A Consititutive Model For Overconsolidated Clays

The naturally deposited clays generally exhibit overconsolidated(OC)characteristic.Compared with normally consolidated(NC)clays,OC clays have lower void ratio,higher strength and show dilatancy and softening behavior.The OC model,which is proposed by studying the stress-strain characteristics of OC clays and UH(Unified Hardening)model,is developed to a model for OC clays with the initially stress-induced anisotropy.Combining the model for OC clays with the initially stress-induced anisotropy and the transformed stress method,the unified undrained strength formula for OC clays with the initially stress-induced anisotropy is deduced.Constitutive model for OC clays:The constitutive model for OC clay,which is based on the modified Cam-clay model and the concept of the subloading surface,is a simple and applied elasto-plastic model and is the same as the modified Cam-clay model for the NC clays.Comparing with the modified Cam-clay model,the OC model requires an additional parameter,which is the slope of the Hvorslev envelope.The reference yield surface and the current yield surface are adopted to describe the tendency that the overconsolidated degree of OC clays decreases gradually in the loading process.The OC parameter R is derived from the functions of the current yield surface and the reference yield surface and the equation of the potential strength M_f is related to the OC parameter R,stress state,the slope of the Hvorslev envelope.By introducing the potential failure stress ratio M_f,characteristic stress ratio M into the unified hardening parameter,the OC clay model can describe many characteristics of OC clays,such as negative and positive dilatancy,hardening,softening and stress path dependency.The evolving rules among the current yield surface,the reference yield surface,the Hvorslev envelope,and the potential strength M_f are discussed by using p=const stress path as an example.Based on SMP criterion and the corresponding transformed stress tensors,the proposed model can be applied to the three-dimensional stress space to describe the deformation of clays under triaxial compression,extension and true triaxial condition.Using the loading criterion,the model can be applied to model the stess strain behaviur under complex loading conditions.OC clay model with the initially stress-induced anisotropy:Compared with the modified Cam-clay model,two additional parameters,which are the slope of the Hvorslev envelopeM_h and material parameter?,are required in the proposed model and can determined by the conventional triaxial test.By introducing the potential failure stress ratio M_f,characteristic stress ratio M and the state stress ratio?_k(newly proposed)into the unified hardening parameter,the model can describe characteristics of OC clays with the initially stress-induced anisotropy,including negative and positive dilatancy,hardening,softening and stress path dependency.The model can be used to describe three-dimensional stress-strain behavior by adopting transformed stress method on the basis of SMP criterion.Undrained shear strength for overconsolidated clays with initially stress-induced anisotropy:Combining the undrained condition at the critical state,which is derived from the referent yield surface,the rotation angle?,and the transformed stress tensors on the basis of SMP criterion,the unified formulation of the undrained shear strengths is given.The undrained shear strengths are proposed according to the stress Lode's angle and the rotation angle under triaxial compression and extension condition,respectively.Based on the plastic theory and the critical state soil mechanics,the undrained shear strengths can be acquired under plane strain condition.Constitutive model for OC clays is included in ADVANCED SOIL MECHANICS edited by Xie,Yao,Dang.