In the southwestern United States, a common cause of foundation settlement is collapsible soil,
which can be broadly classified as soil that is susceptible to a large and sudden reduction in volume
upon wetting. Collapsible soil usually has a low dry density and low moisture content. Such soil can
withstand a large applied vertical stress with a small compression, but then experience much larger
settlements after wetting, with no increase in vertical pressure (Jennings and Knight, 1957). As such,
collapsible soil falls within the second basic category of settlement, which is settlement of the structure
due to secondary influences.