A unified 3-D anisotropic modular elliptic bounding-surface model for soil

The simple unified modular plasticity model presented earlier by the authors is extended from its original triaxial form to cover the three-dimensional case for isotropic soils. The simulated drained response of a sand under combined axial load and torsion is shown to agree well with the results of a hollow cylinder test. The model is also advanced by introducing plastic anisotropy in the form of a rotated and sheared bounding surface. This transformation preserves the stress ratio corresponding to the critical state as anisotropy is induced in cohesive soils. Comparisons are made between the anisotropic model simulations and experimental effective stress paths obtained from the undrained triaxial compression and extension of clay, sand and silt at different levels of overconsolidation or relative density.