Face stability analysis of tunnels under steady unsaturated seepage conditions: Fid-Bau Portal

Face stability analysis of tunnels under steady unsaturated seepage conditions

Li, Z.W. / Yang, X.L. / Li, T.Z.

Highlights • Face stability of tunnels under steady unsaturated seepage conditions is studied. • Considering the role of unsaturated seepage can increase tunnel face stability. • Unsaturated seepage effect size depends on soil type, tunnel geometry, and flow rate.

Abstract Tunnel face stability analyses are generally conducted assuming completely dry or saturated soils. However, a vast majority of practical conditions involve unsaturated soils. This paper presents an analytical framework to estimate the safety factor of a tunnel face with zero supporting pressure under steady unsaturated seepage conditions. The framework is implemented using the kinematic limit analysis method combined with the strength reduction technique. A closed-form solution for the unsaturated steady seepage problem is incorporated into the suction stress-based formulation of effective stress to obtain the profiles of suction stress and apparent cohesion. Based on a three-dimensional log-spiral mechanism of collapse, the safety factor is obtained using an iteration method for finding a zero supporting pressure. Several charts are provided to analyze the effects of unsaturated parameters and tunnel geometry. The results indicate that accounting for the role of unsaturated seepage increases the safety factor. The effect size of unsaturated seepage depends on the soil type, tunnel geometry parameters, and seepage conditions. The main contribution of this work is development of a useful approach for exploring the impact of steady unsaturated seepage on the face stability of tunnels.