Numerical investigation of water inflow and seepage characteristics in a tunnel crossing two overlapped parallel faults

Water inflow is one of the main geohazards that threaten the safety of tunnels and other underground engineering projects. Faulted zone is one of the important geological triggers for such events. Numerical investigations on the evolution of flow behavior in tunnels across fault zones are of significance to the predication and prevention of this type of geohazards. In this work, a numerical investigation model with two overlapped parallel faults is established at a steady stage according to the "Three Zones" fault structure theory. The rapid turbulent flow in the fault zone is simulated by using the improved Darcy-Brinkman seepage model, while the slow laminar flow in ordinary rock zone is described by Darcy equation. The effect of relative position and distance between the tunnel excavation face and overlapped parallel faults to the groundwater pore pressure and flow velocity is studied through several scenarios, and the water inflow rate into the tunnel is calculated. The numerical investigation results reveal that while the tunnel face is excavated into the fault center core, the fractured zone, the ordinary rock zone, and the center of the overlapped faults, the pore pressure value ahead of the excavation face increases while the flow velocity decreases sequentially. The inflow rate is the largest while the tunnel face is excavated to center of the fault center core, which is closely related to the range of the overlapped area. The investigation results offer a practical reference for predicting early warning of water inflow geohazard when a tunnel cross two overlapped parallel faults.