Pre-failure strains as precursors of sliding in a coastal mudslide

A large coastal mudslide at West Down Beacon in East Devon surges occasionally with most of its displacement resulting from basal sliding. Frequent monitoring of surface markers on the mudslide between surges has indicated that, in the months before large surge displacements, there are small displacements caused by shear deformation. These pre-failure strains may be useful precursors of the main movements. The inter-surge conditions of the soil matrix of the mudslide have been recreated in a ring-shear apparatus specially adapted to be stress- rather than strain-controlled. The test results demonstrate the manner in which considerable shear distortions can occur within the mudslide during the early phases of a displacement surge, before the reactivation of failure at residual strength in the basal shear zone.