A geomorphological and geotechnical investigation of the Roughlands Landslide Complex, Southwest Isle of Wight, UK

Abstract

The Roughlands Landslide Complex, comprised of slumped blocks, minor scarps, debris slides and a wide graben, is one of the few contemporary landslides on the Isle of Wight to be without major geomorphological or geotechnical investigation. Furthermore, Roughlands poses a puzzle as to the origin and form of its basal shear surface. If compound coastal landslides in ancient marine sediments are typically controlled by a basal shear surface which is guided by a bedding parallel slide prone horizon, how is the shear surface accommodated into the ancient fluvial Wessex Formation, which is largely without such bedding features? The complex is hence subject to a thorough investigation. This includes a rigorous desk study, geomorphological survey, peg network monitoring campaign using DGPS, comprehensive geotechnical testing program, and the installation of two slip-indicators. Although largely stable during the summer, parts of the landslide complex are recorded moving at up to 6.5m yr-1 during the wet winter months, indicating a high sensitivity to groundwater variation. Following an appraisal of eight possible formation mechanisms, it is concluded that the basal shear surface which underlies the Roughlands Landslide Complex is most plausibly guided by the basal surface of a Cretaceous sheet flood deposit. Support for this comes from palaeo-environment interpretation, which reveals various mechanisms by which sheet floods could entrain swelling clays; the excessive recession at Roughlands in comparison to other parts of the southwest coastline, which indicates that geological dip (which varies about the Brighstone Anticline) is influential; the fossil record; relationships with newly recognized faults; and from field observations. Stability analyses find field mobilized shear strength is greater than laboratory determined residual shear strength, which may be in part due to entrained Cretaceous floodplain debris within the slide mass. The unpredictability of landslides on the Wessex Formation also identifies some geotechnical uncertainties which are a concern for all engineering works in ancient fluvial sediments.

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