Yield Stresses in Soft Contracting Clays

The undrained shear strength su along an arbitrary failure plane can be expressed as: su = 1/2(σ'ULS − σ'LLS) (1) where σ'ULS and σ'LLS denote the upper (U) and lower (L) effective, normal, compressive failure stresses acting at angles of +45 and −45° with the failure plane. These limiting stresses represent values that cannot be exceeded without yield occurring. In this paper, a study was done to investigate how the friction and attraction are mobilised by the plastic and elastic deformations to which the clay has been subjected. It is then possible to express undrained shear strength as a function of the consolidation stresses and two effective stress-strength parameters, the relative material attraction χ, and the effective material friction angle, sinϕ'M. The proposed theory forms the basis for constructing so-called yield envelopes for a clay, based on these strength parameters