Incorporating porosity-dependent hysteretic water retention behavior into a new constitutive model of unsaturated soils

A new unsaturated soil model was recently developed at Ecole Nationale des Travaux Publics de l'Etat (ENTPE), based on the concept of bounding surface plasticity. The first version of this model (by Morvan et al. in 2010) only required 12 parameters for its definition, and yet can simulate a few behaviors typically observed on unsaturated fine sands, which more classical models fail to reproduce. These include post-peak softening, change from contractant to dilatant behavior during shearing, and the smooth transition from elastic to elastoplastic behavior upon yielding. Subsequent to this first publication, additional mechanisms have been introduced in the model previously developed. Among them, the hysteretic phenomenon during cyclic variations of suction and water content, as well as the effects of porosity change on water retention characteristics, are now taken into consideration. To this end, the water content is introduced as an independent generalized strain variable, which is no longer uniquely linked to suction. Thanks to these developments, the model can now account for the bilateral couplings between hydraulic and mechanical quantities, with good precision under very general loading conditions. In the present paper, these new developments are presented in detail, and the experimental basis of this construction is discussed. The model is then validated using existing experimental data on kaolin, by considering two stress paths. The first stress path consists of a single cycle of wetting followed by drying at constant isotropic stress. The second, more complex loading path involves simultaneous variations of suction and isotropic compression or decompression. The results confirm overall the satisfactory performance of the model. The main conclusions are then summarized in the last section, including perspectives for future developments.