Surrounding rock instability mechanism for fault-crossing tunnels in water-rich soft rock

The stability assessment of a potential tunnel, especially a tunnel excavated under complicated geological conditions, is an essential task in tunnel design because it helps to determine the most suitable support system. In this paper, the deformation and failure mechanism of the rock mass surrounding a tunnel driven in water-rich soft rock with a fault zone are investigated. The Youfangping Tunnel, which is located along the Changsha-Kunming section of the Shanghai-Kunming High-Speed Railway, China, is used as a case study. The two main factors influencing the mechanical response of the surrounding rock mass, namely, the existence of a fault (geological structure) and groundwater, are considered in the analysis. Three cases, namely, no fault or groundwater, only a fault, and both a fault and groundwater, are simulated to investigate the deformation and failure mechanism of the surrounding rock and the internal force of the primary support. The results show that the existence of the fault induces shear stress concentration near the fault plane, which causes shear failure of the surrounding rock and contributes to the reduction in the rock mass strength under the action of groundwater, resulting in a failure mode with plastic flow, squeeze-out and shear sliding. The obtained results could assist in the design and construction of tunnels in water-rich soft rock with faults.