An elastic-viscoplastic model for time-dependent behavior of unsaturated soils

Modeling the time-dependent behavior of unsaturated soil is a topic that is poorly addressed by the current literature. This paper presents a novel elastic-viscoplastic constitutive model for reproducing the time-dependent behavior of unsaturated soil. The model is established using the theoretical framework of the Unified Hardening (UH) model and the Barcelona Basic Model (BBM) combined with Perzyna's overstress theory. A unified hardening parameter is introduced to modify the elastic-viscoplastic constitutive model to consider the effect of the loading history. A new expression for the secondary consolidation coefficient considering the effect of suction is presented, and the state variables related to the void ratio of unsaturated soil considering the time effect are derived. Combined with the one-dimensional viscoplastic hardening parameter, the viscoplastic scalar multiplier in overstress theory is presented, and a three-dimensional elastic-viscoplastic constitutive model is then constructed. With 17 parameters, the proposed model can describe the effects of time and suction on the shear, dilatancy, and compression behaviors of unsaturated soil with various initial void ratios and stresses. Comparisons between the model predictions and experimental data are employed to validate the capability of the proposed model in terms of reproducing the time-dependent behavior of unsaturated soil or soft rock.