Stability of Twin Shallow Tunnels in Unsaturated Soils Considering Vertical Steady State Flow

This paper presents an analytical investigation on the stability of twin shallow tunnels excavated in unsaturated soils. The vertical steady state flow is considered to simulate the infiltration or evaporation condition during tunnel construction. The unified effective stress approach is employed to depict shear strength of unsaturated soils and the closed-form solution for the profile of apparent cohesion is established. A multi-block failure mechanism for twin shallow tunnels is then constructed, and on this basis, the analytical expression of required supporting pressure is deduced in accordance of the kinematical approach of limit analysis. The upper bound solution of supporting pressure is then searched through numerical optimization, and parametric analysis is performed to discuss the influence of unsaturated soil strength and vertical discharge on tunnel stability.