Rapid landslides: the importance of understanding mechanisms and rupture surface mechanics

The paper describes methods for assessing the likelihood of rapid landsliding based on the mechanism of landsliding, the mechanics of the surface of rupture and internal and lateral shear surfaces, ground and surface water, and the landslide geometry. For translational landslides in rock the relationship between the slope of the basal surface of rupture and the shear strength of that surface, whether the surface of rupture is strain weakening and whether there is lateral or toe buttressing are critical for post-failure velocity. For compound slides in rock the most important factors are high loads on the passive wedge; strain weakening on the surface of rupture, internal shears and lateral margins; and toe buttressing. For landslides in soil, the potential for loss of strength on shearing is critical in determination of post-failure velocity. This may occur in drained shear with the soil losing strength from peak to residual, but also in undrained shear. The former is important for soils with a high clay-size content. The latter is critical for loose silty sands and sands, which are contractive on shearing and experience what can be regarded as static liquefaction.