Expansive Soils

Expansive or swelling soils, as their name implies, are soils which swell when subjected to moisture. These swelling soils typically contain clay minerals that attract and absorb water. The clay mineral responsible for most expansive damage is smectite, although bentoniteand illite also have some expansive potential. Another category of expansive soil known as swelling bedrock contains a special type of mineral called claystone.

When water is added to these expansive clays, the water molecules are pulled into gaps between the clay plates. As more water is absorbed, the plates are forced further apart, leading to an increase in soil pressure or an expansion of the soil's volume. (Handy, 64)

Identifying Expansive Soils

  Appearance - Soils containing expansive clays become very sticky when wet and usually are characterized by surface cracks or a "popcorn" texture when dry. Therefore, the presence of surface cracks is usually an indication of an expansive soil.   (Noe, 20)

Expansive soil showing cracks
Expansive soil with "popcorn" texture

  Drilling and Laboratory Analysis - In many cases, expansive soils are buried under a layer of topsoil or dense vegetation and cannot be identified at the surface. Therefore, collecting soil samples from various test holes each several feet deep is required. These test holes can be drilled by geotechnical and civil engineering firms or by some construction companies. After the samples are taken, they are sent to a laboratory where the swelling potential is determined. In areas where there is a high concentration of swelling soils, laboratory analysis of the soil is required by law. Consult the real estate agency to find out if a swelling soil report is available for your property.

A sample of an expansive soil with moderate swell potential
The same soil sample after a small
amount of water has been added. Notice the sample has expanded considerably.
The same sample 48 hours later, after the sample has had time to shrink to a smaller volume.

  Inspection of Existing Structures - If construction has already occurred at the site, inspection of the existing structures may help identify the presence of reactive soils. Some common structures are listed below, along with the warning signs that may indicate a reactive soil is present.

Major cracks in exterior walls at doors and windows

  Location - Although expansive soils exist nationwide, certain areas are more severely affected than others. For instance, Colorado, Texas, North Dakota, South Dakota and Montana have very high concentrations of swelling soils. Consult the state Geological Survey, the state Geology Department, or your local city or county building department to obtain maps of swelling soil distribution in your area. The phone numbers of these agencies will be listed in the government blue pages of the phone book.

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NOTE: This information is provided for general education only. For detailed information concerning reactive soils, you should always contact a geotechnical firm.