Shear Strength Anisotropy of Natural Granite Residual Soil

A significant feature of residual soil is the presence of a cemented structure and fissures that dominate its anisotropic behavior. Although the anisotropy of some sedimentary soils is well understood, that of residual soils is not. The present study investigates the strength anisotropy of granite residual soil via undrained hollow cylinder torsional shear tests for which intact and remolded hollow cylinder specimens were sheared in various principal stress directions α. The results reveal significant shear-strength anisotropy of the natural soil, whereas the remolded soil behaved almost isotropically. Expressed using the effective stress ratio at ultimate state t/s′, the shear-strength variation reached 51% for α=0–90°, within which range the highest values of t/s′ was that for α=0°. The torsional shear mode (α=45°) resulted in the lowest soil strength. The anisotropic behavior of the studied soil differs much from that of some normally consolidated sedimentary soils such that new parameters are proposed for evaluating the degree of anisotropy. The proposed parameters confirmed the marked strength anisotropy of intact granite residual soil and found the anisotropy degree of soil significantly reduced after being remolded. This study provides the most direct evidence to date for the strength anisotropy of natural granite residual soil and enhances the understanding of this soil and weathered geomaterials in general.