DEM Modeling of 3D Kinematics in Rock Slope Failure

We compare solutions obtained using 3D Limit-Equilibrium (LE) and the 3D Discrete Element Method (DEM) analyses of rock slope stability and runout to illustrate the importance of kinematics in modeling of rock slides. While 3D LE methods provide a measure of the factor of safety against failure, the failure surface is assumed, and the rock mass is typically represented by vertical columns in the analysis. Thus, the kinematic response of the rock mass is artificially constrained, and the quality of the analysis heavily depends on an accurate capture of the potential failure surface and the failure mechanism. In contrast, a specific mode of failure is not assumed in DEM, since natural discontinuities, joints, shears, and fractures, as observed in the field can be used to create a more realistic representation of the rock mass such that failure can occur along any of the discontinuities. We use a case of rock slope failure in an existing mine to illustrate the difference between 3D LE and 3D DEM analysis results. We also show that with an increasing number of rock blocks in the model (tighter spacing of the joints), the rock mass is less stable. This has implications for rock slope stability evaluations, as rock that is more fractured will be less kinematically constrained and require more mechanical strength to remain stable. Additionally, during rock slide initiation, the rock within the sliding mass may fracture and disintegrate, such that it becomes less constrained as it deforms. The outcome is a progressive rock slope failure and accelerating displacements as the rock blocks within the sliding mass become more fractured.