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

The evaluation of critical state strength parameters is important, especial ly 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 usua lly anisotropic, and the stress paths imposed by external loading could dev iate 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 f rom undrained triaxial and plane strain tests on anisotropically consolidat ed clays. Using Lade's failure criterion and the plastic potential of MCC, a generalized yield surface is determined and thr relationships between the critical state internal friction angles From various triaxial and plane st rain 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 c lays. The undrained strength normalized by the preconsolidation pressure is not a constant but a function of the angle of internal friction and the OC R. For overconsolidated clay, the predicted undrained strength ratio (s(u)/ sigma'(vo)) for the triaxial compression condition is larger than that for the plane strain compression condition. Comparisons of predicted results wi th published data indicate that the procedure is applicable to various comp ression tests on normally and lightly to moderately overconsolidated clays.