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Ask a Pro: Is fat clay a threat to my dream home?

One potential problem that can cause years of aggravation and cost tens of thousands of dollars to remediate is high plasticity clay. The damage manifests itself by sloping floors, sticking windows and doors, and cracking in drywall finishes.

High plasticity clays have microscopic mineral grains that are especially attractive to water. While the clay generally can take on a tremendous amount of water and still maintain its strength, its volume will expand. It only takes one bad event like a broken water line or a large spill to wet the subgrade and begin the swelling process. Little can be done except to prevent new water from infiltrating the subgrade and then waiting to repair the damage, which can be costly. Once the swelling begins, it can continue for years.

Concrete floors and foundations can lift or sink from either the pressures developed by the swell or from when the clay dries and begins to shrink. Depending on the changes in moisture content, the amount of shrinking and swelling can be from a ½ inch to 3 inches during the lifetime of a home.

Some solutions to protect the subgrade from moisture content changes and to prevent swelling and shrinking include: using a plastic liner between the concrete and soil around the home, extending rain gutter downspouts away from the foundation, and avoiding irrigation near the home. Before construction, engineers recommend removing three to five feet of the fat clay soil and replacing it with either low plasticity clay or a dirty sand that is not sensitive to changes in volume.

A geotechnical engineer can help you identify fat clay soils and provide specific recommendations. For more information on how our engineers can help you with your soil problems, contact Wes Dickhut.

Wes Dickhut Principal Engineer

P: 701.355.5430