Multiaxial stress-fractional plasticity model for anisotropically overconsolidated clay

Abstract

Clays in the field can experience various consolidation histories, e.g., isotropic or anisotropic overconsolidation, before subjected to three-dimensional loads exerted by the neighbouring geotechnical infrastructures. Correct representation of the multiaxial strength and deformation behaivour of anisotropically overconsolidated clay is a prerequisite for engineering design. Accordingly, a multiaxial constitutive model is developed in this study, by employing the anisotropic stress-fractional plasticity two surfaces. The consolidation history is represented by using an inclined bounding surface, while the loading behaviour in multiaxial stress space is captured through the stress transformation of an inclined loading surface. The incorporation of such stress transformation method overcomes the difficulty in obtaining the fractional derivative of the Lode's angle. To capture the dilatancy behaviour of anisotropically overconsolidated clay, a multiaxial state-dependant nonassociated plastic flow rule is developed by only using the stress-fractional gradient of the loading surface, which significantly differs from the prior attempts using traditional plasticity. In addition, a multiaxial rotational hardening law for the bounding/loading surface is suggested to characterise the shearing-induced fabric destruction of anisotropically overconsolidated clay. Finally, test results of two different clays under various consolidation histories and stress paths are adopted to validate the developed model. It is found that the predicted strength and deformation behaviour of clay depends the Lode's angle and anisotropic consolidation path, which agrees with the corresponding test results. Specifically speaking, the hardening and softening behaviour of clay induced at seven different overconsolidation ratios ranging from 1 to 10 can be quantified, and the multiaxial strength behaviour of clay loaded from a wide range of Lode's angles ranging from 0° to 60° can be also quantified.