Y. Shi et al. (Eds.): ICCS 2007, Part III, LNCS 4489, pp. 1138–1145, 2007. © Springer-Verlag Berlin Heidelberg 2007 An Elastoplastic Hydro-mechanical Model for Unsaturated Soils De’an Sun and Li Xiang Department of Civil Engineering, Shanghai University, 149 Yanchang Road, Shanghai 200072, China Sundean06@163.com Abstract. This paper presents a coupled elastoplastic constitutive model for predicting the hydraulic and stress-strain-strength behaviour of unsaturated soils. Hydraulic hysteresis in the water-retention behaviour is modeled as an elastoplastic process with the elastic region of the saturation degree. The effect of change in degree of saturation on the stress-strain-strength behaviour and the effect of change in void ratio on the water-retention behaviour are taken into consideration in the model, in addition to the effect of suction on the hydraulic and mechanical behaviour. Model predictions of the stress-strain and water- retention behaviour are compared with those obtained from triaxial tests on unsaturated soil along isotropic compression, triaxial stress paths with or without variation in suction. Keywords: Unsaturated soil, elastoplastic model, water-retention curve, suction, triaxial test. 1 Introduction Since the model was proposed by Alonso et al. [1], several elastoplastic constitutive models for unsaturated soils have been proposed, on both the experimental and theoretical studies. In the early models, the stress state variables employed in most models for unsaturated soils are the net stress (the difference between total stress and pore-air pressure) and the suction (the difference between pore-air pressure and pore- water pressure). In these models, the influence of unsaturation on the soil behaviour is considered through the variable of suction and the degree of saturation is computed from the suction using the soil-water characteristic curve. Hydraulic hysteresis which accounts for irreversible behaviour between the suction and the saturation is usually not considered in these constitutive models. Another shortcoming of these models is related to their implementation into existing finite element codes for saturated soils where constitutive relations are always described in terms of effective stresses. As discussed by Sheng et al [4], using the average skeleton stress and the suction as the stress variables leads to a convenient implementation of unsaturated soil models into finite element codes working with effective stresses, whereas using the net stress and the suction is more appropriate for finite element codes working with total stress.