Geological Sciences and Geotechnologies UNIMIB - … rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012 Federico Agliardi Geological Sciences

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

Landslides:definitions, classification, causes


Landslide

Mass movement of rock, soil, or debris material forming a slope (natural or engineered) towards the lower and external part of the slope, along a defined sliding surface.

Important distinction !

Mass movement: occurs under the effectof gravity

Mass transport: material is transported byan agent (e.g. water flowing in a river, wind)

(debris flows can sometimes by atransitional term….)

Definition of landslide


Landslide classification

Several classifications available depending on the criteria

(kinematics, material, fluid content, geotechnical properties of clays, triggering processes, style of activity)

(Varnes, 1978; WP/WLI, 1993-94)

Cruden and Varnes, 1996

material + movement type

additional: velocity

Materials: rock, debris, earth (soil)

Type of movement: falls, topples, slides, spread, flows

Complex: 2 or more associated types


1. Crown 11. Toe of failure surface

2. Main scarp 12. Surface of separation

3. Top 13. Displaced mass

4. Head 14. Zone of depletion

5. Secondary scarp 15. Zone of accumulation

6. Main body 16. Depletion

7. Foot 17. Depleted mass

8. Tip 18. Accumulation

9. Toe 19. Flank

10. Failure surface 20. Pre-failure topography

1

109 8

54

2

1112

14

15

19

20

VC

L

HC

7

3

H

16

1718

20

Landslide nomenclature


Cu

mu

lati

ve d

isp

lace

men

t

active

dormantsuspended

reactivated

suspended

2 3 4 time (years)

relict

Varnes (1978) and WP/WLI (1993, 1994) :.

- Active: moving (monitoring….)

- Suspended: moving during the last seasonal cycle, presently inactive

- Reactivated: active after a period of inactivity

- Dormant: inactive since more than one seasonal cycle, re-activation possible

- Abandoned: inactive, no more influenced by original triggers

- Stabilized: inactive after artificial protective measures

- Relict: “paleo-landslide”, inactive, occurred in extinct morpho-climatic settings

State of activity (in time)


1

2

3

4

6

5

7

Cruden e Varnes (1993):

1 – Progressive: failure surface propagates

downslope

2 – Retrogressive: failure surface propagates

upslope

3 – Confined: failure surface does not daylight

at the foot (or it is masked)

4 – Moving: landslide body moves without

changes of failure surface

5 – Widening: failure surface propagates

towards landslide flanks

Distribution of activity (in space)


In general terms, a slope fails when shear stress along a potential failure surface equals the available

shear resistance.

In Mohr-Coulomb terms, a «factor of safety» can be defined: :

Then, instability (FS <= 1) can occur in response to:

1) processes increasing mobilized shear stresses;

2) processes decreasing the shear strength of materials

3) combinations of the two types (common)

a) Stress variations for: loading (1), excavation / erosion (2), increase of pore pressure u (3)

b) Strength reduction (c’ e ’): weathering, softening, etc.

mobmob

f ucFS

'tan)('

Causes and controls of landslides


Falls (usually rockfalls)

failure and detachment of rock fragments (i.e.single block, several individual blocks, rockmass volumes) followed by downward motionby free falling, bouncing, rolling, and sliding,without impulsive block-slope interaction.

Volume < 100.000 m3 : limited block-blockinteraction “fragmental rockfall”

Conditioning factors

Onset: presence of rocky cliffs, rock fracturing, favourable rock type (hard, fractured rock), water circulation

Runout: slope macro-topography and roughness, block volume and shape, vegetation

Types: falls


Topples

Rotation of one or more rock blocks or slabsaround a hinge point (line) due to gravity, fluidoverpressure or seismic shaking


Onset: Anisotropic rock mass with sub-verticalbedding or structural pattern

Runout: rockfall

blockflexural

Slope toe(associated to sliding)

Head(associated to sliding

Types: topples


Slides

Shear sliding of a mass of earth or rock, localisedinto single or multiple sliding surfaces, or in narrowshear zones.

