2. Phase Relations of Soils

Soil is not a coherent solid material like steel and concrete, but is a particulate material. Soils, as they exist in nature, consist of solid particles (mineral grains, rock fragments) with water and air in the voids between the particles. The water and air contents are readily changed by changes in ambient conditions and location.

Soil differs from traditional building materials like steel and concrete because it is made up of tiny solid particles instead of being a dense, cohesive substance. These solid particles are mineral grains and rock fragments. Between these particles, there are spaces filled with water and air. The amount of water and air can change easily based on the weather or the location of the soil. For example, when it rains, soil can become more saturated with water; conversely, during a drought, water levels decrease.

Think of soil like a sponge filled with tiny rocks. Just as a sponge can soak up water, soil can hold water in the spaces between its particle. If you place the sponge in water, it absorbs the liquid, making it heavier. If you take it out and let it dry, it releases the water and becomes lighter.

As the relative proportions of the three phases vary in any soil deposit, it is useful to consider a soil model which will represent these phases distinctly and properly quantify the amount of each phase. A schematic diagram of the three-phase system is shown in terms of weight and volume symbols respectively for soil solids, water, and air. The weight of air can be neglected.

In any given soil sample, the amounts of solid particles, water, and air can change significantly. To clarify these relationships, we use a soil model that categorizes these three components. This model visually represents the weight and volume of each component: solids, liquid water, and air. In many cases, especially when calculating the weight of soil, the weight of air can be neglected because it is much lighter compared to solids and water.

Imagine a jar filled with marbles (the solid particles), water (the liquid), and air bubbles in between. The total heaviness of the jar depends mainly on the weight of the marbles and the water. If you shake the jar, the air bubbles shift, but their weight remains negligible in comparison to the marbles and water.

Soils can be partially saturated (with both air and water present), or be fully saturated (no air content) or be perfectly dry (no water content). In a saturated soil or a dry soil, the three-phase system thus reduces to two phases only, as shown.

Soils can exist in various moisture conditions. 'Partially saturated soil' means there are both air and water present, while 'fully saturated soil' indicates that all the available space in the soil is filled with water, leaving no room for air. Conversely, 'perfectly dry soil' contains no water. When soil is either fully saturated or completely dry, we can simplify our model from three phases (solids, water, air) to just two phases, focusing only on solids and water or solids and air.

Think of a wet sponge. When it’s fully soaked (fully saturated), there’s no room for air – it’s all water. When you wring it out (partially saturated), some water is left, but there’s air in the sponge too. Finally, when it’s completely dry, there’s only the sponge material left, similar to dry soil. Each condition has different implications for how the sponge (or soil) behaves.

--