Development of a shear zone in brittle rock subjected to direct shear

Laboratory direct shear tests were carried out on a brittle synthetic rock at various normal loads. The observed fracture patterns changes as the normal load increased. A discrete element code was used to simulate the direct shear tests. The simulation results clearly showed that cracks that developed during the test were predominantly tension cracks. These cracks started to grow from the upper edge of the shear box due to the highly non-uniform stress distribution along the shear plane.The fracture development observed in discrete element simulation models showed that inclined fractures developed parallel to the maximum principal stress direction. En echelon type extensile fractures were also generated and assemblies of theses fractures formed shear bands similar to the process observed in conventional triaxial test using brittle rocks. The discrete element simulations showed that the macro-scale shear band in the direct shear tests resulted from progressive failure of tension-induced micro-cracks.