Elastic-plastic models for fluid-saturated porous media and inception of strain localisation

We formulate an elastic-plastic constitutive model for anisotropic fluid-saturated porous media. Two main assumptions are introduced: (a) the yield function is based on a general effective stress space; (b) the plastic strain rates of the fluid and the solid phases are coupled. These two assumptions introduce two degrees of freedom, namely two symmetric second order tensors. We find that the onset of strain localization in the fluid-saturated porous medium is always governed by the underlying drained solid and so is independent of the choices made for such second order tensor degrees of freedom. Constrained bifurcation modes arising under local undrained conditions are examined.