Modified UH model: Constitutive Modeling of Overconsolidated Clays based on a Parabolic Hvorslev Envelope

Most clays, either naturally deposited or man-made, possess a certain degree of overconsolidation owing to tamping, cyclic loading, erosion, excavation, and/or changes in groundwater tables. An easy-to-use constitutive model for overconsolidated clays is useful for relevant engineering applications. In this paper, a simple model is proposed for overconsolidated clays based on the unified-hardening (UH) model. To evaluate the potential peak stress ratio of overconsolidated clays, a parabolic Hvorslev envelope rather than a straight envelope (used in the original UH model) is adopted. The proposed parabolic Hvorslev envelope passes through the origin of the mean stress-deviatoric stress plane. It has a slope of 3 as the overconsolidation ratio (OCR) approaches infinity and intersects with the critical state line as the OCR reaches unity. This modification leads to more realistic predictions for highly overconsolidated clays than does the original UH model with a straight Hvorslev envelope and is consistent with the critical state soil mechanics in which the higher peak stress ratio in overconsolidated clays is a result of interlocking (or dilatancy) rather than cohesion. The modified UH model retains the same parameters as those in the modified Cam-clay model. Reasonable agreement between the model predictions and experimental data demonstrates that the modified model is capable of addressing the fundamental behavior of overconsolidated clays. The present model is developed for reconstituted clays with an isotropic fabric. The potential improvement of the model, taking into account anisotropy and structural effects, is discussed