The evolution of morphology and fabric of a sand during shearing

Over the past 50 years, experimental studies have repeatedly demonstrated that the mechanical behaviour of sand is sensitive to the material fabric, i.e., the arrangement of the grains. Up until now there have been relatively few attempts to describe quantitatively the fabric of sands. In fact, most of our understanding of the link between the particle movements and interactions and the macro-scale response of granular materials, including sand, comes from discrete element modelling (DEM) and experiments on “analogue” sands with simple, idealized shapes. The aim of this study had been to describe quantitatively the particle morphology and fabric of reals and and their evolution under loading. The material investigated was Reigate sand (from Southeast England), a geologically old sand which, in its intact state, exhibits significant grain interlocking and nobonding. To explore the effects of fabric on the mechanical response of the soil, intact and reconstituted specimens both having similar densities were tested under triaxia lcompression. The specimens were impregnated with an epoxy resin at three different stages of shear deformation and small cores from each specimen were scanned using X-ray micro-tomography. Different systems and scanning parameters were explored in order to obtain three-dimensional high-resolution images with a voxel size of 5μm(0.018d50) and a quality level required for the identification of the individual particles and the surface defining each particle-particle contact.The quantification of particle size and shape has shown that breakage of fractured grains, along existing fissures, occurs both during reconstitution and shearing ofthe intact soil, a phenomenon that cannot be observed using invasive techniques such as sieve analysis. Statistical analyses of the distribution of fabric directional data in terms of particle orientations, contact normals, branch vectors and void orientations were carried out at each loading stage. It has been shown that the initial particle orientation fabric that develops during the deposition of the material tendsto persist during shearing, while the contact normals seem to be reorientated along the direction of the major principal stress in the post-peak regime. Different patterns were observed within the shear band as both the particles and the contact normals appeared to rotate towards the direction of the shear plane. The measurements from the tomographic data were complemented with a qualitative description of the morphology and fabric using SEM and optical microscope images of thin sections.