A soil  as a three phase system consisting of solid particles (called soil grains), water and air. The void space between the soil grains is filled partly with water and partly with air. However, if we take a dry soil mass, the voids are filled with air only. In case of a perfectly saturated soil, the voids are filled completely with water.

In general, the soil mass has three constituents which do not occupy separate spaces but are blended together forming a complex material [Fig.a)],

soil as a three phase system
soil as a three phase system

the properties of which depend upon the relative percentages of these constituents, their arrangement and a variety of other factors. For calculation purposes, it is always more convenient to show these constituents occupying separate spaces, as shown in Fig.(b)(i) and Fig. (b)(ii).

As shown in Fig.(b)

(i), the total volume V of the soil mass consists of

(i) volume of air Va,

(ii) volume of water Vw and

(iii) the volume of solids Vs.

The volume of voids Vv, is, therefore, equal to volume of air plus the volume of water.

Similarly, Fig.(b) (ii) shows the weights.

The weight of air is considered to be negligible. Hence, the weight of total voids is equal to the weight of water Ww. The weight of solids is represented by Wd (or Ws) which is evidently equal to the dry weight of soil sample.

The total weight W of the moist sample is, therefore, equal to (Ww + Wd)