Parametric Study of Water Inrush in a Tunnel Crossing a Fault Based on the “Three Zones” Fault Structure

As tunnelling progresses into the complex geological environment such as fault zones, water inrush has become one of the main geological hazards during tunnel construction. Consequently, understanding the evolution of pore pressure and flow velocity when a tunnel is excavated in a fault zone is crucial to ensure safe working conditions and reduce construction risks. In this work, based on the concept of “Three Zones” fault structure, we simulate the nonlinear water inrush process by solving the Darcy-Brinkman flow equation for the host rock and the fault zone. We examine the impacts of 1) the angle between the tunnelling direction and the fault and 2) the relative position from the tunnel face to the fault on the evolution of pore pressure and flow velocity near the tunnel face. The results show that within 5 m to 20 m ahead of the working face, pore pressure, flow velocity, and water inrush rate are the smallest when the angle is 90°. As the angle decreases, both pore pressure and flow velocity ahead of the working face increase. The pore pressure is larger when the tunnel has not reached the fault zone than when the tunnel has crossed the fault zone. Flow velocity also exhibits similar behaviour as pore pressure. With different relative positions from the tunnel working face to the fault, the closer the tunnel face to the fault, the lower the pore pressure and the larger the flow velocity ahead of the tunnel face. The largest water inrush rate occurs when the tunnel face is excavated to the center of the fault core, and the water inrush rate declines as the distance away from the fault increases. The simulation results provided a new method for simulating water inrush when a tunnel crosses a fault and could provide valuable references for the prediction of water inrush for underground projects.