Elastoplastic Constitutive Model Describing Dilatancy Behavior of Overconsolidated Clay

Abstract

Overconsolidated (OC) clay has a strong dilatancy property, which has a significant effect on ultimate strength and deformation. Compared with normal consolidated (NC) clay, OC clay tends to show a higher strength–stress ratio, lower shear shrinkage, and larger dilatancy, as well as strain hardening and softening. A unified hardening (UH) model is a simple and practical model to describe the stress–strain relationship of OC soil. However, the degree of overconsolidation has a direct effect on dilatancy, which is mainly demonstrated by: (1) the degree of overconsolidation is directly related to the phase transformation stress ratio (Mc) that corresponds to the occurrence of dilatancy. The greater the degree of overconsolidation, the smaller the Mc; and (2) the greater the degree of overconsolidation, the smaller the volume shrinkage strain and the larger the volume dilatancy strain. In a UH model, the stress ratio (η) of the phase transformation is a constant and its value is simplified to be equal to the stress ratio of the critical state (M). A UH model cannot be employed to reflect the fact that the Mc varies with the degree of overconsolidation. To overcome the previous problems, the Mc in the dilatancy equation will be expressed as a power function of the overconsolidation stress ratio parameter (R). To reflect the characteristics of large volume shear shrinkage of underconsolidated (UC) soil, the nonassociative flow rule will be adopted, and the shape of the yield surface modified by state parameters is a water drop surface and the plastic potential surface is an elliptic surface. The improved model could better reflect the double influence of the degree of overconsolidation on dilatancy characteristics.