A soil mass consist of solid particles which form a porous structure. The voids in the soil mass may be filled by with air, with water or partly with air and partly with water. (Fig 1A)

however, for convenience, all the solid particles are segregated and placed in the lower layer of the three phase diagram (Fig 1B). Likewise water and air particles are placed separately, as shown.

3-Phase diagram is also known as block diagram.

Two phase diagram:-

Although the soil is a three phase system,it becomes a two-phase system in the following cases:-

(1) When the soil is absolutely dry, the water phase disappears.(Fig a)

(2) When the soil is fully saturated, there is no air phase.(Fig b)

3-Phase diagram:-

In a 3-phase diagram, it is conventional to write volumes on the left sides and mass on the right side(Fig 2.3a). The total volume of a given soil mass is designated as V . It is equal to sum of volume of solids (Vs). The volume of water (Vw) ans the volume of air (Va) . The volume of voids (Vv) is equal to the sum of volumes of water and air.

M = Total mass of the soil

Ma = The mass of air is very small and is neglected.

Ms = The total mass of the soil is equal to mass of solids

Mw = Mass of water