Morphological transitions for pore water and pore air during drying and wetting processes in partially saturated sand

Water retention characteristics are important for modeling the mechanical and hydraulic behavior of partially saturated sand. It is well known that the soil water characteristic curve shows hysteresis during drying and wetting processes. For a better understanding of the water retention characteristics of partially saturated soil, a microscopic investigation of the morphological transitions for the pore water phase and the pore air phase, such as volume distribution, spatial distribution and continuity during drying and wetting processes, is crucial. In the present study, different water retention states of a partially saturated sand were visualized during water retention tests using microfocus X-ray computed tomography (CT). The CT images obtained from the tests were segmented into the soil particle phase, the pore water phase and the pore air phase. Then, a series of image processing, erosion, dilation and cluster labeling was applied to the images in this order to quantify the cluster volume distributions, the number of clusters and the continuity of both the pore water phase and the pore air phase. The morphological transitions for the pore air phase and the pore water phase, subjected to decreasing and increasing degrees of saturation, were revealed using the results of the image processing, and then, the water retention states were characterized based on the morphologies for the two phases. The influence of the morphologies on the hysteresis was discussed.