Three-Dimensional Face Stability of Tunnels in Unsaturated Soils with Nonlinear Soil Strength

Abstract

Stability analysis of tunnel faces is usually carried out under dry or saturated assumptions. However, in practice, soils are often unsaturated due to the presence of matric suction, and unsaturated components often have nonnegligible effect on soil strength. For a more realistic solution, the influence of unsaturated components on the stability of a three-dimensional tunnel face was considered for the first time. To meet this requirement, the three-dimensional rotational failure mechanism and the extended form of Mohr–Coulomb failure criterion were used to express the face failure in the limit analysis framework. The strength reduction technique was employed to estimate the stability of ta hree-dimensional tunnel face and the safety factors were calculated by the bracketing method. The proposed method is verified by comparing with the results obtained by FLAC3D. The effects of matric suction distribution, tunnel geometry parameters and shear strength nonlinearity on the stability of three-dimensional tunnel face were analyzed in the parameter analysis. The results show that considering the effect of suction stress is beneficial for improving the tunnel face stability, but it is influenced by the matric suction distribution, the tunnel geometry, soil types, and the shear strength nonlinearity. The main novelty of this study is to develop an effective method to evaluate the face stability of tunnels in unsaturated soils.