Effect of degree of saturation on mechanical behaviour of unsaturated soils

The effect of the unsaturated condition in soils is commonly expressed through suction. It is generally accepted that the suction and degree of saturation have a significant effect on the mechanical behaviour of unsaturated soils. However, the effect of degree of saturation is generally not included in the majority of existing elasto-plastic constitutive models. It is believed that inclusion of degree of saturation in constitutive models for unsaturated soils could lead to significant simplification for practical purposes. This paper presents the results of an investigation into the effect of degree of saturation on the behaviour of unsaturated silty soil in the light of a comprehensive set of experiments. The variation of degree of saturation during loading/unloading, wetting/drying and reloading is studied. The results show that the degree of saturation has a direct effect on the behaviour of unsaturated silty soil. The applicability of degree of saturation in an elasto-plastic constitutive model proposed in the literature is examined in the light of the experimental data and a suggestion is made for possible improvement in modelling of unsaturated soils.