Three-dimensional tunnel face stability considering the steady-state seepage in saturated and unsaturated regions with changing water levels

Abstract

For tunnels excavated in the unsaturated soils with changing water levels, there may be steady-state unsaturated flow above the water table and saturated flow below the water table toward the tunnel face due to the excavation. Changing water levels affect the adverse effect of the underwater saturated flow and the contribution of the unsaturated flow to the shear strength. To figure out the regularity concerning the region and magnitude of unsaturated and saturated flow effects, an analytical framework is developed within the kinematic approach of limit analysis combined with the numerical simulation. A suction stress-based equation is employed to express the contribution of unsaturated flow to shear strength. The development of the saturated flow below the water table is numerically simulated, and the resulting pore pressure distribution is extracted and interpolated on a 3D discrete rotational failure mechanism. The work rate balance equation incorporating the apparent cohesion due to unsaturated flow and the pore pressure affected by the saturated flow is established to optimize the critical face pressure. The proposed analytical framework is verified by comparisons with previous literature and numerical simulations of hydromechanical analysis. A parametric analysis is conducted to investigate the effect of groundwater levels, flow types, unsaturated properties, and hydraulic hysteresis on the tunnel face stability. The results show that it is only necessary to consider the effect of unsaturated flow and the differences caused by various unsaturated characteristics when the unsaturated zone is large enough. This work can provide a good reference for the design of face supports in the initial stage for tunnels excavated in unsaturated soils with changing groundwater levels.