Post-mortem analyses of seven slope failures in rock slope are reported. The analysis is based on simple force equilibrium of a block of rock surrounded with several planes of discontinuity and separated from the neighbouring rock mass in a rock slope. Driven by the own weight of the rock block together with the ground water pressure acting on the planes of discontinuity, the stability of rock mass is maintained by the shear resistance of the materials embedded in the thin clearance forming the slip surface. A slope failure takes place when the shear strength is not strong enough to resist the driving forces. Seven case studies indicate that the simple principle of three dimensional force equilibrium is good enough to estimate the factor of safety in the slopes of rock mass containing the cracks filled with soft clayey materials.
Rock slope failures are often triggered by the unusual rise of the ground water level caused by heavy rainfalls or rapid snow melting. Rock masses usually contain a number of discontinuities such as cracks and fissures that may become potential slip surfaces especially when thin seams of soft clay lies within them. This paper presents the post-mortem stability analyses of seven sites where slope failures took place. Undisturbed samples taken from the clay seams which happened to play a role as the slip surfaces are subjected to shear box tests under conditions of constant volume during shear. The driving forces in such a category of rock slope failures arise from the own weight of the rock mass as well as from the water pressure acting on the discontinuity planes around the sliding rock mass. The resisting forces arise from the shear resistance of the clay seams lying along the slip surfaces.