X-ray-aided characterisation of micro-hydro-mechanical behaviour of unsaturated sand using a suction-controlled mini-triaxial system

Despite the advancement of the knowledge of unsaturated soil behaviour developed over the past decades, the understandings remain largely at the macro- or element- scale. Existing testing systems allow simultaneous in-situ loading and X-ray imaging to facilitate the studies at micro- or pore- scale, but the soils of concern are normally two phase (i.e. dry or saturated). This paper develops a new suction-controlled miniature triaxial apparatus for studying the micro-hydro-mechanical behaviour of three-phase unsaturated soils via in-situ quantification of the evolutions of soil microstructures, aided by micro-X-ray computer tomography imaging. The apparatus can independently control combinations of net mean stress and matric suction whilst permitting high-resolution full-field imaging of the entire samples through the image analysis via methods of pore network modelling and discrete digital image correction. Results reveal that at equilibrium, suction at the ‘transition zone’ of the soil does not necessarily mean to have both the air and water phases continuous, as otherwise recognised at the element scale. This phenomenon leads to nonuniform distributions of local void ratio and degree of saturation. Post-peak strain localisation and shear band development are explained by the movements of pore fluid and the associated influences on local soil void ratio change.