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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)
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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)
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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)
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
Long ridges
DSGSD occurrence: topographic settings
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
DSGSD occurrence: topographic settings
Concave or laterally-confined slopes
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
DSGSD occurrence: topographic settings
Slopes bounded by tributary valleys
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
DSGSD occurrence: topographic settings
Convex slopes
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
DSGSD occurrence: topographic settings
Slopes crossing multiple catchments
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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)
Agliardi et al., 2012
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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) -
Gravitational- frequency (N= 697) -
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
field mapping – kinematic interpretation
trenching (stratigraphy, sedimentology, structural
analysis, dating)
Agliardi et al. (2009)
Morpho-structures: characterization
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
1686
1758
Evidence of basal shear zones (Desio, 1962)
Pian Palù reservoir (Peio, TN) Beauregard reservoir (Valgrisanche, AO)
Evidence from deep site investigation data
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Agliardi et al., 2012
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 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)
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Agliardi et al., 2012
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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)
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Agliardi et al. (2009)
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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)
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Agliardi et al (2009)
Role of active tectonics (3)
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Agliardi et al (2011)
Deglaciation: debuttressing, tensile damage, joint opening, groundwater regime
DGPV triggering: Finite Difference model
no interfaceinterface
Counterscarps
Shear bands
Scarp
Agliardi et al. (2012)
Role of glaciation / deglaciation
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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)
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Agliardi et al., 2009
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
Tectonic- frequency (N=3574) -
Tectonic- length (N=3574) -
Gravitational- frequency (N= 697) -
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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
Alpine rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012
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)