What causes shrinkage in expansive soil?

Shrinkage in expansive soil is primarily caused by the removal of water between clay particles. Expansive soils, or clays, contain minerals that swell when water is present and shrink when water is removed. This process occurs because the water molecules are absorbed between the particles, causing them to spread apart and increase in volume. When water is removed, the particles lose this spacing, leading to a decrease in volume and thus shrinkage.

In contrast, the other options do not directly cause shrinkage. An increase in temperature can lead to expansion in some contexts, particularly when combined with moisture, but it is not a direct cause of shrinkage. Freezing of soil usually causes physical changes that may not directly correlate with the shrinkage mechanism in expansive soils. Excess moisture content typically results in further swelling rather than shrinkage, as the soil continues to absorb water. Understanding these dynamics is crucial for managing expansive soils, especially in construction and agricultural settings.