Geotechnical experimental observations of the influence of strain rate on the mechanical behaviour of soft soils express fundamental microscopic processes. The main macroscopical displays are creep, relaxation and strain-rate effect on strength. With these considerations in mind, an innovative constitutive formulation accounting for the time-dependency of the mechanical behaviour of soft soils is developed based on the unique effective stress-strain-strain rate concept. The performance of the proposed model is presented with numerical examples and comparisons with experimental data.