Using a Pressurized Shield to Increase Face Stability of Circular Tunnels in Purely Cohesive Soil

Abstract

To obtain the critical collapse pressure of a circular tunnel driven by a pressurized shield for purely cohesive soils, a two-dimensional analysis model is presented based on limit analysis and slip-line theories. When the ratio of the cover depth C to the tunnel diameter D equals .5, there are three regions, Zones I, II, and III. When the C/D value is greater than .5, the mechanism involves up to four regions, i.e., Zones I, II, III, and possibly IV. Zone II is a transition zone that satisfies the normality condition. Zone IV is a Rankine zone subjected to the influence of the vertical soil arching effect that appears at the top of Zone III. Under Terzaghi’s theory of relative soil pressure, the contribution of Zone IV, including the weight and cohesion of the soils and the uniform surcharge loading, to the critical collapse pressure of the tunnel face is equivalent to an additional load. The proposed failure mechanism improves the existing upper-bound solutions for the face stability of circular tunnels in purely cohesive soils.