A RECONSIDERATION OF COASTAL LANDSLIDES AT FOLKESTONE WARREN KENT

The coastal landslides of Folkestone Warren, involving the Chalk and the Gault, are among the most important in southern England. In the first part of the Paper, their geology, hydrology and history are described and the relevance of these to an understanding of the landslides is explored. All the deep-seated landslides in the Warren recorded during the past two centuries are shown to have occurred within the period of seasonally high groundwater levels in the slipped masses, from December to March. The initiation of the present landslides probably took place between 3500 and 500 B.C. The Warren landslides have not hitherto been considered in terms of effective stresses. The Paper presents such analyses for the deep-seated renewals of movement on non-circular slip surfaces which occurred in 1915, 1937 and 1940. The results are consistent in showing that, in all three landslides, the average strengths mobilized in the Gault on the slip surfaces approximated to the residual and are bounded by the envelopes cr = 0, φ′r= 13·9° and c′r-0, φ′r = 16·6°. This range of φr′ values is intermediate between the greatest and least values of φ′r measured for the Gault in the laboratory. Finally, in the light of the erosion history of the site, the properties of the Gault and certain limited field evidence, a mechanism for retrogression of the rear scarp of the landslips is suggested. This provides an explanation of the features known as ‘vents’ and ‘sets’. Les glissements de terrain à Folkestone Warren, dans la Craie et le Gault, sont parmi les plus importants de l'Angleterre de sud. © 1969, Thomas Telford Ltd. All rights reserved.