Lias clay slope sections and their implications for the prediction of limiting or threshold slope angles

A series of previously published engineering studies of Lias clay slopes in the East Midlands is summarized, along with the description of some further slope sections. All the slopes studied show evidence of landsliding, though some of it may be of periglacial origin. The very disparate nature of the different landslides suggest that various different erosion regimes were responsible for the original slope formation; it is believed that the smaller landslides are more typical of the area. Piezometric observations show that at the present day maximum pore pressures are proportionately less in the larger landslides, the average value of the pore pressure parameter ru varying between 0.48 for shallow landslides to 0.32 in the largest. Stability analyses show that the residual strength is strongly stress dependent, Φr', (c'r = 0) varying from > 16° in shallow landslides to 10° in the case of the largest. These and other factors have important implications for the often used practice of combining laboratory measurements of residual strength with the Infinite Slope method of stability analysis to predict threshold slope inclinations for landsliding.