Most engineering problems involving swelling clays in the United States have occurred in regions of low or markedly seasonal precipitation. The Eastern region (fig. 1), particularly the southeast, is an exception because swelling clays occur in the humid climates characteristic of the region.

Smectite is locally abundant in discontinuous sedimentary geologic units of Tertiary age and younger along the Atlantic Coastal Plain and the Gulf Coastal Plain of Florida (Witczak, 1972; Tourtelot, 1974). These sedimentary units consist of sand, clay, and soft, porous limestone. The following Tertiary units containing swelling clay are extensive in the coastal plains of the southeastern United States: Barnwell Sand (Georgia and South Carolina), Ocala Limestone (Florida and Georgia), Jackson Formation (Georgia), Yazoo Clay (Alabama), and Hawthorn Formation (Florida, Georgia, and South Carolina).

The Eocene Barnwell Sand (eastern Georgia and southwestern South Carolina) and the Ocala Limestone (northern and northwestern Florida and southern and western Georgia) were deposited simultaneously, the Barnwell in shallow water near shore, and the Ocala in deeper water. These adjoining beds are as much as 180 ft thick and, at some locations, clay strata make up one-half of the total thickness. In east-central Georgia, the Barnwell Sand contains the Twiggs Clay, a highly smectitic unit with a maximum thickness of nearly 100 ft in Twiggs County (Brindley, 1957; Rodriguez and others, 1982). Besides serving as a commercial source of fuller's earth, the Twiggs Clay causes foundation problems because of its high volume-change characteristics. Sowers and Kennedy (1967) have noted destructive landslides in central Georgia caused partially by the expansion of a stratum of highly plastic clay, from 6 to 10 It thick, in the Ocala Limestone.

The Miocene Hawthorn Formation, which occurs extensively in northern and southern Florida, southern and southeastern Georgia, and South Carolina, consists mainly of thin-bedded phosphatic and dolomitic soft limestone and partially cemented sand and gravel (Espenshade and Spencer, 1963). Scattered through the formation are lenses of highly plastic clay that swell upon wetting; these are thickest and most abundant along the Florida-Georgia State line, where the clay lenses merge into more-or-less continuous beds from 3 to 30 ft thick. In most cases, however, the clay is found in small pockets less than 10 ft thick (Sowers and Kennedy, 1967). The predominant clay minerals in the Hawthorn Formation are attapulgite and montmorillonite, both of which have swelling potential. In southern Georgia and northern Florida, the attapulgite is mined for commercial use as fuller's earth, drilling mud, absorptive agents and catalysts. Sowers and Kennedy (1967) noted damage to a sewer pipe in central Florida and a building in north-central Florida due to volume change of clay of the Hawthorn Formation. Yon (1972) noted that continual swelling and shrinking of Hawthorn Formation clays in the Tallahassee area may be detrimental to foundations and, when saturated, these clays may be conducive to slope failure.

Schmertmann and Crapps (1980) discussed damage to houses overlying clay with swelling potential in the Hawthorn Formation of north-central and central Florida. In the Gainesville area (Alachua County), the Hawthorn Formation occurs close enough to the ground surface to cause foundation distress to light structures. Foundation distress also has been noted at Lake City (Columbia County) and Ocola (Marion County). Average vertical heave rates of one-story concrete block walls of as much as +2 mm/day, +0.8 mm/day, and +0.4 mm/day have been measured over 1-day, 10-day, and 50-day intervals, respectively. These foundation movements were probably due specifically to swelling and shrinking of underlying clays. The largest heaves occurred during very wet weather, and the largest settlements during periods of drought. Thus, engineers, contractors, and home owners in the area must deal with a clay that can cause either heave or settlement, depending on the amount of water available to the underlying clay.

In southern South Carolina, smectitic clays of the Hawthorn Formation are 15-20 ft thick over a large area in Jasper County (Patterson, 1972). In addition to the Hawthorn Formation, Heron and others (1965) noted that clays generally containing more than 50 percent smectites can be found in the following geologic units in South Carolina: Black Creek Formation (Upper Cretaceous, northeastern South Carolina); Black Mingo Formation (Eocene, central South Carolina); McBean Formation (upper Eocene, southwestern South Carolina); and locally in Pleistocene sediments along the coast. In general, South Carolinašs marine sediments are smectitic and those of nonmarine origin are kaolinitic.

Patrick and Snethen (1976) referred to clays with low swelling potential in sandy shales of the Upper Cretaceous Lumbee Group of Swift and Heron (1969) on the coastal plain of South and North Carolina. Daniels and Gamble (1978) have noted the existence of montmorillonite clay lenses in the Cretaceous Tuscaloosa Formation in central North Carolina.

To the north, in the middle Atlantic States of Virginia, Maryland, Pennsylvania, West Virginia, New York and the District of Columbia, outcrops of swelling clay are generally sparse and poorly documented. An exception has been the study by Obermeier (1984) of montmorillonite clay in the Cretaceous Potomac Group of Fairfax County, northern Virginia. This highly plastic clay has a high shrink-swell potential, and has destroyed houses in Fairfax County due to large vertical movements of foundations. Other swelling-related problems include collapse of basement walls where these clays have been used as backfill. Damage to roads in the area occurs in the form of distorted pavements and curbs and weakened subgrades. To reduce the shrink-swell hazard to house foundations, Fairfax County has enacted ordinances requiring that foundations on highly plastic clays be at least 4 It (1.2 m) beneath grade, that foundations be drained, and that trees be kept away from the foundations (Dallaire, 1976).

Patrick and Snethen (1976) have noted clays with low swelling potential in shales of the Potomac Group in the District of Columbia. They also reported that the Quaternary coastal deposits of the Atlantic Coastal Plain locally may show low swelling potential. Knechtel and others (1966) noted that the clay fractions of the Calvert, Choptank, and St. Marys Formations, in the Miocene Chesapeake Group of southern Maryland, contain large percentages of montmorillonite. Barber (1956) reported on volume-change problems of the marine Tuxedo clay deposits on the coastal plain near the District of Columbia, where buildings were damaged by the swelling of the clay and other structures were cracked by shrinking of the same clays.

Thin potassium bentonite or metabentonite strata, which may cause local foundation problems, have been noted in Paleozoic units in Virginia, West Virginia, Pennsylvania, New York, and adjacent States to the west and southwest by various researchers (Rosenkrans, 1934; Flowers, 1952; Weaver, 1956; Nelson, 1959; Collins, 1979). In general, these strata are too limited in extent to be shown on the accompanying map.

The New England States are nearly devoid of swelling clays in amounts that can cause damage to structures. However, Patrick and Snethen (1976) have noted that local areas of clay in Pleistocene marine deposits may have low swelling potential.