26. Investigations of landslides at St Catherine's Point, Isle of Wight

Abstract

The Undercliff is the local name for a system of coastal landslides on the south coast of the Isle of Wight. Within the Undercliff, the landslides at St Catherine's Point are among the most striking and best developed, and least affected by human activity. An old, deepseated compound landslide, exhibiting a rearward slip, a graben-like feature and a seaward translational slide block, is fronted by an extensive debris apron. Continuing marine erosion is causing renewed sliding in the seaward parts of this apron, and very slight movements of the slide generally. Mudslides are active on the seaward slope of the slide block. The paper describes investigations into the geomorphology, Quaternary history, engineering geology and geotechnics of the slide complex. These indicate that the slides were initiated by marine erosion occurring as a consequence of the recovery of sea level towards its present elevation during the Flandrian transgression. The subsurface investigations show that the slides involve not only the Lower Chalk, Upper Greensand and Gault, but also the upper part of the Lower Greensand, the basal slip surface being seated in a clayey bed towards the base of the Sandrock, approaching 50m below the base of the Gault. Piezometers and inclinometers have been installed in a series of boreholes, and the results from these instruments are summarised, together with those from laboratory testing. Back-analyses of the present slide configuration are carried out. The Late Quaternary history, dating and development of the slide are discussed, and a model for its possible future cyclic development, based partly upon evidence from other parts of the Undercliff, is presented. INTRODUCTION PHYSICAL SETTING GEOMORPHOLOGY OF THE ST CATHERINE'S POINT LANDSLIDES SURFACE WATER AND WELLS SUB-SURFACE INVESTIGATIONS PIEZOMETRIC CONDITIONS LABORATORY TESTING BACK ANALYSES QUATERNARY HISTORY & LANDSLIDE DEVELOPMENT CURRENT MOVEMENTS LANDSLIDE CLASSIFICATION DISCUSSION & CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES