What are expansive soils primarily composed of?

Expansive soils are primarily composed of expansible-layer clay minerals, which are characterized by their ability to absorb water and increase in volume when wet. These clay minerals, such as smectite, are notable for their expansive properties, leading to significant changes in soil volume with moisture fluctuations. This characteristic can pose challenges in construction and land development, as expansive soils can cause movement of the foundation and other structures due to their swelling and shrinkage.

Other materials listed, such as permeable silt, hard rock formations, and gravel and sand mixtures, do not exhibit the same expansive behavior. Permeable silt does not swell like clay minerals and instead allows water to pass through, while hard rock formations and gravel and sand mixtures are typically stable and do not experience significant volume change with moisture content. Therefore, understanding the composition of expansive soils is crucial for proper site evaluation and construction planning in regions where these soils are prevalent.