Numerical analysis of coastal cliff failure along the Pembrokeshire Coast National Park, Wales, UK

Abstract

Cliff stability within the Pembrokeshire Coast National Park was evaluated using a numerical model applied at four sites representative of rock mass failure phenomena and major sedimentary geological sequences. The sites were: Mill Bay, (Old Red Sandstone); St Govan's Head, Carboniferous Limestone (Dinantian); Druidston, Millstone Grit (Namurian) and Lower Coal Measures (Westphalian); and Wiseman's Bridge, Lower Coal Measures (Westphalian). The study integrated a range of geotechnical parameters, measured in the field and laboratory, into a model to predict the likely failure mechanisms. The model is based on the existence of rock prisms delineated by structural parameters, i.e. joints, bedding planes and critical tension fractures behind the cliff face. An iterative approach is used to define the dip of the most probable, stepped failure surface at the base of any potentially unstable multiblock system and to calculate the sliding and toppling forces for each block in the cliff mass. Prediction compared favourably with field observations at three of the four selected sites, i.e. Druidston, St Govan's Head and Wiseman's Bridge. At Druidston sliding is predicted and dominates in the basal blocks, whilst toppling is confined to the upper cliff and is dependent on movement of the lower structural units. St Govan-s Head is shown to have a low risk of toppling and sliding and this was predicted except where basal undercutting reaches a depth of 1·0 m or lateral forces exceed 100 kN m−2 when failure could occur. Copyright © 1998 John Wiley & Sons, Ltd.