Anisotropic elastoplastic model for clays with improved yield surfaces and rotational hardening rule

A new anisotropic constitutive model is proposed based on the critical state theory and the rotational hardening rule. Firstly, an expression for yield surfaces is developed by employing the non-linear logarithmic function capable of describing volumetric deformation characteristics of anisotropic soils in a wide variety based on the results of the constant stress ratio loading tests. The shape of yield surfaces can be controlled by the introduced parameter n. The yield surface is elliptical for n=1, bullet for n < 1 and teardrop for n > 1. Then, the expression for the boundary value with rotational hardening rule under the virgin constant stress ratio loading is proposed. Under isotropic loading and critical conditions, the value of the expression reaches zero eventually. The model can be degenerated to the modified Cam-clay (MCC) when n=1 and the rotation of the yield surface is not considered. Finally, the explicit modified Euler method with automatic error control is used to numeralize the model, and the model is verified by the constant stress ratio loading tests, variable stress path tests, drained triaxial compression tests, undrained triaxial compression tests and undrained triaxial extension tests. The results show that the proposed anisotropic constitutive model can accurately describe the volume deformation, shear and strength characteristics of clays under a series of stress paths.