Face stability analysis of shallow circular tunnels driven by a pressurized shield in purely cohesive soils under undrained conditions

Based on the slip line and limit analysis theories, this paper presents a new 3D collapse mechanism to analyze the face stability of a circular shield-driven tunnel in purely cohesive soil. The new collapse mechanism is a 4 variable multizone mechanism that depends on cover-to-diameter ratios (C/D). The proposed mechanism suggests that failure zones at the tunnel face are primarily formed by shear failure. A comparison of this approach with numerical methods and other kinematic approaches shows that the proposed failure mechanism significantly improves the existing upper-bound solutions for the face stability of circular tunnels in purely cohesive