Numerical study of rockburst damage around excavations induced by fault-slip, A

Abstract

The increasing depths of mining and associated in-situ stresses have made rockburst a serious risk for underground mining and tunneling. Therefore, it is important to understand the energy mechanisms of unstable rock failure to reduce rockburst hazards. In this dissertation, an energy approach is developed by integrating energy equations into UDEC software where energy components including elastic strain energy, plastic strain work, and joint friction work can be tracked in each individual zone or contact of the numerical model at every time step. Rapid and large changes in energy components were used to identify unstable rock failure modes, and the magnitudes of these changes were used to quantify the unstable rock failure intensity. Simulation of compression testing confirmed that unstable rock failure tends to occur in stiff and brittle rock loaded with a soft loading system, resulting from the fact that a small loading system stiffness (LSS) and rock stiffness will increase the amount of stored elastic strain energy in the model, while a brittle rock will require less amount of elastic strain energy for plastic strain work during the rock damage process. Direct shear test modeling results show that unstable slip failure at the same peak shear stress is less likely to occur in discontinuities with smaller shear stiffness, larger roughness, and/or that are embedded in a rock matrix with smaller stiffness. A stiff rock matrix surrounding the discontinuity can store less elastic strain energy to be transferred to the joint during failure, while smaller shear stiffness represents a larger slip-weakening distance and requires more elastic strain energy for the slip failure. Rockburst damage induced by fault slip at the excavation scale was investigated with a circular excavation model with a nearby discontinuity, and a critical discontinuity distance parameter was proposed to quantify the rockburst potentials for this type of rockburst. The critical discontinuity distance refers to the minimum normal distance between the excavation and discontinuity plane for the excavation to stay stable; therefore, a larger critical discontinuity distance represents a higher rockburst potential. The influence of the in-situ stress, discontinuity dip angle, excavation radius, and fault length, are analyzed with the hypothetical base model. Finally, a rockburst event from the drainage tunnel at Jinping II hydropower station is simulated to validate the numerical results with the measured released energy and the estimated normal fault distance.