PHASE RELATIONSHIPS
Soil is composed of solids, liquids, and gases (Figure ). The solid phase may be minerals, organic matter, or both. As mentioned before, we will not deal with organic matter in this textbook. The spaces between the solids (soil particles) are called voids. Water is often the predominant liquid and air is the predominant gas. We will use the terms water and air instead of liquid and gas. The soil water is called porewater and plays a very important role in the behavior of soils under load. If all the voids are fi lled by water, the soil is saturated. Otherwise, the soil is unsaturated. If all the voids are fi lled with air, the
soil is said to be dry.
We can idealize the three phases of soil, as shown in Figure 4.2b. The physical properties of soils are infl uenced by the relative proportions of each of these phases. The total volume of the soil is the sum of the volume of solids (Vs), volume of water (Vw), and volume of air (Va); that is,
is the volume of voids. The weight of the soil is the sum of the weight of solids (Ws) and the weight of water (Ww). The weight of air is negligible. Thus,
The following defi nitions have been established to describe the proportion of each constituent in soil. Each equation can be presented with different variables. The most popular and convenient forms are given. You should try to memorize these relationships. When you work on these relationships, think about a bread dough in which you have to reconstruct the amount of the constituent ingredients—for example, the amount of fl our or water. If you add too much water to a bread dough, it becomes softer and more malleable. The same phenomenon occurs in fi ne-grained soils.
1. Water content (w) is the ratio: often expressed as a percentage, of the weight of water to the weight of solids:
2. Void ratio (e) is the ratio of the volume of void space to the volume of solids. Void ratio is usually expressed as a decimal quantity.
3. Specifi c volume (V9) is the volume of soil per unit volume of solids:
4. Porosity (n) is the ratio of the volume of voids to the total volume. Porosity is usually expressed as a percentage.
5. Specifi c gravity (Gs) is the ratio of the weight of the soil solids to the weight of water of equal volume: