Continuous bidirectional shear moduli monitoring and micro X-ray CT to evaluate fabric evolution under different stress paths

Fabric evolution monitoring of sandy specimens during shearing up to critical state is characterized by continuous, bidirectional shear wave velocity measurements along the vertical and horizontal directions (V&H). The specimens are prepared by water sedimentation methods and then subjected to drained compression and extension loading paths. The results exhibit a significant differences between shear wave velocities in two orthogonal directions, and subsequently shear moduli, as shear develops. Not only do the differences between shear wave velocities in V and H directions illuminate a severe and increasing soil anisotropy during the shearing, but the results also signify promising information related to the current fabric and stress state. Comparison between compression and extension results highlight different fabric evolution trends and consequently dissimilar fabric states at the critical state. Considering the conforming results with recent findings on the basis of the discrete element method (DEM), the proposed method can be used as an experimental method facilitating the macroscopic investigation of the effects of fabric anisotropy on the soil elastic response. The fabric anisotropy and its evolution are assessed consecutively using three methods, including quantitative evaluation of shear moduli, proposing a fabric function to account for the soil fabric, and 3D microscopic inspection of Micro-CT slices. The findings of the mentioned methods agree on the importance of fabric anisotropy in shear wave propagation and microscopic variations towards the critical state evolving from the initial state to dissimilar anisotropic states at the critical state under different shear modes.