Which type of soil is considered undesirable for a solid foundation and changes with moisture content?

Expansion soils, often referred to as expansive clay, are particularly undesirable for a solid foundation due to their high plasticity and significant volume changes that occur with fluctuations in moisture content. These soils can swell when wet and shrink as they dry, leading to dramatic shifts in ground stability and foundation integrity. The expansion and contraction can create forces that exert pressure on structures, leading to cracking, settling, or even failure of foundations if not properly managed.

This behavior makes designing buildings and other structures on expansion soils a challenge, requiring specialized engineering solutions such as deeper foundations, moisture control systems, or soil stabilization techniques to mitigate the risks associated with these dynamic soil characteristics.

In contrast, while plastic soils can also exhibit changes in volume, it's the specific nature of expansion soils to undergo substantial volumetric change that makes them particularly problematic for foundations. Cohesive soils can also have risks regarding stability, but they do not exhibit the same level of expansive behavior. Meanwhile, coarse soils, such as sandy soils, typically have better drainage properties and do not expand significantly with moisture, making them generally more favorable for foundation stability.