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Temporal prediction in landslides – understanding the Saito effect

David PetleyDurham University, United Kingdom

Derek PetleyWarwick University, United Kingdom

Robert AllisonSussex University, United Kingdom

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Worldwide fatal landslides - 2007

Large fatal landslides occur mainly in less developed countriesPrediction may be a key tool to reduce their impact

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The Saito Effect

• During final failure, some slopes show a hyperbolic increase in displacement rate with time

• Manifested by a linear trend in a plot of inverse displacement rate against time

• This is the Saito effect • Can be used to predict the

time of failure if (and only if) we understand the when and why it applies

Time

Dis

pla

cem

ent

ra

te

Time

Dis

pla

cem

ent

ra

te-1

Failure

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Theory one: state and rate dependent frictionFriction inversely related to strain rate

What causes the Saito effect?

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Theory two: crack growth processesBehaviour related to growth of shear surface through subcritical crack

growth

What causes the Saito effect?

Diagram from: Scavia, C. 1995. A method for the study of crack propagation in rock structures. Géotechnique, 45, 447-463.

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Movement analysis in the laboratory

Aim: to simulate movement styles with increasing pore pressures using stress path testing

Apply constant deviator Apply constant deviator stress stress

Increase pore pressureIncrease pore pressure

Allow development of strainAllow development of strain

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Black Ven landslide, United KingdomBlack Ven landslide, United Kingdom

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Conventional drained testing of undisturbed Foxmould

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“Pore pressure reinflation” tests

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p’ = 100 kPa PPR test

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Remoulded and undisturbed samples have different form of strain accumulation

Remoulded Undisturbed

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Different behaviour using a Saito plot

Remoulded Undisturbed

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P’ = 1000 kPa Saito plot

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Relationship with peak strength envelope

Peak strength envelope

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• Saito linearity is a brittle phenomenon– Only occurs in materials in which rupture surface formation is

occurring

• Non-brittle materials do not show this behaviour - therefore it is not possible to use Saito linearity to predict failure in non-brittle materials

• Linearity results from stress concentrations in final stages of failure

• Laboratory testing allows likely behaviour to be understood

Conclusion