The aim of this paper is to model the rate-dependent behavior of soft soil by means of a microstructural approach. A new microstructural elasto-viscoplastic model was developed, based on the granular mechanics approach and the overstress theory of Perzyna. The deformation of a representative volume of the material is generated by mobilizing particle contacts. Thus, the stress-strain relationship can be derived as an average of the mobilization on these local contact planes. The local behavior is assumed to follow a Hertz-Mindlin's elastic law, a Mohr-Coulomb yield criterion for shear sliding and a cone-shaped yield function for normal compression. Only a few triaxial tests, with a stage of isotropic consolidation followed with an undrained shearing up to failure at critical state, are required to calibrate the model parameters. Undrained triaxial tests on Lower Cromer Till are simulated by the proposed model. An investigation also considers the rate-dependent behavior. All test simulations, compared to experimental results, demonstrate the ability of the proposed model to reproduce accurately the rate-dependent behavior of soft soil.