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Expansive soils are soils that contain minerals such as certain types of clay that are able to absorb water. When the clay absorbs water, it will expand. The more water they absorb, the more they expand. Soils containing as little as 5% clay can exhibit expansive characteristics. The more clay that the soil contains, the more expansive characteristics the soil will display. As expansive clay soils absorb water, they can expand in volume by as much as about 10%.

If this change in volume occurs to the soil around a home’s foundation, it will put a tremendous uplift force against the edges of the concrete slab supporting the home. The problem is made worse around the edges of the foundation by two facts: 1) rain water affects to soil near the edge of the house much more so than it affects the soil under the center part of the home, and 2) after a drought, there are normally large cracks in the soil around the edges of the home that can hold large amounts of water.

The amount of damage caused by expansive clay soils to homes is significant. According to some studies, expansive clay soils cause more damage to homes in the U.S. than tornadoes, hurricanes, floods, and earthquakes COMBINED.

According to a study by J. David Rogers, Robert Olshansky, and Robert B. Rogers, expansive clay soils can put pressures as high as 5500 pounds per square foot against a home’s foundation. This amount of pressure can actually cause the slab to bend. Although it may not seem to be the case, concrete is not perfectly rigid. It is flexible to a degree. If it flexes too much, it will crack, but concrete is capable of flexing back and forth a certain amount without cracking. This phenomenon is exactly what happens when a home is built on expansive clay soil. It flexes up and down.

concrete slab- soil expansionThe drawing to the right is exaggerated to show that as the soil under a concrete slab begins to expand, it will put a lot of pressure on the slab, and will actually cause the edges of the concrete to lift up. The edges are affected by moisture more that the center of the slab for two reasons: 1) the center of the slab is supported by the concrete surrounding it, so it is less able to move, and 2) the moisture level in the soil changes much more drastically around the edges of the slab than it does under the center of the slab.

concrete slab- soil expansion

 

As the soil dries out during periods of drought, the soil will contract, essentially allowing the edges of the foundation to sag. Again, the drawing to the right is exagerated to show the edges of the slab sagging as the expansive soil underneath it dries out and contracts.

As the slab under a home flexes up and down, it obviously causes the wood framing, sheetrock, brick veneer etc. to move and twist. It is this movement that causes cracks to occur in the interior walls and ceiling and in the exterior brick veneer on homes built on expansive clay soils. As long as the slab does not move too much, it will continue doing this year and year without any significant effects other than the cracks developing and some doors and windows possibly sticking.

Over time, these cycles of upward pressure followed by loss of support/sagging can wreak havoc on a home’s foundation. If they are excessive or not controlled, they can result in the concrete foundation cracking. This will often result in the need for a very expensive repair. This is why it is important to maintain the moisture level as constant as possible in expansive clay soils.

See our other blogs about foundation watering and foundation settling.

This map from https://ngmdb.usgs.gov/Prodesc/proddesc_10014.htm shows the areas of highest expansive soil in the US. The areas in red are the areas with the highest levels of expansive clays in the soil, and the blue areas are the next highest.

map of expansive soil

© 2020 Mike Morgan

This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at mike@morganinspectionservices.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.


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