Simulation of yield characteristics and principal stress rotation effects of natural soft clay

A three-dimensional structural anisotropic bounding surface model with a modified shape function of Lode angle is introduced to describe the effects of the principal stress rotation. One-dimensional consolidation tests and undrained triaxial tests under isotropic and anisotropic consolidation modes and a series of drained stress probe tests and hollow cylindrical undrained torsion shear tests with different coefficients of the intermediate principal stress are performed on natural undisturbed samples of Shanghai soft clay to study the yield characteristics and the effects of the principal stress rotation. The test results show that the properties of natural Shanghai soft clay are characterized by structural yield and anisotropic plasticity. The initial state bounding surface of Shanghai soft clay can be described by an inclined ellipse on the  p′-q plane, and the limit state surface is not symmetrical as well with respect to the K₀ line. The ratio of the critical stress decreases with the increase of coefficient of the intermediate principal stress. It shows that the equation of three-dimensional yield surface in the model is reasonable. Furthermore, the stress-strain relationship and strength of soft clay are influenced remarkably by the principal stress rotation. The parameters of the proposed model are calibrated by the undrained isotropic and anisotropic triaxial test results fornatural Shanghai soft clay. The validity of the model is verified by drained stress path tests and hollow cylindrical torsion shear test results. Comparison between the simulated and experimental results shows that the effects of loading paths and the principal stress rotation are well captured by the model.