CAPILARITY AND SOLUTE TRANSPORT IN SWELLING AND SHRINKING SOILS

Abstract

The Aranca Low Plain (SW of Romania), with accumulative relief, a slow dip and an obvious subsidence, is covered with fluvio-lacustrine deposits with various grain sizes, from gravels to smectite clays. The phreatic waters are mineralized and sodium ions reach into the colloidal complex, destroying soil structure. The rise of water in the soil from a free water surface has been termed capillarity rise. The maximum height to which water may rise through capillarity varies between 1.5 and 4 m in Fluvisols and Vertisols. Capillarity rise of soil water takes place above the ground water level and is due to matric suction. The complex geometry of soil pare space creates numerous combinations of interfaces, capillaries and wedges in which water is retained. In addition, water is absorbed on to solid surfaces, with considerable force at close distances. Solutes can interact strongly with soil surfaces and their transport can be slowed in a process known as retardation. In Fluvisols, like in Chernozems and Phaeozems, soluble salts are concentrated in the first 50 cm of the soil profile, while in Vertisols most soluble salts are concentrated at the bottom of the profile. The water potential in a Vertisol is the sum of gravitational potential, capillarity potential and overburden potential. The overburden potential is related to civil engineers effective stress. The phenomenon of capillarity is thus dependent on solid and liquid interfacial properties such as surface tension, contact, angle and solid surface roughness and geometry. These phenomena, partial attributed to capillarity, determine retention and movement of water and solutes through soils.