A Definition

Heaving bedrock and swelling soils contain clay minerals that can attract and absorb water. As a result these materials swell in volume when they get wet and shrink when they dry. Heaving bedrock contains claystone that is composed of the clay minerals that shrink and swell.

Legal definition

H.B. 1041, 106-7-106 (6): “Expansive soil and rock” means soil and rock which contains clay and which expands to a significant degree upon wetting and shrinks upon drying.

Descriptive definition

Sedimentary rocks and surficial soils are composed of gravel, sand, silt, and clay particles. In order to visualize the relative grain sizes of these particles, an example using familiar objects can be given. Although the average diameter of a gravel particle is approximately ¾ in., suppose an average gravel particle were the size of a basketball. An average sand particle would then be the size of a baseball and a silt particle the size of a pea. The average clay particle, however, would be almost invisible, with a pencil dot representing a large clay particle. These clay particles may consist of a variety of minerals—quartz, feldspar, gypsum, and clay minerals. Common clay minerals in Colorado are montmorillonite, illite, and kaolinite. To return to the previous analogy, gravel, sand, silt, and some clay particles are often round, three-dimensional objects. Clay minerals, however, are generally flat, nearly two-dimensional plates just as the above mentioned pencil dot is flat and two-dimensional.

The clay minerals in rocks and soils are responsible for their expansion, or “swell”, as it is generally called. This swelling is caused by the chemical attraction of water to certain clay minerals. Layers of water molecules can be incorporated between the flat, submicroscopic clay plates. As more water is made available to the clay, more layers of the water are added between the plates, and adjacent clay plates are pushed farther apart as shown in the simplified diagrammatic sketch below.

This pushing apart, or swelling, occurs throughout the mass of soil that is being wetted, and causes increased volume and high swell pressures within the mass. The opposite effect, called shrinkage, may occur if a previously wet swelling clay is dried. Although no large positive pressures are exerted, shrinkage will cause a volume decrease of the soil mass. These processes of swelling and shrinkage may occur any number of times for a single soil mass. Either swell ore shrinkage may cause damage to streets and buildings, but swell accounts for nearly all such damage in Colorado.

The clay mineral responsible generally for swelling is montmorillonite, often called “bentonite”. A sample of pure montmorillonite may swell up to 15 times its original volume. However, most natural soils contain considerably less than 100 percent montmorillonite, and few swell to more than 1-1/2 times their original volume (a 50 percent volume increase) (Jones and Holtz, 1973). A small load may decrease the actual swell to less than 1-1/4 times the original volume (a 25 percent volume increase). However, as 25 percent increase can be extremely destructive because volume increases of 3 percent or more are generally considered by engineers to be potentially damaging and require specially designed foundations.