A critical review of the State Boundary Surface concept in light of advanced constitutive modelling

The State Boundary Surface (SBS) concept was introduced in the late sixties by the Cambridge soil mechanics group in order to formulate and illustrate some of the basic hypotheses of the Critical State Soil Mechanics framework. The first part of this paper illustrates some experimental results to show the apparent non uniqueness of the SBS for some natural soils, indicative of the need for more complex hypotheses on the hardening of these soils. This is achieved by a constitutive model, recently proposed by the authors. The model was developed in the context of multi-surface plasticity and includes isotropic and kinematic hardening to account for the effects of structure degradation in cemented soils and non-isotropic consolidation histories. The model is used as a theoretical tool to propose a revised definition of the SBS and to illustrate strategies for the normalisation of experimental data in a way consistent with this definition. Finally, the paper compares selected experimental data from two stiff clays and the corresponding numerical results in an attempt to validate the proposed approach.