SCLAY1S-BS: an anisotropic model for simulation of cyclic behaviour of clays

Abstract

Natural soils are usually lightly overconsolidated to overconsolidated clays that have a considerable degree of anisotropy in their structure. They also exhibit significant inter-particle bonding and the degradation of these bonds during straining largely influences their behaviour. Several constitutive models have been developed to capture the anisotropic response of soft natural clays (e.g., Wheeler et al. 2003, Dafalias et al. 2006), among which S-CLAY1 model proposed by Wheeler et al. (2003) and its extension S-CLAY1S (Karstunen et al. 2005) have been widely accepted to provide very good results in simulating the plastic anisotropy combined with destructuration effects observed in soft sensitive soils. However, the main focus of these models is on large plastic strains at primary loadings, and like every single yield surface model, they provide little or no flexibility in describing the change of the plastic modulus with loading directions. Hence these constitutive models, which are based on the classical concept of a yield surface, imply only a purely elastic stress range within the yield surface and therefore are best applicable to clays at normally consolidated to lightly overconsolidated states under monotonic loadings. To overcome the limitations of classical elasto-plastic models in simulating the loading unloading cycles, Dafalias (1986) modified the “Bounding Surface Plasticity” theory, initiallydeveloped for metals, for geomaterials. The salient features of a bounding surface formulation are that plastic deformation may occur for stress states within the yield surface, and the possibility to have a very flexible variation of the plastic modulus during a loading path. Using bounding surface plasticity concept, in this paper a new constitutive model is proposed for enhanced predictions of the cyclic behaviour in natural soils by capturing non-linearity and plasticity from the early stages of loading. The new model is based on further extension of the S-CLAY1S model through incorporation of a Bounding Surface formulation, hence it is referred to as SCLAY1S-BS model. The paper studies the variations of additional parameters on model performance; it also compares the simulation results from the new model with those from original S-CLAY1S model using experimental data of compression and extension tests over Kaolin Clay.