The implications of assuming isotropic elasto-plasticity to model the behaviour of soil under simple shear conditions arc considered. For small strains, usc of such a model implies the following threc consequences:
(1) strains and strain increments at any stage of shearing may be expressed as the sum of elastic and plastic components; (2) principal directions of stress and of plastic strain increment are collinear. (3) principal directions of stress increment and of elastic strain increment are collincar. These consequences arc used in order to establish relationships between the stresses, stress increments and strains which develop in a simple shear test. No additional assumptions with regards the form of the yield function, the flow rule or the hardening function arc required for this development. By defining the ratio of the plastic to the total shear strain increment on the horizontal plane (the plane of zero extension) as L, it is possible to define the horizontal normal stress u, in terms of 1 and other stresses and strains which are normally known during simple shear loading. As a result, all components of the stress tensor in the simple shear plane may be defined. Results of some direct simple shear tests on soft clay have been interpreted using the model and found to be generally consistent with some of the observations reported in the literature from tests in which boundary stresses were measured.