M. Dunbaven writes: The geometrical forms of the slips described in the paper bear many similarities to failures in mining waste dumps reported by Blight (1969). The important feature of the slides is the coincidence of the lower parts of the slip surfaces with a weaker layer of soil; in this case the Oldhaven Beds of silty fine sand. The example chosen for discussion is the Mirmar landslide where the weak layer forms the base of a simple slope. A careful study of some similar landslides has shown that the graben block and the en masse movement of the lower slope are common characteristics.

Recent research at James Cook University has reproduced such failures in a small physical model. A slope made from a well graded cohesionless silica sand was built over a layer of bentonite saturated to give low yield strength. Overall dimensions of the slope were: height 400 mm, width 750 mm, base length 1100 mm. The kinematics of the failure have been determined by photography which revealed basically a movement of two ‘rigid’ wedges.

A computer study based on a wedge analysis was conducted using both force balance and energy balance equations, with the condition that one slip-plane pass through the base material. The situation which created the highest force or energy imbalance was calculated by numerical optimization. For both methods, the predicted slip-planes closely followed those observed in the laboratory, however, different answers would be expected for a material which exhibited significant dilatancy.

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