Application of X-ray Tomography to the Characterisation of Grain-Scale Mechanisms in Sand

Abstract

X-ray micro-tomography allows 3D imaging at sufficiently high spatial resolution to distinguish all the individual sand grains in a small sample (10mm diameter), as well as the distribution of air and/or water at this scale. Since this imaging technique is completely non-destructive, an imaged sample can be made to evolve by controlling some relevant variable (e.g., imposed deformation, suction), and can subsequently be re-imaged. This allows processes to be followed in 4 dimensions (3D + relevant variable). This paper shows the application of this technique and philosophy to the study of two different phenomena: localised deformation resulting from imposed triaxial compression, and the water retention behaviour of sand. The experimental techniques and setups for these two studies are detailed, and the fundamental steps of image treatment are outlined. Some key results are given to demonstrate the power of this “full-field” characterisation approach, such as rotations and displacements for each of the 50,000 grains of a sample in which a shear band occurs as well as the evolution of local measurements of porosity and degree of saturation in a sand where suction is being varied.