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.

This paper presents a semi-analytical solution for calculating soil stresses and pore pressures in the vicinity of an expanding cylindrical cavity. The main features of the solution are: i) realistic modelling of time-dependent soil behaviour, by means of an elastic-viscoplastic constitutive model, and ii) accurate prediction of soil strength under the stress path followed during plane-strain undrained cavity expansion, by using the transformed stress method to formulate the solution in the three-dimensional stress space and adopting an appropriate failure criterion. Predictions of the presented solution are benchmarked against a published solution, and are comprehensively analysed to identify the effect of the failure criterion and of the cavity expansion rate on the soil stresses and pore pressures developing in the vicinity of the cavity, and the associated stress paths.