Upper-bound solutions for face stability of circular tunnel in purely cohesive soil using continuous rotational velocity field

This paper presents a 2D analysis model based on limit analysis and slip-line theories for the face stability of a circular tunnel in purely cohesive soil. The analysis model depends on the ratio of cover depth C to tunnel diameter D. When C/D is equal to 0.5, the mechanism consists of three blocks, namely, Zone I, Zone II, and Zone III. When C/D is greater than 0.5, the mechanism consists of four blocks, namely, Zone I, Zone II, Zone III, and a possible Zone IV. Zone II is a transition zone satisfying the normality conditions. The possible Zone IV is a Rankine zone that is subjected to the influence of the vertical soil arching effect appearing at the top of Zone III. The criterion for the collapse thickness limit of the tunnel is proposed based on Terzaghi’s theory of relative soil pressure. The results show that significant improvements have been made to the existing solutions using the proposed failure mechanism for the face stability of circular tunnels in purely cohesive soil.