Rotational v.s. translational sliding dependingon material anisotropy, stratigraphy, structure

Soils: shallow (soil slips) or deep-seated (slumps)

Rocks:

block slides (planar, wedge) of different size

deep-seated rock slide, usually complex


Slope gradient

Material (soils, weak or fractured rocks, flysch, marls)

slope-scale persistent structures

Rotational

Translational (planar)

Cesana BrianzaPonte Sesa (BS)

Types: slides


Lateral spreading

Seep-seated lateral extension over low-angle shear zones and formation of associated tensile fractures and gravitational normal faults.

Develop in sub-horizontal slabs of stiff materials(rocks, over-consolidated cohesive soils), overlying layers of soft rocks or soils subjected to liquefaction


Soft materials (e.g. clays, argillite) of soilswith high liquefaction potential (e.g. siltysands, fine sands)

Water circulation

Seismicity

Peru

Types: lateral spreads


Slumgullion earthflow, Colorado

Most complex group, including:

flows in both granular material and cohesive soils

− Debris flows (fast)

− Earthflows (slow, fast)

− Rock avalanches (fast)

wet: Newton or non-linear flow (mud flow, flow slide, debris flow)

dry: granular flow (debris / rock avalanches)Alpisella, Valdidentro (SO)

Denali earthquake, Alaska

Cavallerizzo, Calabria

Types: flows


Debris flows

Rapid mass movement of mixtures of granular solids, water and air, in an intermediate position between flooding and landslide processes (Costa, 1984).

Triggered by: soil slips, debris remobilization in steep channels or on talus slopes

High mobility, velocity, and runout potential


Channel or slope steepness

Water availability

Relative abundance granular / cohesive soils

Slope material and erodibility

Valtellina, 1987

Val Tartano, SO, 1987Types: flows


SHEAR RATE u/y

SH

EA

R S

TR

ES

S

Pseudoplastic with strength

Bingham

Dilatant with strength

Pseudoplastic (shear thinning)

Newton

Dilatant (shear thickening)

R

Rc

PLUG

LAMINAR FLOW

u

LAMINAR FLOW

PLUG

T

Tc

x

y

Surges

Snout

Longitudinalsection

Plan view

Cross. sections

Lateral levees

Channel deposits

Lobe

Inverse grading

tail

D

C

B

A

Debris flows: morphological and

sedimentological features

Depend on mixture rheology:

- newtonian fluid

- bingham fluid (with strength)

- dilatant fluid

Rheology =

f (% fine, sediment concentration)

Types: flows


Earth-flows

Slow flow (associated to sliding) of cohesive soils or clayey weak rocks. Triggering modes and mechanics(e.g. flow, flow + sliding) depend on water content and fine fraction


Materials with abundant fine fraction (clay, argillite, marl, flysch, weathered rock)

Water availability / groundwater

Lavina di Roncovetro

Fra

na d

i Cor

nigl

io

Types: flows


Las Colinas, Salvador

Rock avalanches

Rapid granular flows (dry or wet) of large volume of fragmenting rock masses or pyroclastic materials

Triggered by: rockslide collapse on high slopes, earthquakes, permafrost degradation, very large rockfalls (bergsturz)

High mobility, velocity, and runout potential

Larger volume, larger runout (scaling)

Runout, morphology and features of the deposit controlled by topography

Denali earthquake, Alaska

Types: flows


Types: flows

Bualtar Glacier rock avalanche (Hewitt et al., 2008)

Rock avalanches: morphological and

sedimentological features

coarse, poorly sorted debris

fragmented material + boulders

Lobes, transversal and longitudinal

ridges, levees

«bedrock» erosion / deformation


Complex landslides

Varnes (1978): association of two or more types of movement either in space or in time

e.g. compound (mixed rotational-translational) landslides

Cruden e Varnes (1993): association of two or more types of movement in time (evolution stages)

e.s.: rockfall debris flow;rockslide rock avalanche, etc.topple rockfall

Complex landslides

Val Pola rockslide / rock avalanche

Documents

Geological Sciences and Geotechnologies UNIMIB - … rock slope failures: mechanisms, controls, characterization F. Agliardi – Matrei, 19 June 2012 Federico Agliardi Geological Sciences