An Analytical Framework for Assessing the Tunnel Face in Unsaturated Soils Under Transient Infiltrations and Its Application

An accurate assessment of the tunnel face excavated under rainfalls is commonly desired in the design and construction. The infiltrating rainfall usually needs to pass through unsaturated layers before reaching the water table. During this dynamic process, the response of the soil is complex due to the rainfall intensity and duration, hydraulic properties of soils, and soil geometries. To assess the tunnel face in unsaturated soils under transient flow conditions, an efficient general framework is established in this paper. An analytical suction stress-based formula for one-dimensional transient flow was derived, which is adaptive to express the tempo-spatial profiles of the suction head and suctions stress. A three-dimensional (3D) rotational failure mechanism was established with the discrete spatial technology, allowing the tempo-spatial changed unsaturated effect incorporated into the calculation. Then the proposed framework using the kinematic approach of limit analysis was applied to assess the tunnel face in three hypothetical soils, including the sands, silts, and clays. A numerical simulation was also conducted to validate the established analytical framework. Finally, several design charts of normalized safety factors obtained by the dichotomy and strength reduction method were provided for practical reference.