Experimental and numerical analyses of an opening in a jointed rock mass under biaxial compression

Tunnel construction in a rock mass produces damage around the tunnel by concentration of in situ stress and by construction activity such as blasting. The generated damage changes the mechanical and hydraulic properties of the rock mass. In this study, the rock fracture and joint sliding behaviors of jointed rock masses with an opening under biaxial compression are investigated through experimental and numerical analyses. The tested rock models have a persistent joint set with dip angles of 30°, 45°, and 60° to the horizontal. Under the applied biaxial compression, tensile crack initiation and propagation are the dominant fracture behaviors around the hole in a low joint dip angle rock model (i.e., 30° to the horizontal). The propagation direction of the tensile cracks is roughly normal to the joint surface, and with propagation of tensile cracks, removable rock blocks are generated. The experimental results are simulated using a discrete element code. The numerical analysis simulates several aspects of rock mass cracking and the joint sliding processes around an opening: progressive fracture behaviors in a low joint angle rock model, abrupt initiation and propagation of tensile cracks and joint sliding in a high joint angle rock model (i.e., 60° to the horizontal), propagation of tensile cracks normal to the joint surface, generation of removable blocks in rock segments, an increase of lower hoop stress threshold inducing tensile fractures with a decrease in the joint angle, and an increase of the damage zone around the hole with a decrease in the joint angle.