UH model: three-dimensional unified hardening model for overconsolidated clays

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.