Engineering and Construction

Expansive soils are a worldwide problem, causing extensive damage to civil engineering structures.

Jones and Holtz estimated in 1973 that the annual cost of damage in the United States due to expansive soil movement was $2.3 billion (Jones and Holtz, 1973). A more up-to-date figure is about $9 billion in damages annually to buildings, roads, airports, pipelines, and other facilities (Jones and Jones, 1987). Previously discussed the consolidation of clay, which is basically the compression of soft clays that have a high water content. Expansive clays are different in that the near-surface clay often varies in density and moisture condition from the wet season to the dry season. For example, near- or at- surface clays often dry out during periods of drought but then expand during the rainy season or when they get water from irrigation or leaky pipes.

There are many factors that govern the expansion behavior of soil. The primary factors are a change in water content and the amount and type of clay size particles in the soil. Other important factors affecting the expansion behavior include the type of soil (natural or fill), condition of the soil in terms of dry density and moisture content, magnitude of the surcharge pressure, and amount of nonexpansive material such as gravel or cobble size particles (Ladd and Lambe, 1961; Kassiff and Baker, 1971; Chen, 1988; Day, 1991b, 1992a). These main factors are individually discussed below:

Change in Water Content. An important factor for expansive soils is a change in water content.

For example, expansive soils cause extensive damage to structures located in the desert southwest of the United States. Because of the lack of rain, the near-surface clays are often in a desiccated or dry powdery state. After a structure has been built on desiccated clay, water is often introduced through irrigation or leaky pipes, and the clay absorbs the water and expands causing extensive damage.

Other areas may have significant surface deposits of clays, but there is enough yearly precipitation so that the clays stay in a permanently wet condition. Since there is always enough rainfall to keep the clays in a wet state, they tend to remain relatively dormant and they neither swell nor shrink. There can be exceptions to this rule, such as an area that normally has a wet climate, but a severe drought occurs which causes the clays to become desiccated and shrunken, resulting in expansive soil movement.

Although most states have expansive soil, Chen (1988) reported that certain areas of the United States, such as Colorado, Texas, Wyoming, and California, are more susceptible to damage from expansive soils than others. These areas have both large surface deposits of clay and have climates characterized by alternating periods of rainfall and drought.

Amount of Clay Size Particles. The more clay size particles of a particular type a soil has, the more swell there will be (all other factors being the same). Clay size particles attract water to their particle faces due to the double layer effect. Water is also drawn into the soil due to the negative pore water pressures associated with dried clay. Thus, the more clay size particles in a dry soil, the greater the need for water to be drawn into the soil and hence the higher the swell potential of the soil.



Type of Clay Size Particles. The type of clay size particles significantly affects swell potential.

Given the same dry weight, kaolinite clay particles (activity between 0.3 and 0.5) are much less expansive than sodium montmorillonite clay particles (activity between 4 and 7) (Holtz and Kovacs, 1981).

Montmorillonite is a much smaller and more active clay mineral than kaolinite, and this results in much more attracted water per unit dry mass of clay particles. Once again, the need for more water results in a greater amount of water drawn into the dry soil and hence higher swell potential of the soil.

Density and Water Content. The dry density and water content are important factors in the amount of expansion of a soil. In general, expansion potential increases as the dry density increases and the water content decreases. The most expansive condition is when the soil has a high dry density and a low moisture condition. This often occurs when near-surface clays becomes desiccated, such as during a hot and dry summer season.

Clays that have a low dry density and high water content may not have additional swell, but they could still cause the structure to experience downward movement if they should dry out. Thus it is not unusual for near-surface clays to experience changes in dry density and water content throughout the season, depending on whether they have absorbed water and swelled-up during the rainy season, or shrunk and dried-out during the dry season.

Surcharge Pressure. Laboratory and field studies have shown that the amount of swell will decrease as the confining pressure increases. The effect of surcharge is important because it is usually the lightly loaded structures such as concrete flatwork, pavements, slab-on-grade foundations, or concrete canal liners that are often impacted by expansive soil.