Most natural and engineered soils usually exhibit a state of partial saturation in water. Indeed, fast and heavy water flowing into the soil is known to cause collapse or soil plasticization which may be at the origin of harmful settlements. The mechanical loading conditions and the amount of confining have a pre-eminent influence on the intensity of these plastic phenomena. The presented work sets up an innovative constitutive framework for modelling soils partially saturated in water. The framework features (i) a stress-strain model with two mechanisms of plasticity, (ii) an advanced definition of the coupled stress and strain variables, and (iii) an accurate model of the soil water retention curve. The model is validated over different sets of experimental data, in particular with swelling pressure tests. The innovative concepts are intended for instance for the modelling of earth dams and landslides.