This paper develops a new time-dependent hypoplastic model for normally consolidated and overconsolidated clays. A novel viscous strain rate formulation is derived from the isotach concept and incorporated into the total strain rate of the hypoplastic framework, allowing for viscous deformation at the onset of loading. The hypoplastic flow rule is defined for the direction of the viscous strain rate and its intensity directly linked to the overconsolidation ratio (OCR) and secondary compression coefficient. The Matsuoka-Nakai criterion is further introduced into the strength parameter through the transformed stress technique, enabling the model to describe the stress-strain-time behaviour of clays in general stress space. In addition, a new scalar function is proposed and implemented into the model to consider the OCR effect on the initial stiffness. The model predictive ability is finally examined by simulating laboratory tests on three different clays with various OCRs and stress paths, demonstrating that the model can capture the rate dependency, stress relaxation, and creep behaviours for both normally consolidated and overconsolidated clays under various loading conditions.
