Impact of loading rate on the mechanical behavior of jointed rock

The impact of loading rate is a vital issue in the study of the time-dependent behavior of rock masses. A sample containing a single inclined joint is represented by the particle flow code and used as an analog to examine the rate-dependent behavior of jointed rock. A series of numerical triaxial compression tests are completed at various loading rates on specimens containing this single joint inclined at 30°, 45°, and 60° with respect to the orientation of the confining stress of 5 MPa. Observations are recovered for the evolution of stress–strain, deformation and energy release together with resulting failure mode. Rate sensitivities of four parameters defining a smooth joint model (normal stiffness, shear stiffness, stiffness ratio and friction coefficient) are used to represent observed response. We find that competition between mechanical damping and inertial force results in the strain rate effect and that the peak strength of the rock specimens increases with increasing loading rate. The step-wise form of both the stress–strain and kinetic energy can be used as indicators of the onset of dynamic failure. These numerical experiments are consistent with observations from laboratory experiments on identical samples where rate effects in the jointed rock are accentuated over those apparent in intact rocks. Change in the loading rate has a significant effect on the failure mode for specific joint angles relative to the confining stress. The joint friction coefficient is the main rate dependent factor controlling behavior and is an important factor in defining the significance of loading rate effects.