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Capillary Phenomenon in Unsaturated Soils - IntroductionSurface tension is a phenomenon, at the water-air interface, that allows insects to float on water. It varies with temperature and the type of fluid. This surface tension can be measured using mechanical probes such as a dew wire ring. Vapour bubbles formed on water surfaces are spherically shaped in order to minimise the surface area of the water. Surface tension causes the development of negative pore water pressure in soil systems. When a capillary tube is inserted in water, the water rise in the tube because of the adhesive force between the water molecules and the glass surface. Fundamental Principles of Unsaturated Soil Mechanics - Summary To define any given phenomenon, you must understand and distinguish the state variables, constitute relationships and material constants of that phenomenon. The governing equations are basic principles that represent fundamental physical laws that are independent of matter or independent of material. The dependent variables in these governing equations are the state variables. The constitutive equations are a relationship between two different state variables. The basic steps in predicting a given phenomenon are: Observing the physical phenomenon Develop governing equations, Identify the state variables, Develop the constitute relationships, Identify the material constants, Predict the phenomenon, Compare the predicted phenomena with observed physical phenomenon. Unsaturated soils are three-phase systems: Air, water, and solid phases. The air phase contains water vapour while water phase contains some dissolved air.The density of air is sensitive to pressure and temperatures. The density of air increases with an increase in air pressure but decreases with an increase in the temperature. Water density varies with temperature and physical-chemical effects of soil. As temperature decreases, the density of the water content of soils increases. Relative humidity is defined with respect to the partial pressure of vapour. It is the ratio of absolute humidity in equilibrium with some solution to the absolute humidity in equilibrium with pure water. Relative humidity decreases as the salt concentration increases. Saturated vapour pressure can be reached by decreasing temperature or by increasing the vapour pressure whilst keeping the temperature the same.