Rock joints often exhibit shear resistance softening and a tendency to dilate to a steady joint opening when sheared under constant normal loads. In addition, rock joints sheared while keeping the joint opening constant exhibit an increase in normal stresses to a steady state value. From a micromechanical viewpoint, these phenomena are related to the material mechanical properties, the interface roughness and the evolution of interface roughness as the joint is sheared. Here we utilize a micromechanical approach that explicitly considers asperity interactions on joint surface to study these rock joint phenomena. Elastic deformations and inelastic frictional sliding are considered at inclined asperity contacts. Rock joint roughness is modeled via distributions of asperity heights and asperity contact orientations. Evolution laws for asperity heights and asperity contact orientations are introduced to account for the change in interface roughness resulting from asperity damage under shear. The micromechanical approach developed in this paper shows a clear link between the softening and dilatation behavior of rock joints and the evolution of asperity heights and asperity contact orientations with joint shear. The micromechanical model is also shown to replicate the experimentally measured rock joint shear behavior culled from the literature.
