The hardening soil model: Formulation and verification

A new constitutive model is introduced which is formulated in the framework of classical theory of plasticity. In the model the total strains are calculated using a stress-dependent stiffness, different for both virgin loading and un-/reloading. The plastic strains are calculated by introducing a multi-surface yield criterion. Hardening is assumed to be isotropic depending on both the plastic shear and volumetric strain. For the frictional hardening a non-associated and for the cap hardening an associated flow rule is assumed. First the model is written in its rate form. Therefor the essential equations for the stiffness modules, the yield-, failureand plastic potential surfaces are given. In the next part some remarks are given on the models incremental implementation in the PLAXIS computer code. The parameters used in the model are summarized, their physical interpretation and determination are explained in detail. The model is calibrated for a loose sand for which a lot of experimental data is available. With the so calibrated model undrained shear tests and pressuremeter tests are back-calculated. The paper ends with some remarks on the limitations of the model and an outlook on further developments.