Undrained trapdoor and tunnel face stability revisited

Abstract

In this study, closed-form solutions are presented for two classical problems: the load exerted on strip or circular trapdoors and the stability of tunnel headings in purely cohesive soils, considering both the case of a constant undrained shear strength and of a linear vertical strength profile. The solutions are based on a minor modification of Gunn's lower-bound plasticity solution. According to comparative numerical stress analyses and finite-element limit analyses, the proposed equations accurately estimate trapdoor loads and face support pressures in soils with uniform strength, while they are slightly conservative in the case of depth-dependent shear strength. Thus, they are appropriate for the assessment of the stability of underground structures in saturated, low-permeability soils subjected to short-term loading and exhibiting depth-dependent strength – which, so far, has only been possible numerically.