Microplane model for triaxial deformation of saturated cohesive soils

A microplane model, in which the constitutive properties are characterized independently on planes of various orientations (microplanes), is presented. It is found that the basic scheme previously developed for concrete is also valid for clays, but with certain modifications. The micro-macro constraint is kinematic, and the stresses on each microplane are defined as explicit functions of the volumetric and deviatoric normal and shear components of the macroscopic strain tensor on the microplane. To account for undrained behavior, a pore water pressure term that affects only the volumetric equations is introduced. This makes possible the uncoupling of the stress-strain and the pore water pressure formulations. The model is calibrated and verified by comparisons with numerous data for both drained and undrained tests, and good agreement is attained, including volume changes, pore water pressure evolution, and various stress-strain diagrams. Although the model involves nine material parameters, four (or five in some cases) can be fixed constant for all soils, and only five (or four) need to be determined by data fitting. The fact that the stress is given as an explicit function of strain makes the model suitable for finite element codes.