Fabric evolution of carbonate and silica sand under simple shear using synchrotron X-ray microtomography

Fabric anisotropy governs the mechanical behaviour of granular soils, yet direct microstructural observations during Direct Simple Shear (DSS) remain scarce, particularly for carbonate sands. Synchrotron X-ray microtomography was adopted to image full-scale (50 mm-diameter) DSS specimens of angular carbonate sand, sub-rounded silica sand, and rounded glass spheres in situ at pre-shear, intermediate strain (20%), and large strain states (30%), at 80% relative density and 25 kPa vertical stress. For this purpose, a radiolucent shear device was developed to enable scanning of full-scale specimens without miniaturisation or resin fixation, mitigating boundary artefacts. Image analysis quantified scalar and tensorial fabric descriptors. The results demonstrate that (i) coordination number ranked in the order of carbonate, silica sand, and glass spheres, reflecting decreasing particle angularity; (ii) carbonate sand exhibited the strongest contact-normal and branch-vector anisotropy, whereas silica sand exhibited the strongest tangent-vector anisotropy; (iii) fabric anisotropy remained the highest for carbonate sand and lowest for glass spheres. Image-derived particle-size distributions showed no systematic shift towards finer sizes, indicating no evidence of appreciable particle breakage at 25 kPa. These particle-scale observations provide quantitative evidence of material-dependent differences in fabric evolution among granular materials with contrasting morphology and mineralogy, supporting the development of anisotropy-informed constitutive models