Bounding surface model for overconsolidated clays with new state parameter formulation of hardening rule

The paper presents a critical state bounding surface constitutive model for describing the behaviour of overconsolidated clays for a wide range of overconsolidation ratios, called the Hardening State Parameter - HASP model. Keeping the simplicity and the same set of the parameters as Modified Cam Clay, the HASP model provides a more realistic description of numerous elements of clay behaviour. With the new state parameter formulation of the hardening rule, elasto-plastic behaviour is enabled from early stages of loading. In undrained conditions, general shape of effective stress paths for various overconsolidation ratios and negative pore pressure is well predicted. In drained conditions, strain hardening, strain softening, as well as smooth transition from contractive to dilatant behaviour before the peak strength is reached are described. Verification is done against published results of drained and undrained tests in triaxial compression and extension, on the samples with various overconsolidation ratios.