FACTORS THAT AFFECT SWELLING OF CLAYS

Volume changes in clay, because of variations in moisture content, occur within about 30 ft of the ground surface (Jones and Holtz, 1973), and most changes that cause engineering problems take place at depths of less than 10 ft (Hamilton, 1963; Gromko, 1974); thus, further discussion will pertain to factors that affect swelling clays at these shallow depths.

Swelling potential, as used here, refers to the amount of volume increase due to swelling that is possible in a clay body in its natural environment. It is influenced by many factors, some of which are indigenous to the clay body and others that are related to its environment. Inherent factors determine the maximum increase in volume that can take place under optimum conditions. They include clay mineral composition, amount of nonclay material present, density, void ratio, size and orientation of clay particles, cementation, macrostructure, size and thickness of the clay body, and depth below ground surface.

The most significant of the swelling factors is clay-mineral composition. Most naturally occurring clays are composed of two or more clay minerals plus fine nonclay particles. The swelling potential and pressures generated by the swelling of a particular clay body are, to a large degree, functions of the combined inherent swelling capacities of all of its clay-mineral components (White and Pichler, 1959, p 2). The presence of silt, sand, and other nonclay materials will reduce the swelling potential in proportion to their amount. The nonclay materials are dilutants that in effect reduce the clay-mineral content per unit volume and hence reduce swelling potential (Komornik and David, 1969). A key to the amount of clay is generally provided by dry-density and void ratio. Clays with high dry densities and low void ratios usually contain greater proportions of clay minerals than do those with low dry densities and high void ratios. Other factors being equal, clays with high dry densities commonly have higher swelling potentials than do those with low dry densities.

Clay particle size (size of an individual clay crystal) determines, to a large degree, the total surface area accessible to moisture within a mass of clay (Barshad, 1955). The total surface area increases as the particle size decreases. The orientation of the particles, most of which are thin sheet-like crystals, affects their access to moisture; the surface area accessible to moisture increases as degree of preferred orientation decreases. Hence, swelling potential increases with decrease both in particle size and in degree of preferred orientation. Some clays composed of fine-grained, randomly oriented, nonexpandable clay minerals have swelling potentials as great as those clays composed of expandable clay minerals. Such a clay occurs in the Decorah Formation (Ordovician) of southeastern Minnesota. The swelling of parts of the Decorah Formation consisting primarily of randomly oriented, fine-grained particles of illite is largely due to capillary pressures (Parham and Austin, 1969).

Clays that are cemented, usually with carbonate, silica, or iron oxide, have little or no swelling potential, because the clay particles are bonded by the cement and have low porosity. For reasons not fully explained, macrostructures produced by roots, microbial activity, and soil-forming processes may reduce swelling potential (Hamilton, 1963).

Maximum volume change for a clay body is directly proportional to the size of the body; however, even thick clay bodies composed of highly expandable clay minerals will have no swelling potential if the weight of the overburden is sufficient to counter swelling pressures.

Clays beneath the water table, which may be at the ground surface or many feet below, have no swelling potential, as they are completely saturated and have no capacity for Moisture. Clays above the water table are generally unsaturated and will have capacities for Moisture and swelling that will differ according to their degree of saturation. Generally, saturation levels are high and swelling potentials are low for clays just above the water table, because, due to capillary attraction, they have access to abundant moisture.

Clays in the weathering zone, which may extend a few inches to more than 30 ft below the surface, usually have minimum moisture contents that are determined by climate. Variations in moisture content above minimum amounts will occur in response to fluctuations in the weather. Thus, swelling potential and the actual amount of swelling of clays of similar composition may differ, depending on the climates of the regions in which they occur. Minimum moisture content will be lowest and swelling potentials highest in regions of warm or hot, and climates with low precipitation/evaporation ratios, and minimum moisture levels will be highest and swell potentials lowest in regions of cool, humid climate with high precipitation/ evaporation ratios. In such regions, however, the magnitude of swelling due to natural causes is generally small because of scarcity of moisture in regions of and climate and low swelling potentials in regions of cool, humid climate.

Variations in moisture contents and volume changes are greatest in clays that occur in regions of moderate to high precipitation where prolonged periods of drought are followed by long periods of rainfall. It is in these regions, which include many of the Southern, Central, and Western States, that swelling of clays due to climatic fluctuations causes the most severe engineering problems. A map of the conterminous United States, showing climatic ratings based on precipitation characteristics, and a discussion of the relations of swelling potential and soil activity to the ratings has been published by the Building Research Advisory Board of the National Research Council (1968, p. 35-39).

Activities of man that disturb the local environment may drastically alter local moisture conditions and cause large volume changes in near-surface swelling clays. Construction of concrete slabs and buildings that protect underlying sediments from the weather in regions of humid climate may result in a decrease in moisture levels and cause underlying clays to shrink, whereas, in and regions or in regions where prolonged droughts are common moisture contents beneath covered areas may increase and underlying clays may swell as a result of moisture migration in response to the gradient in the relative humidity of the sediments (Mielenz and King, 1955, p. 233). Moisture increases and swelling may also be caused by watering of lawns and shrubbery, breakage of water and sewer lines, and modifications of the surface that produce ponding. Shrinkage of clays may result from loss of moisture by evaporation from beneath heating units such as fireplaces, boilers, and furnaces.

The stability of clays at the site of an engineered structure may be affected by chemical or organic waste that comes in contact with the clay. The exchangeable ion chemistry of swelling clay minerals may be changed, which, in turn, may alter the water-retention capacity of the clay and adversely affect its volume and strength. This also may occur where concrete construction contacts a clayey foundation, because of an increase in the calcium ion concentration. Research is needed on the possible effects of deleterious ground water and chemical wastes on high-quality stable rock used in fills or foundations overlying weak clays.

Moisture levels in clays may be considerably reduced and swelling potentials increased because of removal of moisture by root systems. The amount of moisture removed from the ground can be large; a single tree can transpire the equivalent of 100 gallons of water on a sunny day (Perpich and others, 1965). The root systems of some plants extend to depths of as much as 30 ft.

Time is important in determining the amount of swelling that may take place in response to local changes in the environment and moisture conditions of a clay body. Swelling clays have low permeabilities and, when wetted, the clay minerals expand and further reduce permeabilities. Because of the slow migration of water through swelling clays, it may take several years for moisture levels to reach a state of near equilibrium in clays beneath covered areas (Means, 1959; Blight, 1965). The time required will depend in part on climate, moisture content at time of construction, and the type of sediments beneath the structures (Carothers, 1965). Because of differences in permeability, a sandy clay with low swelling potential and fair permeability may expand more during a wet season than a sand-free clay with low permeability and a very high swelling potential (Gromko, 1974).