Collapse Mechanism and Mitigations of Mountain Tunnel Crossing Fault Fracture Zone: A Case Study from Southeast China

Due to the randomness of fault occurrence as well as the loose and broken internal surrounding rock, the fault zones often become the area with the worst stability of surrounding rock and the highest frequency of distresses during the entire tunnel project. It is crucial to clarify the deformation characteristics of the surrounding rock and the collapse mechanism during the tunnel crossing the fault zone. In this paper, a tunnel passing through seven fault fracture zones in Southeast China was selected as a case study, where the fault zones in this region presented a unique construction challenge due to their high frequency of distresses and the complexity of the fault occurrence. The spatial temporal characteristics of surrounding rock deformation and the collapse mechanism under various spatial positions between tunnel and fault during the entire process of crossing the fault zone were analyzed in detail by carrying out the field investigation, discrete element simulation method (DEM), and limit equilibrium method. It was found that the existence of the fault fracture zone and the insufficient strength of advanced support measures were the main reasons of the collapse. The safe factor of the excavation face decreased significantly during the tunnel crossing the fault zone. Because the continuous changed of the relative spatial positions between tunnel and fault, the surrounding rock deformation presented a significant time-spatial effect and the collapse displayed an obvious progressive nature. The collapse occurred only in the advanced support section and the sharp increase in axial force in the forepoling bolts both suggested that more robust preemptive reinforcement methods were essential in preventing structural failures in fault zones. Finally, specific emergency treatment and mitigation measures were proposed to address the collapse, including the treatment of the collapse area, the optimization of support measures, and the strengthening of monitoring. The simulation and monitoring data confirmed that the proposed measures effectively stabilized the tunnel, reducing further structural distress during excavation. This study highlights the critical importance of adjusting support measures based on real-time monitoring data or fine numerical simulation results when tunneling through fault zones. According to the significant spatiotemporal effects of the surrounding rock deformation and stress when the tunnels pass through the fault zones, studying the deformation control technology of tunnel key parts (local support optimization technology) or more resilient support systems has good economic benefits in the future.