The existence of three phases in soils is very common in the nature. Therefore the degree of saturation is commonly less than unity in the shallow depths of ground surface as illustrated in Fig. 1.2.

Fig. 1.2 Spatial distribution of moisture content within the field soil

The Fig. 1.2 shows the spatial variation of soil matrix in the field soil consisting of different amounts of water and air in the pore spaces. The top soil is usually unsaturated with a varying degree of saturation from zero to unity from season to season. The deeper layers of the soil but above the ground water table (the capillary zone), the degree of saturation is “full” ( S r = 1). The unsaturated zones in the environment can extend (above the ground water label) from meters to hundred of meters in depth. The presence of partial saturation in soils in our environment varies from place to place and season to season. Several engineering activities usually take place on such soils. The behavior of the soils due to changes in the saturation levels can greatly influence the stability of the structures constructed on such soils. Climatic (Environmental) variables influence the variation of the saturation levels of soils to a great extent. Therefore, the unsaturated soils play an important role in the natural hydrological cycle as shown in Fig. 1.3. The amount of water in the unsaturated zone located between the water table and the ground surface represents only ~ 0.01% of the total water involved in the hydrologic cycle.