Some of the state variables and material constants; their importance in the unsaturated soil mechanics are detailed here. Two important constitutive relationships, namely soil-water characteristic curve and hydraulic conductivity function are discussed in the next two chapters. The sate variables viz. total head, effective stress, net normal stress, shear stress, and matric suction are commonly used in defining several geotechnical phenomena. These variables are discussed in-detail when the geotechnical aspects were discussed in the later chapters. Some of the important state variables such as density of air, density of water, and relative humidty; few important material constants such as viscosity of air and water are discussed in three chapters.

Multi-phase system

Fig. 2.4. Soil as a multi-phase system

Unsaturated soils are analyzed as a multi-phase system as dipicted in Fig. 2.4. The liquid phase in the soil pore space is generally circumscribed by the pore space not occupied by air. The liquid phase could be water, oil, non–aqueous phase liquids, or combination of two or more solutions. Similarly, the air phase is circumscribed by the pore space not occupied by liquid pohase. However, water can exist in air phase as vapor and air can exist in water in dissolved condition under the given atmospheric conditions. The solid phase consists of the soil solid grains and may range from fine-grained materials such as clays to coarse-grained materials such as gravels. It is important to consider the properties of the different phases existing in the soil matrix. The important state variables that are responsible for air flow and vapor flow through unsaturated soils are presented herein.