Blast-Induced Fracture Behavior of a Rock Mass with Discontinuity Subjected to In Situ Stress

Deep rock masses are subjected to high in situ stress and also characterized by a significant presence of discontinuities such as joints and faults. Many studies have focused on the blast-induced fracture in intact rock under in situ stress or in shallow discontinuous rock masses without in situ stress. The blasting fracture behavior of deep rock masses under the combined actions of in situ stress and discontinuities remains insufficiently understood. In this study, the split Hopkinson pressure bar tests under confining pressure are conducted to validate the Riedel–Hiermaier–Thoma (RHT) model parameters of granite. A series of dynamic FEM simulations with the RHT model are performed to investigate the explosion stress wave propagation and crack distribution of single-hole blasting under various in situ stress and discontinuity conditions. Based on numerical modeling, the mechanism of blast-induced fracture in a rock mass with discontinuity subjected to in situ stress is exposited, and the blasthole arrangement for utilizing the discontinuity to facilitate rock fragmentation by blasting is discussed. The results show that blasting in discontinuous rock masses mainly generates three types of cracks, namely radial cracks emanating from the blasthole, spalling cracks resulting from explosion stress wave reflection on the discontinuity surface, and wing cracks derived from the discontinuity ends. With increasing in situ stress, the radial cracks and wing cracks are significantly reduced; particularly, wing cracks completely disappear when the in situ stress reaches 30 MPa. The spalling cracks are the primary mechanism for the discontinuity to influence the blast-induced rock fracture under in situ stress. The area of the cracks generated between the blasthole and the discontinuity expands with the increase of the discontinuity length. For maximizing the facilitation of the discontinuity on the blast-induced rock facture, the blasthole is recommended to be arranged on the midvertical line of the discontinuity. The blasthole–discontinuity distance should be carefully designed to avoid excessive rock fragmentation near the blasthole and ensure the formation of the effective spalling cracks. Under high in situ stress, a smaller blasthole–discontinuity distance is advisable.