Erosion and stability of the till cliffs on the Holderness coast

Abstract

The Holderness coast, located in the county of Humberside, is undergoing severe erosion because the cliffs are composed of soil, predominantly clay tills, and are exposed to strong marine attack. The cliffs are subject to several erosion mechanisms and modes of failure, which vary with location on the coast. A general mapping survey of the coast and detailed field and laboratory investigations at two sites were undertaken, to study them and the physical behaviour of the cliff as it recedes. The research has shown that the main controls on the mode of failure are the cliff height, state of the beach and presence of minor soil types within the tills. The cliffs stand at a steep angle (40-500) on unprotected coast, because pore water pressures are low, as a result of drainage to the underlying Chalk and sand layers. Where the cliff toe is protected, the slope angle decreases to between 200 and 300, by shallow slips and mudslides. Indirect evidence of the stress relief caused by unloading was given by observation of ground movements adjacent to the cliff. Depressed pore water -pressures resulting from undrained unloading were not observed. However stability analyses indicate that it is likely there are small depressions of pore water pressure in the cliff. Stability analyses have shown that a more realistic factor of safety is obtained if strength parameters are measured by tests which take account of the slip geometry. Erosion and removal of cliff debris from the toe, particularly when the beach is absent, is necessary for the continued occurrence of deep seated slips and overall recession of the coast.