What is a reason why soil might swell upon disturbance?

Soil swelling upon disturbance is primarily associated with the introduction of water. When water is added to certain types of soil, especially clay soils, it interacts with the particles, causing the soil to expand. This phenomenon occurs because water molecules occupy spaces between the soil particles or are absorbed by the soil structure, leading to an increase in volume.

Clay soils are particularly susceptible to this reaction due to their plate-like particle structure, which allows for significant water retention. When water is introduced, the layers of clay particles expand as they absorb the water, resulting in swelling. This swelling effect can cause issues in construction and landscaping, as structures built on such soils can experience shifts or even damage due to the changes in soil volume.

Other factors, such as increased density, air exposure, and temperature changes, do not typically lead to swelling in the same manner that water introduction does. Increased density usually results from compaction, which would not cause expansion. Air exposure primarily can lead to drying and cracking, while temperature changes can influence moisture levels but do not directly result in swelling without the presence of water. Thus, the introduction of water is a clear and direct cause of soil swelling upon disturbance.