A time dependent constitutive model for soils with isotach viscosity

This paper presents a three-dimensional elastic viscoplastic constitutive model that is able to reproduce the time dependent behaviour of soils with isotach viscosity. This constitutive law is based on the overstress theory and incorporates some important features, namely: (i) a non-linear creep law with a limit for the amount of creep deformation under isotropic stress conditions; (ii) a flexible loading surface that is capable of reproducing a wide range of shapes in p ′ − J stress space and incorporates the Matsuoka–Nakai failure criterion in the deviatoric stress space and (iii) assumes that the viscoplastic scalar multiplier is constant on a given loading surface to ensure that critical state conditions are reached. A full description of the model, its governing equations and implementation in a finite element program are presented. The model is then used to simulate the results of laboratory tests, highlighting the model’s abilities and shortcomings in reproducing the time dependent behaviour of soft clays. Furthermore, the paper investigates the importance of accounting for creep non-linearity and the consequences of isotach viscosity on the critical state line.