Stability of plane strain tunnel headings in soils with tensile strength cut-off

Abstract

This paper focuses on stability analysis of plane strain tunnel headings in soils governed by the modified Mohr-Coulomb failure criterion. In terms of a very small or even zero tensile strength of soils, the tensile strength cut-off is introduced to modify the classical Mohr-Coulomb failure criterion which allows a nonlinear strength envelope of soils in tensile regime. An improved rotational failure mechanism of tunnel face is constructed to search the portion that is subjected to tensile stress along the whole failure block. It is discovered that the face pressure is the most critical when the tensile stress only appears at the top of the failure block. The numerical results obtained by the proposed approach are compared with those without tensile strength cut-off to highlight its influence on tunnel face stability. The finite difference method of FLAC3D is employed to further validate the proposed approach. The variations of cohesion, internal friction angle and unit weight of soils as well as the dimensionless coefficient ξ are investigated using the proposed approach to gain some insight into the improved failure mechanism. The elementary energy analysis is performed by dividing the failure block into numerous infinitesimal elements with respect to the rotation angle to give some extended discussions. It is shown that the top portion of the failure block that plays a role of anti-sliding is cut away due to the tensile strength cut-off, which eventually yields the critical face pressure. Conclusively, the proposed approach is an improved method for the stability analysis of plane strain tunnel headings in soils with tensile strength cut-off.