Optimal mine pitwall profiles in jointed anisotropic rock masses

A new methodology based on the limit analysis upper bound method for the topological optimisation of slopes is presented, to determine geotechnically optimal slope profiles in anisotropic jointed rock masses. The methodology accounts for the effects of discontinuities such as joints, bedding planes, and tension cracks. We applied this methodology to the context of open pit mines, with the goal of achieving geotechnically optimal pitwall profiles. The optimal profiles maximise the Overall Slope Angle (OSA) while maintaining a prescribed Factor of Safety (FoS) and satisfying the geometric constraints imposed by benches and ramps. The method, implemented in the software OptimalSlope, utilises direction-dependent cohesion and internal friction angle parameters to replicate the effect of joints on slope stability. Key inputs include joint orientation, non-persistence, and probability of occurrence. We tested the methodology on a Mexican open pit mine to be excavated into Cretaceous siltstone featuring eight different joint sets and a primary bedding plane. Optimal pitwall profiles were determined for various combinations of bedding dip angles (0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees, 90 degrees) and mine pitwall orientations (hanging wall, footwall, side walls), considering the three-dimensional kinematics of the joints through anisotropic functions of cohesion and friction angle. Results indicate that the optimal pitwall profiles generally exhibit higher OSA compared to planar profiles with the same FoS, except in one bedding dip direction. Additionally, stability analyses performed using Rocscience Slide2 independently verified the FoS values of the optimal profiles.