Landsliding in London Clay coastal cliffs

Factors controlling rotational landsliding in the unprotected London Clay coastal cliffs at Warden Point, Isle of Sheppey, Kent, are described. Surveys of the landslides have been carried out at regular intervals since 1971, and these are supported by an extensive subsurface investigation that included the installation of 56 piezometers. These studies have resulted in an understanding of the pore water pressures in the cliffs, including the role of undrained unloading resulting from slope formation, and pore pressures within the landslide mass 'carried down' the slope. The zone of depressed pore pressures is carried inland with the slope as the coastline retreats. Factors controlling the shear strength mobilized during first-time slides, and rates of movement, include geological control on the position of the slip surface and reduction in shear strength to close to the residual value along the basal section of the slip surface. This strength reduction results from stress relief controlled pre-failure deformations. A simple model is proposed to allow calculation of pore water pressures associated with the degrading slide mass. These have enabled back-analyses to be carried out which extend the residual shear strength envelope for London Clay to high effective normal stress levels, and explain the rate of re-activated slide movements.