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A recent large landslide at the Spittles, Lyme Regis, Dorset and its implications for the stability of the adjacent urban area

By R.W. Gallois

Abstract

The Black Ven-Spittles landslide is an old, probably Pleistocene, complex of interacting coastal landslides that are in the process\ud of being reactivated as a result of a combination of man-made works and marine erosion. The upper part of the complex is\ud underlain by Cretaceous rocks and the lower part by the Jurassic Charmouth Mudstone Formation. Large-scale rotational and\ud translational failures have occurred in the Cretaceous rocks at less than 10-year intervals during the past 60 years, almost always\ud during or shortly after prolonged periods of rainfall. In contrast, large-scale failures have been infrequent in the Charmouth\ud Mudstone and have been restricted to areas where a low (<1.5°) seaward dip has resulted in bedding-plane-initiated failures. Two\ud such failures have been recorded, in 1908 and 2008, both in the same area at the western end of the landslide complex in the area\ud closest to the Lyme Regis urban area. The first of these occurred at 1.15 pm on June 10th 1908 and involved the collapse, or\ud partial collapse, of 450 m of cliff. It involved an estimated total of more than 300,000 tonnes of rock in what was probably the\ud culmination of three separate failures that occurred in rapid succession. The second, involving an area of c. 40,000 m2 and\ud c. 500,000 tonnes of material, occurred over a period of a few hours starting at 8 pm on May 6th 2008. In addition, the new\ud landslide intersected part of the former (c. 1920-1973) town rubbish tip with the result that glass, metal, other wastes and possible\ud pollutants were deposited on the beach. Before-and-after geological surveys of the area and the availability of pre- and\ud post-failure photographs and LiDAR surveys have made it possible to determine how the 2008 failure was initiated, and how it\ud progressed. Both the 1908 and 2008 failures appear to have started as relatively small rock-block collapses in a fracture zone\ud associated with a minor fault. At its western end, the new landslide is <300 m from the Lyme Regis built-up area and separated\ud from it by similar mudstones with small faults that may be equally prone to failure

Publisher: Ussher Society
Year: 2009
OAI identifier: oai:nora.nerc.ac.uk:10025

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