Failure potential of a circular tunnel face under steady-state unsaturated flow condition

Abstract

In natural geotechnical structures, especially for those in the vicinity of water bodies or subjected to rainfalls, the soil properties are more likely governed by the unsaturated soil constitutive model rather than the fully saturated or dry one. In this contribution, the theory of unsaturated soil mechanics is introduced to tackle the stability problem of a circular tunnel face under steady-state unsaturated flow condition. By making reference to the existing findings, a generalized analytical solution for the vertical suction stress profile is discussed which permits to deal with unsaturated soil problems with the classical limit analysis method. An extended Mohr-Coulomb yield criterion is presented to characterize the tunnel face failure in unsaturated soil where the effect of suction stress on tunnel face stability is interpreted as the apparent cohesion. The increase in shear strength of unsaturated soil due to the existing of apparent cohesion is considered as a combined effect of n, α, ks and q, which respectively correspond to the pore size parameter, the inverse of air entry value (AEV), the saturated hydraulic conductivity and the flow rate. Within the framework of limit analysis, an advanced three-dimensional (3D) failure mechanism is used to include the unsaturated shear strength parameters into the work rate equation which eventually yields the critical retaining pressure for tunnel face stability. Parametric analysis is performed to show the influence of different unsaturated soil parameters on tunnel face stability. Four representative soil types are investigated to give some suggestions for tunnel excavations and designs in unsaturated soil.