Fracture Evolution Analysis of Rock Bridges in Hard Rock with Nonparallel Joints in True Triaxial Stress States

The excavation of a deep jointed rock mass in a true triaxial stress environment can easily lead to catastrophic consequences, and the failure of a rock bridge is directly related to the stability of the jointed rock mass. Most previous studies have focused on experimental and simulation studies related to parallel joints, while few studies have considered nonparallel joints due to their complexity. In this paper, the mechanical behaviors and acoustic emission (AE) characteristics of nonparallel joints with different rock bridge lengths (RBLs) at different intermediate principal stresses (sigma(2)) are studied. The results show that the peak strength increased with the increasing sigma(2) and RBL, while the peak strain decreased with increasing sigma(2) and increased with the increasing RBL. The longer the RBL was, the greater the influence of sigma(2) on rock failure, and the smaller sigma(2) was, the more significant the influence of the RBL on rock failure. The increase in the AE energy lagged behind the ringing count, and the cumulative AE counts of the specimens all showed a good pattern of decreasing with the increasing sigma(2). The longer the RBL was, the more sensitive the AE frequency was to the change in sigma(2). With the same RBL, the high-amplitude signals in the low-frequency range gradually attenuated as sigma(2) increased. The "quiet period" in the AE signals during rock failure could be specifically characterized by a comprehensive AE analysis, including AE time-frequency signals, AE ringing counts, AE event numbers, and AE b values. Finally, a modified Mogi-Coulomb-type criterion considering the effects of the true triaxial stress and RBL is proposed. The research results can provide a useful reference and help in the identification of precursory information of joint rock mass failure and early warning of sudden instability failure.