| The term 'rock slumping' describes a compound failure mode of jointed hard rock slopes that involves back-rotations and inter-layer slip between slender blocks comprising the unstable rock mass. Rock slumping typically occurs in footwall environments, having repetitive discontinuities dipping in the same direction but steeper than the slope face. Rock slumping can occur in foliated rock masses, such as slates, phyllites, schists and gneisses, together with interbedded sedimentary sequences and massive, sheet-jointed granites and sandstones. Rock slumping represents a fairly widespread slope failure mode, occurring at many scales in nature, that has been poorly understood and often unrecognized. It is a potentially deep-seated mode, and can be a particularly important mechanism in the footwall slopes of open pit mines. Field investigations led to the concept of rock slumping, and a limit equilibrium solution was based on subsequent physical and numerical model results. The models indicate rock slumping failures can progress rapidly. As topping is routinely considered a potential failure mode in headwall slope environments, rock slumping should similarly be considered in footwall environments. The morphologic similarity between rock slumps and traditional soil slumps has led to the incorrect use of traditional soil-mechanics limit-equilibrium methods in the hard rock slope environment. Traditional soil mechanics methods do not consider kinematics, are incapable of modeling rock slumping, and will produce unconservative results. Cases studied show that a slope possessing an apparent factor of safety between about 1.4 and 2.0 when determined with soil mechanics methods, may prove to be only marginally stable when rock slumping represents the actual mode of failure. Discontinuous Deformation Analysis (Shi & Goodman, 1988; 1989), however, is a kinematically correct numerical model that is consistent with the rock slumping limit equilibrium solution. This research has addressed fundamental aspects of rock slumping, and much remains to be discovered regarding this compound mode of failure. As more case histories of rock slumping are evaluated, our understanding will evolve, and the limit equilibrium solution should prove a useful predictive engineering analysis and design tool. |