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

Soil is a multiphase material with irregularly shaped particles, the behaviour of which is highly nonlinear, anisotropic, and inelastic. Anisotropy and its evolution characteristics are crucial in soil's constitutive modelling, particularly for clayey soil. A new anisotropic constitutive model for remoulded clay is proposed based on improved yield surfaces and rotational hardening rules for initial and induced anisotropy. The logarithmic interpolation function is adopted to describe the volumetric deformation characteristics of clay in an arbitrary stress state, and a flexible yield surface shape is obtained by changing the newly introduced parameter n. This new parameter can be determined by fitting the undrained shear strength or stiffness of stress–strain curves. A key feature of the proposed evolutional rule for anisotropy is that the principal direction of the yield surfaces in the stress plane can be consistent with the isotropic stress axis when the critical state is reached. The proposed elastoplastic model shows good capacity for describing laboratory results from oedometer tests with constant stress ratios, reconsolidation tests with various stress ratios, triaxial compression tests, and triaxial extension tests under both undrained and drained conditions.