Stability of 3D tunnel face under steady unsaturated seepage coupled multi-layer soil suction head distributions

This paper proposes a new discrete approach for analyzing tunnel face stability in stratified soils under unsaturated seepage conditions. Traditional methods have primarily focused on homogeneous soils, neglecting the spatial variability of unsaturated and soil parameters in multi-layer systems. To address this, the study incorporates principles from unsaturated soil mechanics and kinematic limit analysis to develop a model for two horizontally layered soils. A model of two layers of horizontally layered soils is developed, and a framework which involves a combination of principles from unsaturated soil mechanics and a kinematic approach to limit analysis focuses on estimating the stability of tunnel surface excavation influenced by soil layering in unsaturated seepage conditions is proposed. The distribution of suction stress in two layers of soils with flow in the unsaturated soil is deduced from the discrete suction head profile, and then the analysis provides profiles of apparent cohesion within the two layers of unsaturated soils by integrating the closed-form solution and the expression of effective stress which contains the suction stress. Based on logarithmic helix curve in three-dimensional plane which imitates collapse using spatial discrete technology, an iteration method is utilized to find the critical condition at which the supporting pressure reaches zero. During the iteration process, the work done can be claculated by suction stress at each point within the discrete failure mechanism using the discrete suction stress formula. The influences of the unsaturated parameters and the horizontal soil are discussed. Based on the no suction condition, this framework is ultilized to obtain the influence on the stability of the tunnel face when changing relevant unsaturated parameters for different soil layers, as well as altering the tunnel's burial depth and diameter ratio. Furthermore, we analyzed the distribution of suction head and matric suction in layered soil layers under different seepage conditions. The results reveals that the variation of unsaturated parameters of the upper soil in layered soils has the greatest effect on avoiding the collapse of tunnel face and infiltration or evaporation can obtain vary results with the consideration of variations in unsaturated soil parameters of different layers.