Certain clay minerals if present in a soil can cause it to expand when wet and shrink upon drying, a phenomena known as shrink-swell potential. Such “expansive soils” can pose a hazard to structures founded upon them if not appropriately engineered. The effects of expansive soils are most evident in areas of moderate to high precipitation, where prolonged drought is followed by prolonged periods of rainfall. Recent estimates put the damage from expansive soils as high as $7 billion annually across the U.S. (source: FEMA). However, because this hazard develops gradually it is seldom a life-safety threat.
This condition can be aggravated by the way a property is maintained or irrigated. Excess irrigation of landscaping, for example, or removal of significant amounts of native vegetation could create or worsen this condition. Where shrink-swell potential is known to be significant, certain engineering and construction standards may be required for new buildings and remodeling, which could affect project cost or approval. California law does not mandate disclosure of expansive soils zones; however, many cities and counties include such a map in their General Plan Safety Element, and a prospective buyer may consider this to be material if the sale property is located in the locally mapped zone.