3D stability analysis of tunnel face with influence of unsaturated transient flow

The analysis of tunnel face stability is commonly implemented with the assumption that the soil is dry or fully saturated. However, many practical cases occur in partially saturated soils, and the unsaturated component of the shear strength warrants consideration for a more realistic solution. This paper extends the current understanding of the tunnel face stability, which, for the first time, incorporates the contributions of unsaturated transient flow into the analytical framework of the three-dimensional (3D) tunnel face stability. A closed-form describing the spatio-temporal variability of saturation, suction stress, and apparent cohesion was derived and introduced into the Mohr-Coulomb criterion to consider the unsaturated transient flow. Within the framework of the kinematic approach of limit analysis, the work rate balance equation including the unsaturated component was established based on a 3D horn-like failure mechanism. A parameter analysis concerning the hypothetical fine sand, silts, and clay was performed to study some parameters of interest. The results show that unsaturated transient flows play a positive role in reducing the required face pressure. The magnitude of the influence is controlled by factors including soil types, thickness of the soil layer, and infiltration time. Compared with neglecting the unsaturated effect, the reduction of normalized critical face pressure caused by transient flows can reach 16.22% in fine sands, 24.98% in silts, and 85.62% in clays. The results suggest that the contribution of unsaturated transient flow to clays, particularly in the early infiltration stage, should be considered. Findings of this work can provide an insight to quantitively capture the influence of the unsaturated transient infiltration and the resulting transient changes of the 3D tunnel face stability.