08 06 Petley Round Table

http: daveslandslideblog.blogspot.comhttp: daveslandslideblog.blogspot.com

Strength criteria and design approaches for difficult soils and rocks

Slow moving rock slides

David PetleyDurham University, United Kingdom

Fatal landslide distribution 2005Fatal landslide distribution 2005

Cumulative landslide fatalities since Sept 2002Cumulative landslide fatalities since Sept 2002

2003 2004 2005 2006 2007 20082003 2004 2005 2006 2007 2008

Flims, SwitzerlandFlims, Switzerland

• When and why do large, creeping rockslides transition into sudden failure?

• What geotechnical and geometrical parameters do we need to collect to allow behaviour to be forecast?

• Does a standard factor of safety approach apply in this case?

• If not, what does?

Key issuesKey issues

• Seismic events

• Intense rainstorms

• Debuttressing

• Nothing?

What triggers sudden large rock slope failure?What triggers sudden large rock slope failure?

Three key states:-1.Movement on fully formed shear surface(s)

at residual strength• Rate of movement determined by stress

2.Movement associated with the ongoing formation of critical shear surface (progressive failure)• Rate of movement determined by stress

state and state of shear surface development

3.(Rheological) creep• Rate of movement determined by stress

The mechanical state of slow moving rockslidesThe mechanical state of slow moving rockslides

Two key issues:-• Type of movement• Geometry

Most difficult issue is differentiating between creep and progressive failure

Potential for transition to catastrophic collapse

Landslide managementLandslide management

Bjerrum (1967) model (ASCE) of progressive Bjerrum (1967) model (ASCE) of progressive failurefailure

• Based upon propagation of the shear surface through the slope

• Locally, the shear stress exceeds shear strength, which allows crack growth

• Slope progressively weakens to failure

Peak strengthPeak strength

Residual strengthResidual strength

Rupture surface Rupture surface growthgrowth

Time to failure is inversely related to slope Time to failure is inversely related to slope angle in progressively failing rock slopesangle in progressively failing rock slopes

Scatter Scatter due to due to lithology lithology and size?and size?

After Kodera After Kodera et al.et al. (2005) with additional data (2005) with additional data

Rockfall activity also increases prior to collapseRockfall activity also increases prior to collapseRosser Rosser et al.et al. 2007: 2007: Journal of Geophysical Research Journal of Geophysical Research and this conferenceand this conference

Volume of final failure (mVolume of final failure (m33))

0 20 40 60 80

Time (days)

Tertiary phase of progressive failureTertiary phase of progressive failure

Peak strengthPeak strength

Can occur if:• Global shear stress >

residual strength• Local shear stress >

peak strength

Time depends on size and stress state

Is safe design approach to use residual strength?

Progressive failureProgressive failure

An indication that progressive failure could An indication that progressive failure could occuroccur

Main, I.G. 2000. A damage mechanics model for power-law creep and earthquake aftershock Main, I.G. 2000. A damage mechanics model for power-law creep and earthquake aftershock and foreshock sequences. and foreshock sequences. Geophys. J. IntGeophys. J. Int. 142, 151–161. 142, 151–161

0 20 40 60 80Time (days)

Primarycreep (?)

Secondarycreep

Tertiarycreep

Landslide movement record: New Tredegar, Landslide movement record: New Tredegar, 19301930

Data from Jon Carey, Durham University and Halcrow LtdData from Jon Carey, Durham University and Halcrow Ltd

200 250 300 350 400 450Time (min)

BPS testing of BPS testing of Tertiary slate from JapanTertiary slate from Japan

BPS resultBPS result

250 300 350 400 450

Time (mins)

Displacement dataVolume change data

DilationDilation

• State of development of the sliding surface

• Properties of the materials• State of stress

– Global factor of safety – Local stress at crack tip

• Geometry

Presentation available for download Presentation available for download at:at:

http: http: daveslandslideblog.blogspot.comdaveslandslideblog.blogspot.com

Key issues highlightedKey issues highlighted

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