Three Phase System
Soil is typically considered a three-phase system because it comprises solid particles, liquid, and gas. For most civil engineering applications, the liquid in the soil voids is assumed to be water, while the gas is usually air. When soil is fully saturated, it only contains two phases: the solid particles and water. In this state, all the void spaces between the particles are filled with water. This concept is crucial for understanding how soil behaves, as the properties of saturated soil—such as strength and stability—differ significantly from those of unsaturated soil.