SSUH model: A small-strain extension of the unified hardening model

A small strain unified hardening (SSUH) model is proposed in the present study to tackle the small strain behavior of clay. The model is an extension of the unified hardening (UH) model for overconsolidated (OC) clays accounting for the small strain stiffness. The new features of the SSUH model over the UH model include: (a) a new elastic hysteretic stress-strain relationship to evaluate the stiffness degradation at small strains and to generate the hysteresis loop under cyclic loading condition; (b) a revised unified hardening parameter to enhance the plastic stiffness at small strains; and (c) a new overconsolidation parameter, which is crucial to make the UH model working with the elastic hysteretic stress-strain relationship effectively. With these enhancements, the SSUH model can describe a high initial stiffness and the highly nonlinear stress-strain relationship at small strains, in addition to the shear dilatancy and strain hardening/softening behaviors of OC clays at large strains. In comparison with the Modified Cam-clay (MCC) model, the proposed model needs two more small strain related parameters, which can be easily obtained from laboratory tests. Finally, some drained triaxial compression tests at large strains, drained triaxial compression/extension tests at small strains, an undrained compression test at small strains and a drained cyclic constant radial stress test are employed to validate the new model.