What is Phase Diagram?
In general, soil is a three-phase system composed of solid, liquid and gaseous matter. The voids in between solid soil particles are filled with either water or air or both. In completely saturated soil, the voids are filled with water only whereas, incase of completely dry soil voids are filled with air only. In case of partially saturated soil, the voids are filled with both water and air. The representation of different phases of soil with diagrams is called as phase diagram. Figure 2.1(a) shows the phase diagram of partially saturated soil (three-phase system: solid, water and air). Figure 2.1(b) and Figure 2.1(c) show the phase diagram of completely saturated (two-phase system: solid and water) and dry soil (two-phase system: solid and air), respectively. In the figures, weight of air (Wa) is assumed to be zero. The weight of the voids is equal to the weight of water (Ww). The weight of solid is represented by Ws. The total weight of the partially and completely saturated soil is W = Ws + Ww whereas, the total weight of completely dry soil is W = Ws. The volume of solid, water and air are represented by Vs, Vw and Va, respectively. The total volume of the soil is V = Vv + Vs, where Vv is the volume of voids (Vv = Vw + Va). In case of completely saturated and dry soil the volume of voids Vv = Vw and Vv = Va, respectively.