Inherent and stress-induced stiffness anisotropy of natural granite residual soil

Because of the weathering pedogenesis of natural granite residual soil (GRS), its mechanical responses are dominated by the parent rock and weathering process, a situation that differs fundamentally from that of sedimentary soil. Natural GRS is characterized by inherent anisotropy and anisotropic in situ stress conditions, but how this affects its stiffness behavior is not well understood. In this study, systematic bender-element and modified Hardin-type resonant-column tests were performed under various stress conditions (isotropic or anisotropic), thereby enabling the shear wave velocities and shear modulus in different directions to be measured. The degree of inherent stiffness anisotropy of natural GRS is quantified and compared with those of well-studied soils. Natural GRS is found to have apparent stiffness anisotropy, with the shear wave velocity in the vertical direction being higher than that in the horizontal direction. This unique stiffness anisotropy behavior can be attributed to the unique structural characteristics of natural GRS, in which cementation and minor fissures play important roles. Under stress anisotropy, natural GRS has the same shear wave velocity hierarchy as that under isotropic conditions, but vertical loading alters the soil structure as reflected by the change of soil fabric constants. In addition, a higher stress ratio corresponds to a larger shear modulus of both natural and reconstituted GRS, but it affects them differently, with cementation again playing an important role. This study provides a fundamental dataset regarding the stiffness anisotropy of natural residual soil and improves the understanding of its stiffness behavior.