Based on the relationship between the current yield surface and the reference yield surface, a new model, called the three-dimensional unified hardening model for overconsolidated clays (the UH model), is proposed in this paper. A current yield surface is used to describe overconsolidated behaviour, and a reference yield surface to describe the yield characteristics corresponding to normally consolidated clays. The UH model can model many characteristics of overconsolidated clays well, including stress-strain relationships, shear dilatancy, strain-hardening and softening, and stress path dependence behaviour. The key feature of the model is the adoption of a unified hardening parameter that is independent of stress paths. Based on the SMP criterion and the corresponding transformed stress method, the proposed model can be applied conveniently to three-dimensional stress states. Compared with the Cam-clay model, the UH model requires only one additional clay parameter, the slope of the Hvorslev envelope. The validity of this new model is confirmed by data from triaxial drained and undrained compression and extension tests for clays with different overconsolidation ratios, true triaxial tests with different Lode's angles, and cyclic loading tests.

No abstract

No abstract

No abstract

a b s t r a c tExperimental evidence shows that the strength of geomaterials, such as soils and rocks, is significantly influenced by inherent anisotropy and other factors such as shear banding and the intermediate principal stress, which cannot be properly described by an isotropic failure criterion. This paper presents a generalized failure criterion for geomaterials with cross-anisotropy. To account for the influence of cross-anisotropy, we introduce an anisotropic variable in terms of the invariants and joint invariants of the stress tensor and the fabric tensor into the frictional coefficient of the failure criterion. The anisotropic failure criterion is formulated in both the deviatoric plane and the meridian plane which collectively offer a general three-dimensional description of strength anisotropy. All the parameters introduced in the criterion can be conveniently determined by conventional laboratory tests. We demonstrate that the new criterion is general and robust in describing the variation of strength with loading direction for a wide range of materials. The failure criterion has been applied to the prediction of strength for several clays, sands and rocks reported in the literature. The predictions compare favorably with available experimental data. Further discussion is made on possible improvement of the new criterion to address other materials with complex strength characteristics, as well as its potential usefulness for constitutive modeling of anisotropic geomaterials.

No abstract

No abstract

A constitutive, non-isothermal unified hardening (UH) model is presented to interpret the thermo-elasto-plastic behaviours of normally consolidated and overconsolidated clays. Two yield surfaces are adopted in the proposed model: the current yield surface and the reference yield surface. A UH parameter (H) is developed to describe the evolution of the current yield surface, and the plastic volumetric strain is employed to quantify the hardening of the reference yield surface. The similarity ratio (RT) between the current yield surface and the reference yield surface, which is a function of the temperature and the plastic volumetric strain, is developed to govern the volume change behaviour and the shear strength of soils with different stress histories and at varying temperatures. The performance of the proposed model is then discussed in five typical scenarios: isotropic heating and cooling, drained/undrained triaxial compression with constant temperatures, and heating under constant non-isotropic states (drained/undrained). The mechanisms for thermal contraction/swelling and thermal failure are interpreted within the framework of the proposed non-isothermal UH model. Finally, the proposed model is validated through test results in the literature: heating/cooling tests, temperature-controlled drained triaxial compressions, and temperature-controlled undrained triaxial compressions.