telli

na

M.Vallecetta

Corno SobrettaPunta di S assalb o

Cima Bianca

Valfurva

S. Ca terinaValfur va

M.Confinale

D oss o del Ruinon

M.Pietra Rossa

Val Zebrù

M.Cris tallo

Stelviopass

Bra

ulio

va

lleyM.Solena

M.Slimbrada

M.Fo rcolaP.Limbrail

Ca ncano

Laghi di

Gravitational morpho-structures associated to DSGSD or large landslides

28 DSGSD areas:

- Valfurva NW-SE trend

- Valtellina N-S trend

- Val Viola NE-SW

Over 30 large landslides

- locally active (e.g. Ruinon)

- frequently inside DSGSD

- higher degree of evolution

(clear boundaries, developed

shear sufaces, etc.)

DSGSD along major recent structures (large scale structural control)

DSGSD control on rock slide/avalanche phenomena


Morpho-structural map

The Bosco del Conte DSGSD

Main features:

- metapelites (Bormio phyllite)

- close to a regional overthrust

- poorly-defined boundaries

- NE-SW morpho-structures

lithological and structural controls

- displaced glacial erosion features

post-LGM activation

- SAR evidences present-day activity

Area: 2.6 km2

gneiss

phyllite


DSGSD vs. glacial history

LGM ice surface elevation(after Florineth and Schluchter, 1998, 2000)

Ice

Rock mass

Pre-failure topo

Set-up Deglaciation - debuttressing Strain localisation - Shear band formation

LGM surface elevation > 2500m a.s.l.

deeply incised glacial valleys

numerical models

DSGSD triggering by deglaciation

Possible mechanisms of glacial actions:

- post-glacial rebound

- valley erosion / slope debuttressing

2D continuum modelling of the Bosco del Conte area (FLAC)

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Geological Sciences and Geotechnologies UNIMIB - University of Milano ... · Geological Sciences and Geotechnologies UNIMIB - University of Milano-Bicocca Milano, Italy Czech Geological