Experimental assessment of 3D particle-to-particle interaction within sheared sand using synchrotron microtomography

The paper presents a quantitative assessment of particle orientation, contacts and spatial variation of void ratio in a sand specimen loaded under plane-strain conditions. The specimen has a prismatic shape, and measures 57 mm wide X 121 mm long X 180 mm high. It was prepared using a dry natural silica sand known as F-75 Ottawa sand at a dense packing, and was sheared until failure. The specimen failed along a well-defined single shear band. It was then stabilised by slowly percolating a low-viscosity epoxy. Cylindrical subsamples 8 mm in diameter and 50 mm high were cored from the stabilised specimen at regions of interest for further investigation. A synchrotron X-ray microtomography technique was used to acquire high-resolution three-dimensional images of the cores. The scans have a voxel size of 15.06 mu m, which is high enough to visualise individual sand particles. Post-scanning analyses included mapping the spatial distribution of void ratio and particle contacts inside and outside the shear band. A good correlation was found between void ratio and particle contacts. Statistical distributions of particle orientation within and outside the shear band were calculated: they show that most particles inside the shear band orient in the x-y plane towards the y-axis, which is perpendicular to the shear band with angle in the range 70-90 degrees. Particles outside the shear band have no preferred orientation in the x-y plane, and form chains with preferred angles of 30-60 degrees from the direction of the applied major principal stress.