Constitutive Model for Describing the Fully Coupled Hydromechanical Behavior of Unsaturated Soils

Many constitutive models have been developed to describe hydromechanical characteristics of unsaturated soils. However, most models lack the capability to adequately simulate the change of the yield stress with suction, the deformation of a soil over the drying process, and the soil volume change as soil transitions from a saturated state to an unsaturated state. The Sheng, Fredlund, and Gens (SFG) model was developed to address these issues. This model has been shown to perform better than most models in the literature in modeling the yield surface, as well as the consolidation and shear behavior of the unsaturated soils. However, two deficiencies are associated with the SFG model. The SFG model utilizes a constant small-strain shear modulus to estimate the elastic shear strain, although the small-strain shear modulus changes with hydrological variations in the environment. In addition, the hysteresis function used to govern the water content-volume change relationship under loading conditions requires at least one set of unsaturated constant suction consolidation data. These data must be obtained via performing very advanced and time-consuming tests. In this current study, the original SFG model was modified such that a unique hydrologic behavioral model was developed to predict the small-strain shear modulus of the unsaturated soils. In addition, the hysteresis equation was modified based on a redefined water content-volume change function. The proposed modified-SFG model was found to perform very well in simulating the hydromechanical behavior of various unsaturated soils over several loading and suction conditions.