Microscopic investigation into density dependence of water retention characteristics of sand during drying-wetting process

The density dependence of the water retention characteristics of sand during the drying-wetting process was microscopically investigated. Water retention tests on both loose sand and dense sand were conducted with X-ray micro computed tomography (CT). The frequency distributions of the local porosity and degree of saturation within each sand specimen, the principal curvature of the air–water interface, and the volume distribution of pore water were analyzed using image processing techniques. The microscopic observations revealed that the density dependence of the water retention characteristics during the drying-wetting process primarily lies in the morphology of the pore water, the number of pore water clusters, and the volume distributions of the pore water clusters. The types of distributions of the local degrees of saturation and the volume distributions of the pore water clusters within the sands remained similar despite the drying-wetting history, and were found to be independent of the sand density. The present study confirmed the essential factors of the density dependence of the water retention characteristics which have been conceptually interpreted. The findings will surely contribute to the development of a theoretical model for water retention curves considering the porosity and particle size distribution based on an explicit mechanism, offering an alternative to conventional models (e.g., the van Genuchten model) that empirically consider the effects of porosity on water retention curves.