The National Resources Conservation Service defines expansive soils as those soils with a significant shrink-swell potential. The presence of clay in soil, its quantity and type determine the shrink-swell potential. According to "Geology: A Self-Teaching Guide," the two types of clay with high and moderate shrink-swell potential are smectite and montmorillonite. Expansive soils found in the US include xererts, xerults and xeralfs.
Xererts are a suborder of vertisols found in areas of extreme precipitation interspersed by long dry spells, otherwise known as a Mediterranean climate. Large deposits of xererts are documented in California, Oregon and Idaho. To be a xerert, the soil must contain a minimum of 3/5 smectite or montmorillonite. Also, if not irrigated during the year, xererts must have cracks that remain 5 mm wide down to a depth of 50 cm for 60 days or more following the summer solstice, and those cracks must remain closed for 60 or more days after the winter solstice.
Xerults, a suborder of ultisols, can be found throughout most of the Southeastern United States. To be a xerult, the soil must have an argillic or kandic horizon, that is a subsurface layer composed of mineral clays with high to moderate cation exchange rates and shrink-swell capacities. There must be a sandy layer 125 cm below the clay horizon or 180 cm beneath the soil, whichever is deeper. Typically this layer contains high quantities of nutrient minerals, which help to enrich the soil for plant life above. Finally, the base water saturation of the clay horizon must be less than 35 percent at any time of measurement.
Xeralfs, a suborder of alfisols, are what's called xeric in their moisture exposure. They swell heavily with winter rains and dry out in summer, being found primarily in California but with smaller deposits in Idaho, Washington and Utah. They display all the exact same traits as xerults except the base saturation of xeralfs' clay horizon never drops below 35 percent.