2.1. Three-phase System

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).

Soils exist in different moisture conditions which define their phases. When a soil is fully saturated, it means that all the pores in the soil are filled with water; hence, there is no air. In a partially saturated soil, there is a mixture of both air and water in the soil pores. A perfectly dry soil has no water at all, meaning it's completely filled with air. Understanding these phases is essential for various applications in agriculture, civil engineering, and environmental studies.

Think of a sponge. When you immerse a sponge in water and then lift it out, it becomes fully saturated, just like a fully saturated soil. If you squeeze the sponge lightly, it retains some water but also lets air into the pores; this is similar to partially saturated soil. Finally, if you leave the sponge out to dry completely, it becomes perfectly dry, resembling dry soil.

In a saturated soil or a dry soil, the three-phase system thus reduces to two phases only, as shown.

The three-phase system of soil - consisting of solid particles, water, and air - simplifies under certain conditions. When soil is saturated, the air phase disappears, leaving only solids and water. Similarly, in dry soil, all water is removed and only solids and air remain. This reduction to two phases is useful for simplifying calculations and understanding characteristics such as soil strength and permeability.

Consider preparing a recipe that calls for only two main ingredients: flour and water. If you add the water to the flour, it becomes a dough (saturated condition, with only solids and liquid). If you let the dough air dry, it will turn into a solid cake (dry condition, with only solids and air). In both cases, you've reduced the mixture to just two phases, making it easier to manage.

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.

Soil engineers and scientists need to understand the proportions of solids, water, and air in soil because these ratios influence the soil's stability, fertility, drainage, and overall behavior. By using a soil model that quantifies these phases, it becomes easier to predict how a soil will perform under different conditions, which is crucial for construction projects, agriculture, and environmental management.

Imagine a bowl where you mix three different colored beads representing solid particles, water, and air. Depending on how many beads of each color you add, you can predict how the mix will look or behave. If there's a lot of water (blue beads), the mixture is wet and tends to stick together, which is beneficial for certain plants. If you only have solid and air beads (red and yellow), the mix becomes dry and loose, which can be good for other types of plants. By learning to count and visualize these proportions, you can apply that knowledge to grow healthy plants or build stable structures.

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