Bounding Surface Plasticity Model for Clay under Cyclic Loading Conditions Considering Fabric Anisotropy

Abstract

Cyclic loading is encountered in several practical geotechnical problems. Understanding and modeling cyclic soil responses are key to engineering analysis and design. As one of the most distinct features of soils, fabric anisotropy plays an essential role in the soil response to cyclic loading, such as pore-pressure development dependent on the interplay between fabric and loading direction, effective stress path inclination related to initial fabric, and stiffness variation associated with bedding-plane orientation. However, most previous clay models developed within the bounding surface framework use the rotational angle, a stress-ratio-type scalar, to describe fabric anisotropy and its evolution, which fails to comprehensively capture the anisotropic responses. In this study, a deviatoric fabric tensor, instead of the commonly used rotational angle, was used to describe the internal microstructure within the framework of anisotropic critical state theory. A scalar-valued anisotropic fabric variable quantifying the interplay between the fabric tensor and loading direction was used to account for the impact of anisotropy on both dilatancy and strength, aimed at simulating the typical ‘butterfly-shaped’ stress loops, and varying rates of stiffness degradation and pore-pressure accumulation of samples with different bedding-plane directions. The initial fabric tensor was also introduced into the elastic expression to replicate the inclined undrained stress paths, as well as the variational degree and direction of inclinations due to different bedding-plane orientations. The predictive capability of the proposed model was demonstrated by simulating three typical clays in undrained and drained conditions, with varying stress and strain amplitudes. The model can capture the major influences from the initial fabric anisotropy and its evolution of clay, i.e., the typical ‘butterfly-shaped’ stress loops and the bedding-plane direction-dependent effective stress path, the pore-pressure generation, stiffness variation, and strain accumulation.