Water in unsaturated soils

Water plays a major role in the behavior of unsaturated soils. Its cohabitation with air in the pores takes various forms, depending on whether water dominates and air cannot move freely, or, on the contrary, whether air occupies most of the pores and water is blocked at particle contacts, or whether both water and air can move. In addition to capillary action, which occurs where the two fluids come into contact, water can interact with the minerals in the soil particles: these physico-chemical interactions are particularly important in clays. Finally, the concentration of salts dissolved in water is at the origin of osmotic phenomena, which influence the migration of water molecules in soil pores.

This article describes the different states of water in unsaturated soils, the physical phenomena that determine the interaction of water and minerals, and the laws of water movement in unsaturated soils. In particular, it presents experimental techniques for measuring or imposing suction, and for characterizing water retention and transfer capacities in these soils.