A model for predicting groundwater level response to meteorological changes

The most characteristic feature of the Barton Clay cliffs of Christchurch Bay is the presence of a number of preferred bedding plane surfaces of shearing. Most of these are contained within clay horizons and the reason for their preferential use by the coastal landslides is unknown. One surface, however, is at the base of the 2.7 m thick A3 Zone which consists of inter-bedded sand and clays. This surface is well exposed and forms a distinct feature in the cliffs throughout nearly the whole of the 1.5 mile coastal outcrop of the A3 Zone.The paper discusses the reason why a shear surface is preferentially developed at this stratigraphic location. Three possible causes are discussed under the headings of (i) pore pressure fluctuation, (ii) seepage erosion and piping and (iii) equilibration response time. The first two are well established theories but the third is a new hypothesis derived from the original Bishop and Bjerrum theory of delayed equilibration. It is considered that the discussion is useful in the general context of landsliding at sand/clay junctions in overconsolidated soils