Mobilization of Cohesion and Friction Angle of Intact Rocks in the Shearing Process

The strength of intact rock materials is often linked to their cohesion and friction angle described by Mohr–Coulomb criterion. A common assumption in the Mohr–Coulomb criterion is that these two strength related components are mobilized simultaneously at the same displacements. Review of previous studies on the brittle failure of rock materials shows that the cohesion degrades, while friction mobilizes during the failure process. In this study, brittle, brittle–ductile and ductile failure modes were numerically simulated in a large number of direct shear tests (around 290 experiments) under different normal stresses to study the mobilization of cohesion and friction angle in the failure process. It was shown that an increase in normal stress results in an increase in cohesion but a decrease in friction angle. The Mohr–Coulomb criterion was consequently modified, to consider both the cohesion and friction angle as a function of normal stress and displacement (strain). The proposed criterion successfully reproduced the results of direct shear tests in different types of failure modes. Results of this study demonstrate the evolution of simultaneous onset of plastic strain for residual cohesion and fully mobilized friction angle. It was further revealed that there is no considerable change in the mobilized friction angle at low normal stresses when the shear stress is in the range of crack initiation and crack damage thresholds. Finally, it was shown that the mobilized cohesion cannot be overlooked in the residual stage of loading process.