Critical state strength parameters of saturated clays from the modified Cam clay model

The evaluation of critical state strength parameters is important, especially with the introduction of limit state design. The modified Cam clay (MCC) model is often used, but it is suitable mainly for evaluating the critical state strength parameters from triaxial compression tests on isotropically consolidated soils. The initial stress condition of a natural soil is usually anisotropic, and the stress paths imposed by external loading could deviate from that of a simple triaxial compression. The use of MCC in practice deserves careful consideration. This paper describes a proposed extension of the MCC model for the evaluation of critical state strength parameters from undrained triaxial and plane strain tests on anisotropically consolidated clays. Using Lade's failure criterion and the plastic potential of MCC, a generalized yield surface is determined and the relationships between the critical state internal friction angles from various triaxial and plane strain tests are obtained. By relating the isotropic overconsolidation ratio to the conventional overconsolidation ratio (OCR), a procedure is suggested for the prediction of critical state undrained shear strength (s u ) for clays. The undrained strength normalized by the preconsolidation pressure is not a constant but a function of the angle of internal friction and the OCR. For overconsolidated clay, the predicted undrained strength ratio (s u /σ VO ) for the triaxial compression condition is larger than that for the plane strain compression condition. Comparisons of predicted results with published data indicate that the procedure is applicable to various compression tests on normally and lightly to moderately overconsolidated clays. Key words: stress anisotropy, clay, constitutive model, critical state, laboratory test, shear strength parameters.