Expansive Soils

There are three basic types of soil naturally occurring in this area: sand, silt and clay. Clay soils are generally classified as "expansive." This means that a given amount of clay will tend to expand (increase in volume) as it absorbs water and it will shrink (lessen in volume) as water is drawn away. One of the most expansive of the clay soils is called "adobe".

Expansive soils can create damaging movements shrink and swell to foundations and structures. These movements originate from changes in soil moisture. Providing uniform soil moisture next to and under your foundation is the single best thing you can have to reduce or minimize the effect expansive soil movements have on your structure.

Indicators of Expansive Soil Movements

The items below are signs you can look for to determine possible expansive soil related movement. The probability that your foundation has experienced some movement increases with the number of indicators observed, their frequency and location in the structure.

Exterior Indicators:

Interior Indicators:

Soil Moisture Changes

Observing soil moisture changes around your foundation is possible, but what about under it? Moisture can move from outside to under your foundation through a property of soils known as suction. Soil suction is similar to placing just a corner of a dry, compressed sponge in contact with a puddle of water. In a short time, the sponge has drawn water throughout itself and grown in volume. While a water source is present, the sponge will continue to absorb water until it is saturated. If the water source is cut-off, then water already in the sponge will distribute itself evenly, but the sponge will not reach saturation.

Water can move horizontally and vertically through the soils under your foundation in a similar manner. As clayey soils draw water to themselves, they too grow in volume (swell or heave) causing your foundation to move. Drying outside your foundation reverses the process. The moist soils will lose volume (shrink) as soil moisture moves out from under your foundation causing the foundation to settle. Shrinking and swelling soil motions can lead to damaging your foundation and structure. Uniform changes in soil moisture are less damaging to your structure than localized changes.

Several sources of soil moisture changes are provided in the following table. You should review the list and possible actions to control or minimize the various sources. Begin practicing the suggested actions as soon as possible to improve your foundation and structure performance. Many of these actions can become a routine part of your ongoing conscientious owner maintenance activities. Annually inspect the area within 5 feet of all sides of your foundation after a rain to determine if proper drainage is maintained away from your structure. Monitor existing cracks for progressive or seasonal movements. Some of the possible actions, suggested in the accompanying table, will require an expert for assistance.

What Can I Do to Minimize the Effects of Expansive Soil?