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Copyright© 2011
By: Fred Willcox
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Worried about the foundation of your house during the drought in southeast Texas? You probably should be. However; the odds of your having foundation problems depend on several factors. The first thing to determine is the type of soil your foundation is sited on.

Most people who reside in the Greater Houston area believe that their house is sited on an expansive clay commonly referred to as “gumbo”. In fact, only 20.7% of the surface soils in Harris County have been classified as expansive clay. The remaining 79.3% of the surface soils in Harris County are made up of silt, sand and a small section of gravel.

According to the “Soil Survey of Harris County” published by the Soil Conservation Service of the United States Department of Agriculture, if you live south of Interstate 10 from downtown Houston west to Highway 6 and south of Highway 59 from downtown Houston northeast to Humble, the odds are that you live on an expansive clay.

Another way to tell if your house is on an expansive clay is to look at the trees. Pine trees do not reseed well in expansive clays. If you have naturally growing pine trees in your area, the odds are that your house is located on a silt soil or on a sandy soil. Pine trees will grow in expansive clays if the tree is planted in a clay soil. Pine seeds are very small. Pine seeds require an open, cohesionless soil to bed and take root. Hardwoods grow more commonly in cohesive (clay) soils as hardwood trees rely on squirrels and other animals to bury their seeds (nuts).

Expansive clays shrink and swell with variations in water contents. The drought may have an effect on your foundation if your house is located on an expansive clay.

Silts and sands are cohesionless materials that are lubricated by the application of water. Watering your foundation in silty or sandy can cause a loss of friction between the soil particles. This can cause consolidation of the soil mass that can result in a loss of support for your foundation. Simplified, this can mean that your watering your foundation in an attempt to prevent differential foundation movements (settlement) may actually cause your foundation to ‘settle’.

If your house is located on a sand or silt, your best bet is to do nothing except to water your grass and vegetation enough to keep them alive and in good condition.

If you own an older home on an expansive clay, your odds of having differential foundation movements are fairly great. The engineering sciences at the time your foundation was constructed had determined that few grade beams were needed in your foundation and that these beams could be shallow in depth. Some grade beams were only designed and built to a depth of 12 inches. These foundations lack rigidity and can deform with the surface of the soil as the soil shrinks down and away from your foundation. If you see indications of differential foundation movements such as diagonal cracks in the interior drywall covering materials, in the brick veneer, out of square door frames and particularly if your begin to notice slopes in your floors tilting down toward the exterior of your house, you probably need to begin a foundation watering program.

If you own a newer home on an expansive clay, your odds of having differential foundation movements are not nearly as great as older homes. Engineering studies of expansive soils have led to much more rigid foundation designs with much deeper grade beams that are spaced much closer together. Your foundation has a greater ability to support itself even when the soil shrinks away from the foundation. While you need to be observant for signs of differential foundation movements, you don’t need to be panicked.

In any event, it is strongly recommended that you do not do anything unless you observed indications of differential foundation movements. Expansive clay soils expand when exposed to water. However; the clay does not know when to stop expanding. I have seen a number of foundations that have been damaged from ‘edge lift’ movements caused by foundation watering. As with many things, ‘rules of thumb’ and ‘common knowledge’ can do a lot of damage. Make sure you know what you’re doing before you damage your own foundation.

No one knows what the long term effects of the drought will be on your foundation or on your house. It will be something that is studied in the future.