The term "clay" is used for a natural, earthy, fine-grained material that develops plasticity when mixed with a limited amount of water. The term also applies to a definite group of hydrous aluminum silicate minerals and to grains or particles that are either <2um or <5um in diameter, depending upon which grain-size classification is used (see Truesdell and Vames, 1950, for comparison of grain-size definitions of sedimentary materials). The most common clay minerals can be classified into five groups: smectite (montmorillonite, beidellite, nontronite, and others); illite (illite and glauconite); kaolinite (kaolinite, halloysite, and others); chlorite (chlorite and others); and sepolite (sepolite and palygorskite). Many of these clay minerals do occur as the dominant mineral in some areas and almost all of them occur as minor constituents with other clay minerals. Discrete clay crystals may consist of alternating layers of two or more different clay minerals, which are referred to as mixed-layer clays. Common combinations contain the expandable clay mineral montmorillonite or beidellite, interlayered with chlorite or with a mica.
The inherent swelling potential of aggregates of clay minerals is closely related to the total external and internal surface areas of clay-mineral particles. Clay minerals are capable of adsorbing water on their outer surfaces, and water so adsorbed will cause a small amount of swelling related to enlargement of the capillary films. Some clay minerals, however, such as montmorillonite and beidellite of the smectite group, are capable of absorbing appreciable amounts of water between the individual silicate layers of the structural lattice, which results in a high swelling potential. Vermiculite, attapulgite, nontronite, and degraded mica (illite) and chlorite, though less common, are also capable of absorbing appreciable amounts of water. These expandable clay minerals consist of atoms arranged in lattice structures in which sheets or layers of the lattices are bound together by atomic physicochemical forces. The strengths of these forces vary with the types of clay minerals and with the water-retention properties of the cautions that occur between the lattice layers.
The swelling potential of clay is dependent upon the amount and kind of clay minerals present, their exchangeable ions, the electrolyte content of the aqueous solution, particle-size distribution, void size and distribution, water content, superimposed load, and probably other factors.
Clay units composed almost entirely of Na-, Ca-, and Mg-montmorillonite are commonly referred to as bentonites. They have a number of commercial uses, many of which are related to their absorptive and swelling properties.
Volume changes of 1400 to 2000 percent are reported from laboratory tests in which samples of dry Na-montmorillonite were immersed in water, and changes of 45 to 145 percent are reported for Ca-montmorillonite (Mielenz and King, 1955, p. 232). The inherent swelling capacities of vermiculite, attapulgite, illite, and degraded chlorite have not been fully investigated. Available evidence suggests that they have low to moderate swelling capacities.
The amount of volume change and pressure generated by a swelling clay under laboratory conditions is usually considerably greater than that generated by the same clay in the field under natural conditions. This is possibly due to changes in diagenetic and environmental factors as a consequence of collecting and preparing specimens for testing (Holtz and Bara, 1965; Parcher and Liu, 1965; Woodward-Clyde and Associates, 1967, p. 75).
The results of specific field measurements of volume changes in swelling clays generally are lacking, because of difficulties in obtaining and recording relevant data. The most obvious volume changes are those that affect the land surface; in many cases these changes are manifest as a result of damage to structures. Uplift of land surface due to increase in volume of swelling clay generally amounts to no more than a few inches; however, uplift of more than a foot has been reported (Mielenz and Okeson, 1946, p. 271). Horizontal displacement also results from swelling; however, information on the amounts of displacement is meager. Laboratory tests conducted on undisturbed specimens of swelling clay indicated a vertical to horizontal displacement ratio varying between 0.3 and 0.7 (Parcher and Liu, 1965). Dry swelling clays absorb much larger quantities of water before becoming plastic than do dry, non-swelling clays. They also remain plastic over a wider range of moisture content. This range in moisture-content is referred to by engineers as the plasticity index (PI) and is expressed as the numerical difference between the plastic limit (the percent moisture content at which a clay passes from the solid to the plastic state) and the liquid limit (the percent moisture content at which clay passes from the plastic to the liquid state). The PI bears a direct relation to the amounts and types of clay minerals present and to the orientation and size of clay particles. Other factors remaining constant, the PI increases with (1) increase in amount of expandable clay minerals, (2) decrease in degree of parallel orientation of the platy clay minerals, and (3) decrease in clay-particle size. The plasticity indicies of 1 is for an artificially saturated Na-kaolinite.
Plasticity indices range from as low as 1 for artificially saturated sodium kaolinites to more than 600 for some Na-montmorillonites (Grim, 1962, p. 213). The PI of most expandable clay minerals is usually greater than 50, whereas for nonexpandable types it is generally less than 50. High plasticity indices for nonexpandable clay minerals generally indicate small particle size and lack of common orientation of platy clay-mineral particles.
The PI is generally a good indicator of swelling potential. Seed and others (1962, p. 87), who found the PI to be the single most useful indicator of swelling potential, noted ". . . that this parameter alone can provide an assessment of swelling that is probably accurate to within 35 percent," and Sowers and Kennedy (1967, p. 117) found the PI to be "the most reliable working tool" in identifying potentially troublesome clays in the humid coastal plains of the southeastern United States.
Pressures generated by swelling clays can be very great. Pressures exceeding 15 tons/f? (14.6 kg/cm2), and normally ranging from 1 to 6 tons/f? (1-5.9 kg/ cm2), have been measured in laboratory tests conducted on bentonitic clays (Dawson, 1953; Mielenz and King, 1955). The pressures generated by a clay with moisture content near the plastic limit are much greater than pressures generated by the same clay with moisture content near that of the liquid limit.
Numerous laboratory testing methods have been devised for quantitatively evaluating swell and swelling pressures to be anticipated under field conditions. The advantages and disadvantages of many of these methods were reviewed by Woodward-Clyde and Associates (1967, p. 75-107), who noted: "No one test is universally suited for all conditions." In most cases, the tests provide indications of the general magnitude of problems to be anticipated in the field: however, where little is known of the environmental conditions that influence swell, the test indications may be drastically misleading (Hamilton, 1965).
Information is meager on swelling pressures generated under actual field conditions because of difficulties in recording data where the clay may be less confined and other factors that affect swelling are uncontrolled. Lateral and vertical pressures of as much as 0.86 and 2.3 ton/ft2 (0.8 and 2.2 kg/cm2) are reported from field measurements for a bentonitic clay in central Texas, and a pressure of 50 lb/in2 (3.5 kg/cm2) is reported for a bentonite near Ontario, Oregon (Mielenz and King, 1955).
The general physical appearance of swelling clays in outcrop varies widely, and no fixed group of criteria can be established for identification that is universally applicable. Clays that contain high percentages of Na-montmorillonite, such as bentonites of the Cretaceous Mowry and Belle Fourche Shales of Wyoming, and many bentonites with a high Ca-montmorillonite content, such as those of the Paleocene Midway Group of Mississippi and the Eocene Jackson and Claiborne Groups of Texas, are hard and brittle when dry, dense and waxy when moist, and highly plastic and sticky when wet. Dry fragments immersed in water slake rapidly and increase in volume. Dry surfaces of outcrops usually exhibit intricate patterns of shrinkage cracks that may be coated with white alkali staining. From a distance, the surface of such an outcrop resembles popcorn in appearance, and is aptly referred ,o as a "popcorn surface," or, in the weathering process, as "popcorn weathering." Popcorn weathering occurs on outcrops of clays and bentonitic shales containing both Na- and Ca-montmorillonite.