Stability of an undrained plane strain heading revisited.

The stability of an idealised heading in undrained soil conditions is investigated in this paper. The heading is rigidly supported along its length, while the face, which may be pressurised, is free to move. The problem approximates any flat wall in an underground excavation. Failure of the heading is initiated by a surface surcharge, acting with the self-weight of the soil. Finite element limit analysis methods, based on classical plasticity theory, are used to derive rigorous bounds on load parameters, for a wide range of heading configurations and ground conditions. Solutions for undrained soils with constant strength, and increasing strength with depth are presented. Recent improvements to finite element limit analysis methods, developed at the University of Newcastle, have allowed close bounds to be drawn in most cases. Previous research in this area has often been presented in terms of a stability ratio, N that combines load and self-weight into a single parameter. The use of a stability ratio for this problem is shown not to be rigorous, a finding that may be applicable to other stability problems in underground geomechanics