DAMAGE CAUSED BY SWELLING CLAY

Most engineering problems caused by volume changes in swelling clays result from activities of man that modify the local environments; they commonly involve swelling clays beneath areas covered by buildings and slabs or layers of concrete and asphalt such as are used in construction of highways, canal linings, walkways, and airport runways. Damage results from differential vertical movement that takes place as moisture levels in the clay adjust to the changed environment. In a highway pavement, differential movement of 0.4 in. within a horizontal distance of 20 ft is enough to pose an engineering problem if high standards for fast travel are to be maintained (Williams, 1965). Buildings are capable of withstanding even less differential movement before being structurally damaged. Generally, a differential movement of 0.25 in. between adjacent columns will cause cracking in load-bearing walls of a 20-ft-wide bay and, with differential movement of 1.5 in. over a span of 20 ft, beams or stanchions are likely to be structurally damaged (Skempton and McDonald, 1956).

Swelling clays are a source of problems in the design, construction, and maintenance of highways. In response to a questionnaire submitted to 50 highway departments in 48 States, Puerto Rico, and the District of Columbia, 36 departments acknowledged the occurrence of swelling clays in their areas, and 19 "... recognize swelling soils as part of their pavement design criteria" (Lamb and Hanna, 1973, p. 12). According to Lytton (1974, p. 416), "... general pavement roughness... due to swelling clays, is one of the most costly single elements in any pavement design." Annual losses due to damage of U.S. highways because of swelling clays have been reported to be more than one billion dollars (Jones and Holtz, 1973).

Houses and one-story commercial buildings are more apt to be damaged by the expansion of swelling clays than are multistory buildings, which usually are heavy enough to counter swelling pressures. However, if constructed on wet clay, multistory buildings may be damaged by shrinkage of the clay if moisture levels are substantially reduced, such as by evapotranspiration or by loss of moisture due to evaporation from beneath heated areas. The most obvious manifestations of damage to buildings are sticking doors, uneven floors, and cracked foundations, floors, walls, ceilings, and windows. If damage is severe, the cost of repair may exceed the value of the building.

Probably the greatest amount of damage to small buildings has been caused by volume increases in swelling clays that, as a consequence of prolonged droughts followed by long, soaking rains, were dry at the time of construction and were wetted following construction. Typical cases have been reported by Means (1959), Carothers (1965), Sowers and Kennedy (1967), and Schmertmann and Crapps (1980). Other reported cases of damage involved volume increases due to moisture derived from broken or leaking water and sewer lines, watering of lawns and shrubbery, and ponding of surface drainage. The desiccation of soil by trees and the resulting effect on foundation settlement were discussed by Perpich and others (1965).