Undrained collapse of a shallow plane-strain trapdoor Available to Purchase

The problem considered here is the plastic collapse of a strip trapdoor overlain by a relatively thin layer of homogeneous undrained clay (idealised as a rigid–plastic Tresca material). Although the simple ‘vertical slip' solution is intuitively appealing, and provides an upper bound that previous authors have found difficult to improve, it cannot be exact, because it implies an inadmissible stress field. This note describes the stress and velocity fields of a new slip-line solution that improves the upper bound slightly, but more importantly, satisfies all the boundary conditions of the problem (and is therefore a candidate for exactness). A comparison with closely bracketed bounds obtained using finite element limit analysis suggests that for cover ratios up to about 1·3, the new analytical solution is indeed the exact one.