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What are Expansive Soils?

Every summer in California, water becomes headline news. In addition to the unavailability of water, summer or drought conditions can cause expansive soil problems around homes. "Expansive soil" is the "adobe" soil that we hear so much about. These are soils that have a relatively high percentage of clay minerals and are subject to changes in volume with changing moisture conditions.

Expansive clay particles are invisible to the naked eye and swell by absorbing large amounts of water relative to their volume. When these particles dry out, they can shrink considerably. Most of us have seen expansive clays at work when they dry out and crack open in the bottom of a mud puddle in the summer heat. When winter rains fall on the dry, cracked ground, the clays swell; the cracks close; and the ground can heave up as much as several inches.

Each year in the United States, expansive soils cause billions of dollars in damage to buildings, roads, pipelines, and other structures. This is more damage than that caused by floods, hurricanes, tornadoes, and earthquakes combined!

Much of California is underlain by expansive soils. However, expansive soil doesn't cause problems unless poorly designed structures are built on it. According to W.G. Holz from the Colorado Geological Survey in the publication "Home Construction and Swelling Soils Report", despite the fact that 250,000 homes are built on expansive soils each year, only 10% of them will experience significant damage.

When can expansive soils be a problem?

A house built on expansive soil will probably move if the foundation was not designed to take this soil type into account. Movement occurs because the soils expand so forcefully, the foundation actually moves. Different parts of the house can move at different rates and distances, thus cracking the foundation. Significant cracks often appear at the corners of windows and doors, in walls, garage slabs, walkways, and driveways. Doors and windows may become jammed. The "looks" of a home could be affected. During extreme drought conditions, even homes that are not normally affected by expansive soil problems may experience slight cracking.

Expansive soils can affect homes with suspended wood floors as well as those with concrete slab floors. Most of the movement occurs at the outside walls of a building, but the inside of a house can move if water finds its way under the house when it rains, through landscape watering or through a plumbing leak.

Is there any way to minimize the effects of "adobe" soils?

There are some easy and inexpensive ways to help minimize most problems associated with expansive soils without resorting to redesigning the home's foundation. The following recommendations involve keeping expansive soils from either expanding or shrinking too much around the foundation of a house. The goal is to maintain a relatively constant moisture content which results in minimal movement.

  • Remove large trees and bushes that grow within about ten feet of the house. Large plants tend to dry out the soil unless a drip irrigation system is installed (see below).
  • Utilize drip irrigation systems to water vegetation. Drip irrigation minimizes the amount of water used and maintains a more even soil moisture content.
  • See that downspouts and roof gutters don't deposit water close to the foundation. If possible, direct roof water into closed pipes that empty onto the street or other suitable location. Keeping water away from the foundation is the single most important step that can be taken to help minimize an expansive soil problem.
  • Build at least a three foot wide concrete walkway out from the foundation, or compact the soil around the foundation into a hard surface.
  • Slope the soil or concrete away from the foundation. Such construction aids runoff and helps prevent water from puddling or seeping into the ground.

If there are major uncertainties about the soil or if the soils have already severely affected the house, a soils engineer should be consulted. In the long run, consulting a soil engineer before mitigating an expansive soils problem can save energy and expense.

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