Time-dependent unified hardening model: Three-dimensional elastoviscoplastic constitutive model for clays

A new three-dimensional (3D) elastoviscoplastic (EVP) constitutive model for both normally consolidated and overconsolidated clays is presented. In developing the new model, first the time effects on clays were connected with the change of overconsolidation degree according to the concepts of aging time and the instant normal compression line. This made it convenient to combine a logarithmic creep function with the reloading function of the unified hardening (UH) model (a model for overconsolidated clays), and, thus, an isotropic EVP relationship was built. Second, a time variable derived from the isotropic EVP relationship was embedded into the current yield function of the UH model, and, thereby, a time-dependent current yield function is proposed. Based on the time-dependent current yield function and the flow rule, an EVP model for triaxial compression stress states was built. The EVP model for triaxial compression stress states then was extended to a 3D EVP model for general stress states through the transformed stress method. The proposed 3D EVP model can describe time-dependent behaviors, such as creep, relaxation, and loading rate effect, for both normally consolidated and overconsolidated clays, with two additional parameters compared with the modified Cam-clay model. Finally, numerous experimental results in the literature (such as the results of triaxial undrained creep tests, triaxial undrained compression and extension tests at the constant strain rate, and triaxial undrained tests at the stage-changed strain rate) were used to validate the new model.