Separable Yield Surfaces to Correlate Axi-symmetric, Plane Strain, Simple Shear and Multiple Stage Tests

Separable yield surfaces were first considered in 1971; no particular expression for plastic work is needed. Plastic straining of 'wet' clay is taken as (a) anisotropic consolidation steps, using the normality law, plus (b) straining in one mode of deformation while there is no strain in one or more other modes. The stress-strain curve for each mode is empirically related to the corresponding flow rate function. The yield surface is assumed unchanged by the deformations. In plane strain and shearing, when a single mode of deformation is applied, two or more cohesive resistances are reduced; the new steepest-ascent principle shows how the reduction might be shared. Examples of different volume change conditions are given. It is hoped that improved forecasts of three-dimensional deformations under working stresses will be made possible by this fully-computable 'Bir-Clay' model.