Significance of anisotropy and structure in simulating natural clays

In geotechnical design, the simulation of soil behaviour is the most important facet. Often the behaviour of clay soils is modelled using the modified-cam-clay model (MCC), because it is readily available in many commercially available finite element programs used by practitioners for geotechnical design. The MCC model is formulated considering the experimentally observed remoulded clay behaviour considering isotropic consolidation conditions and developed within the critical state framework with an elegant formulation with a clear physical interpretation. However, natural clays in their in situ undisturbed state are anisotropic and have a structure, which gives an additional strength compared to the same soil in the reconstituted state. Hence, the simulation capability of the MCC model is considered inadequate for natural clays. The focus of this paper is to investigate the significance of the anisotropy and structure of natural clays using the SANICLAY family of soil models implemented in the finite element modelling program ABAQUS. The basic model behind all SANICLAY models is the MCC model. In SANICLAY models, the anisotropy of natural clays is introduced by rotational hardening where yield and plastic potential surfaces are rotated and distorted to an ellipse. Two distinct types of destructuration included in SANICLAY are isotropic and frictional. Isotropic destructuration is introduced through the softening of the yield surface. The frictional destructuration replicates the reduction in friction angle due to the breakage of soil structure. The paper presents numerical simulations for triaxial tests conducted on undisturbed Bothkennar clay samples to show the capability of the chosen soil model in simulating the experimentally observed natural clay behaviour. Then a parametric study will be presented emphasising the importance of anisotropy and structure of natural clays comparing results from the MCC and SANICLAY family of models. Results of this study show that the anisotropy and structure are important parameters to simulate the behaviour of natural clays.