Tunnel failure in hard rock with multiple weak planes due to excavation unloading of in-situ stress

Natural geological structures in rock (e.g., joints, weakness planes, defects) play a vital role in the stability of tunnels and underground operations during construction. We investigated the failure characteristics of a deep circular tunnel in a rock mass with multiple weakness planes using a 2D combined finite element method/discrete element method (FEM/DEM). Conventional triaxial compression tests were performed on typical hard rock (marble) specimens under a range of confinement stress conditions to validate the rationale and accuracy of the proposed numerical approach. Parametric analysis was subsequently conducted to investigate the influence of inclination angle, and length on the crack propagation behavior, failure mode, energy evolution, and displacement distribution of the surrounding rock. The results show that the inclination angle strongly affects tunnel stability, and the failure intensity and damage range increase with increasing inclination angle and then decrease. The dynamic disasters are more likely with increasing weak plane length. Shearing and sliding along multiple weak planes are also consistently accompanied by kinetic energy fluctuations and surges after unloading, which implies a potentially violent dynamic response around a deeply-buried tunnel. Interactions between slabbing and shearing near the excavation boundaries are also discussed. The results presented here provide important insight into deep tunnel failure in hard rock influenced by both unloading disturbance and tectonic activation.