However, because this zone forms the essential transition between the atmosphere and larger groundwater body, the movement of water within this small portion of the cycle is very significant. The hydrological processes such as infiltration, evaporation, and transpiration occurring in the near-surface unsaturated soil zone influence the behavior of the soils. Thus it is important to consider the concepts of interfacial physics such as equilibrium among different phases of air, solid, and water; the transition of water from one phase (vapor) to another. The application of interface physics makes the understanding of unsaturated soils different from the saturated soils. It will be shown in the following lectures that the traditional framework of soil mechanics fails to adequately describe and predict the behavior of the natural soils as the saturation was not considered as a state parameter. In partly saturated soils, equilibrium of different phases is important for understanding the Soil behavior. Interfacial physics deals with the equilibrium between different phases. Similarly, the clay behavior is dictated by the environmental factors which can be understood by physico-chemical equilibrium. Therefore, the classical definition of soil mechanics is extended to define the unsaturated soil mechanics as 'the application of the laws of mechanics, hydraulics, interfacial physics, and physico-chemical equilibrium for understanding the engineering behavior of partly saturated soils'.

Fig. 1.3 Hydrological processes occurring in the near-surface unsaturated soil zone