Multiscale structural characterizations of anisotropic natural granite residual soil

While the anisotropy of sedimentary soil—particularly the underlying role of soil structure—is well understood, similar knowledge about granite residual soil formed by weathering is scarce. In particular, the evolution of soil structure during the hollow cylinder torsional shear tests (among the most appropriate for studying soil strength anisotropy) remains largely unknown. This study systematically investigates the multiscale structural properties of granite residual soil using stereo microscopy, scanning electron microscopy, energy-dispersive spectrometry, and computed tomography. Furthermore, the structural evolution during hollow cylinder torsional shear tests is traced. Results indicate the strength anisotropy of residual soil is associated with horizontal fissures which cause low shear strength when the soil is sheared with a principal stress direction of 45°. The cementation via Fe is characterized by a uniform distribution. Different evolutionary patterns are observed for soils within the shear band, inside the necking area, and in other regions. It is not always the case that particles are rearranged with their long axis perpendicular to the major principal stress. This study improves the understanding of natural residual soil structure and provides some insights into its anisotropic behavior.