Soil is a complex physical system. A mass of soil includes accumulated solid particles or soil grains and the void spaces that exist between the particles. The void spaces may be partially or completely filled with water or some other liquid. Void spaces not occupied by water or any other liquid are filled with air or some other gas.
 |
| Figure 1. Soil as a three phase system |
‘Phase’ means any homogeneous part of the system different from other parts of the system and separated from them by abrupt transition. In other words, each physically or chemically different, homogeneous, and mechanically separable part of a system constitutes a distinct phase. Literally speaking, phase simply means appearance and is derived from Greek. A system consisting of more than one phase is said to be heterogeneous.
Since the volume occupied by a soil mass may generally be expected to include material in all the three states of matter-solid, liquid and gas, soil is, in general, referred to as a “three-phase system”. (Fig. 1)
 |
| Figure 2. (a) Saturated soil (b) Dry soil represented as two-phase systems |
A soil mass as it exists in nature is a more or less random accumulation of soil particles, water and air-filled spaces as shown in Fig. 2 (a). For purposes of analysis it is convenient to represent this soil mass by a block diagram, called
‘Phase-diagram’, as shown in Fig. 2 (b). It may be noted that the separation of solids from voids can only be imagined. The phase-diagram provides a convenient means of developing the weight-volume relationship for a soil.
When the soil voids are completely filled with water, the gaseous phase being absent, it is said to be
‘fully saturated’ or merely
‘saturated’. When there is no water at all in the voids, the voids will be full of air, the liquid phase being absent ; the soil is said to be dry. (It may be noted that the dry condition is rare in nature and may be achieved in the laboratory through oven-drying). In both these cases, the soil system reduces to a ‘two-phase’ one as shown in Fig. 2 (a) and (b). These are merely special cases of the three-phase system.