Undrained Shear Strength Of K₀ Consolidated Soft Soils

The Undrained shear strength of soft soils is not only the classic issues of soil machanics, but also the key parameter for slope stability analysis. Based on the research findings of the other scholars, the paper discusses the undrained shear strength of K₀ consolidated soft soils both from theoretical and experimental way.First, a brief review of the shear strength theory of soft soils was made and several factors which affect the undrained shear strength were analyzed. All kinds of measurement of the undrained shear strength were recited, including in-situ tests and laboratory tests, merits and shortcomings of both were analyzed. Available theories for analysis of the undrained shear strength were presented and compared.Secondly, the classic model of critical state soil mechanics—Cam-clay model was introduced. As it is an isotropic constitutive model, the paper extends it to an anisotropic constitutive model which involves K₀ consolidation, then theoretical formulas of the undrained shear strength of soft soils under triaxial compression and extension conditions were derived. After comparison with the theories of Ohta and Nishihara (1985) and Li-zhong Wang et al(2005), it is found that there is a better agreement with the available test data and the theories presented by the paper. Based on investigation of soil properties, the paper proposes a simple formula for calculating the mean undrained shear strength, which is meaningful for engineering practice.Thirdly, it is pointed out that the shear strength at peak state, critical state and residual state are totally different, we must pay attention to them both in study and engineering practice. 8 series of triaxial tests of Ningde clay were performed on GDS triaxial system, based on the available test data, the critical state of saturated soft clay was analyzed, after that, the paper concludes that the critical state is single and has no relationship with the initial stress state.Finally, the constitutive model proposed in the paper was extended to three dimensional principal stress space using transformed stress method, and the yield locus was revised by SMP criterion. The new model can solve problems not only under axial symmetrical conditions, but also under general stress conditions such asplain strain state. On the other hand, it is found that the effective frictional angle under plain stain state is larger than that of triaxial compression, which is in accordance with the most extant research findings.