Link between the macroscopic behaviour and the micro-structural changes in partially saturated sand studied by X-ray tomography

Partially saturated soils are three phase mixtures of soil particles, pore water and pore air. Interaction among the phases at the grain scale, in particular the suction due to water menisci, is a key issue to understand the overall mechanical behaviour of such materials.

This project aims to reveal some microscopic mechanisms of partially saturated soil behaviour: microstructural changes during deformation up to failure as well as water distribution in pore space and its relation to suction levels. This will lead to better understanding of the contribution of suction to the stiffness and strength in geomaterials.

These microscopic mechanisms are investigated through triaxial compression tests and water retention tests on sands. The samples are imaged during these tests using computed tomography (CT) in order to quantify local evolution in these sample: trinarisation of the images are performed to distinguish the therr phases, grain kinematics are determined and the water distribution is characterised. These techniques are non-destructive, quantitative and can provide very detailed analysis by giving the evolution of local density, grain positions, local degree of saturation, liquid bridges characteristics. These evolutions at the local scale are induced by the macroscopic loadings in terms of stress (triaxial compression) or suction (water retention tests).

The effect of suction on the macroscopic mechanical characteristics and their changes during deformation have the potential to be a fundamental physical background for modelling deformation and failure of partially saturated soils.