Swelling clays occur in all 48 States of the conterminous United States. Where they are at or near the surface and within the zone of weathering, they constitute a potential source of damage to foundations and structures. Particularly susceptible are lightweight types of construction such as concrete slabs for highways, canal linings, spillways, and basement floors and foundations of buildings, as well as walls supported by these foundations. Slope materials containing swelling clays are especially susceptible to land-sliding. Annual loss due to damage resulting from swelling clays within the United States is estimated to be at least 2.3 billion dollars, and during this century these losses have been twice as great as those from floods, earthquakes, hurricanes, and tornadoes (Jones and Holtz, 1973).

A swelling clay, according to the American Geological Institute Glossary of Geology (Bates and Jackson, 1980, p. 631), is a "clay that is capable of absorbing large quantities of water, thus increasing greatly in volume . . ."

Dry clays that are capable of absorbing water, if unconfined, will increase in volume in an amount proportional to the amount of water absorbed. However, the amount of water absorbed and the degree of expansiveness for particular types of clay are dependent on many variable and interrelated factors that have not been, and probably cannot be, formulated in such terms as to be applicable to all possible combinations that exist in nature.

For purposes of these maps, the term swelling clays is applied to those clays that, due to swelling caused by absorption of moisture, are known to cause damage to foundations and structures; it also includes those clays that, on the basis of their mineralogical composition and physical properties, are believed to have the potential to do so. These same clays also may be a source of engineering problems due to shrinkage resulting from a loss of moisture.

Distribution of swelling clays in the conterminous United States is shown on the accompanying 1: 7,500, 000- scale map. For ease of description of the swelling clay units, the authors have subdivided the 48 conterminous States into four regions (fig. 1): Pacific Coast/Western Mountain, Rocky Mountain, Mid-Continent, and Eastern. The Pacific Coast/Westem Mountain Region includes the States of Washington, Oregon, California, Idaho, Nevada, and Arizona. The Rocky Mountain Region is made up of Montana, Wyoming, Utah, Colorado, and New Mexico. The Mid-Vontinent Region is comprised of North Dakota, South Dakota, Nebraska, Kansas, Oklahoma, Texas, Minnesota, Iowa, Missouri, Arkansas, Louisiana, Wisconsin, Illinois, Michigan, Indiana, Ohio, West Virginia, Kentucky, Tennessee, Mississippi, and Alabama. The eastern section consists of the New England and eastern seaboard States: Maine, Vermont, New Hampshire, Massachusetts, Connecticut, Rhode Island, New York, Pennsylvania, New Jersey, Delaware, Maryland, Virginia, North Carolina, South Carolina, Georgia, and Florida. The following sections of this report discuss the distribution of swelling clays and the severity of swelling-clay problems in the four regions.