Limit equilibrium of asymmetric laterally constrained rockslides

Existing limit equilibrium methods, routinely used to calculate the stability of rock slopes, do not satisfy rotational equilibrium of the potential slide around a vertical axis. This is inconsequential for slides that are symmetrical with respect to a vertical plane oriented in the direction of motion. In this paper, methods are developed to analyse the stability of strongly asymmetric slides, defined by a weak sliding surface dipping in an oblique downslope direction and a stronger vertical plane forming one of the lateral boundaries (“constraint surface”). It is shown that tensile bending stresses can develop on the constraint in some cases. If the rock mass cannot sustain tension and if tensile rock mass failure destroys the cohesion (e.g. by the formation of cracks), the sliding Factor of Safety can be very substantially reduced. Under certain conditions, a rotation failure can occur irrespective of the shear strength available on the constraint. Routinely used analysis, such as the wedge stability method, can err substantially on the unsafe side. Two new methods are presented: a simple closed form solution of a quadrilateral block and a numerical “Method of Columns” solution applicable to general surfaces. The results are compared with an advanced discontinuous stress–strain solution of an asymmetric wedge example.

► A new limit equilibrium solution for rockslides.
► Asymmetric sliding surfaces.
► Rotational equilibrium around vertical axis.