By and large, the mechanical property of a soil mass is the materialization of the resistance to displacement of soil particles in the soil mass by external stressors. The different kinds of actions resulting from application of the external stressors on a soil mass include such phenomena as compression, shearing, cutting, and twisting (torsion). The three-phase nature of soils (solid, liquid, and gaseous) requires one to consider how the various phases react and interact with each other to provide the response to the external stressors responsible for provoking compression, shearing, cutting, and so forth of the soil mass. Each of the three phases is highly variable, with their compositions and proportions in a soil mass varying continuously because of regional weathering and environmental conditions (see Chapter 10).