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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
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