Three-phase system of soil
Soil is 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 the case of perfectly saturated soil, the voids are filled completely with water.
In general, the soil mass has three constituents that do not occupy separate spaces but are blended together forming a complex material [Fig. a)],
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 the 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 the soil sample.
The total weight W of the moist sample is, therefore, equal to (Ww + Wd)
The properties of soil are as follows-
1. Void Ratio, e
Void ratio is the ratio of the volume of voids to the volume of solids.
2.Porosity, n
Porosity is the ratio of the volume of voids to the total volume of soil.
3.Degree of Saturation, S
The degree of saturation is the ratio of the volume of water to the volume of voids.
4.Water Content or Moisture Content, w
Moisture content, usually expressed in terms of percentage, is the ratio of the weight of water to the weight of solids.
5.Unit Weight, γ
Unit weight is the weight of soil per unit volume. Also called bulk unit weight (γ), and moist unit weight (γm).
6.Dry Unit Weight, γd
Dry unit weight is the weight of dry soil per unit volume.
7.Saturated Unit Weight, γsat
Saturated unit weight is the weight of saturated soil per unit volume.
8.Effective Unit Weight, γ'
Effective unit weight is the weight of solids in a submerged soil per unit volume. Also called buoyant density or buoyant unit weight (γb).
9.Specific Gravity of Solid Particles, G
The specific gravity of solid particles of soil is the ratio of the unit weight of solids (γs) to the unit weight of water (γw).
G=γsγw