Effect of height on delayed collapse of cuttings in stiff clay

The ‘delayed collapse’ of cuttings in stiff high-plasticity clay is known to be significantly affected by long-term recovery of pore pressures and progressive failure resulting from brittle material behaviour. Limit equilibrium methods are not well equipped to model these effects, but the increasing use of numerical modelling offers new potential to improve understanding of these failure mechanisms. A series of numerical analyses incorporating brittle material behaviour has been undertaken, specifically considering deep-seated first-time failure of cuttings in weathered London Clay. Because of progressive failure, the average mobilised strength is between the peak bulk strength and the residual strength. In current practice, parameter selection is often based on Chandler and Skempton's back-analysis of several deep-seated failures in the ‘brown’ weathered London Clay. This paper considers variation of the critical (minimum) height for delayed deep-seated failure in cuttings in weathered London Clay with slope angle, and variation of mobilised strength with cutting height.