Your House Foundation Is On What Kind of Soil?

My concrete slab foundation sits on Branyon clay.

A couple of blocks away there are areas of Houston-Black and Lewisville silty clay.

None of this is happy news.

How did I aquire this fascinating knowledge? Why, from the Web Soil Survey tool on the USDA’s Natural Resources Conservation Service website.

The survey says the “linear extensibility”, engineer speak for the amount of shrink-swell my clay soil can produce, is very high. That is very bad for my foundation. To quote the soil survey report:

Linear extensibility is used to determine the shrink-swell potential of soils. The shrink-swell potential is low if the soil has a linear extensibility of less than 3 percent; moderate if 3 to 6 percent; high if 6 to 9 percent; and very high if more than 9 percent. If the linear extensibility is more than 3, shrinking and swelling can causedamage to buildings, roads, and other structures and to plant roots.

My Branyon clay is over 9 percent. Ouch. Well now, what is a soil survey exactly?

One of the main tools available to help land users determine the potentials and limitations of soils is a soil survey. Soil surveys are available through the USDA, Natural Resources Conservation Service (NRCS). The surveys are made by NRCS in cooperation with other Federal, State, and local agencies. Our offices can provide this information, but more and more soil surveys are also available on the Internet. Web Soil Survey allows you to produce a customized soil survey for your own area of interest.

A soil survey generally contains soils data for one county, parish, or other geographic area, such as a major land resource area. During a soil survey, soil scientists walk over the landscapes, bore holes with soil augers, and examine cross sections of soil profiles. They determine the texture, color, structure, and reaction of the soil and the relationship and thickness of the different soil horizons. Some soils are sampled and tested at soil survey laboratories for certain soil property determinations, such as cation-exchange capacity and bulk density.

Like any tool, a soil survey is helpful only if you know what it can and can’t do, and if you use it accordingly. The survey does not replace careful onsite investigation or analysis by a soil scientist

(Found in “From the Surface Down,” NRCS)

This is the kind of information a structural engineer incorporates into his report when hired to inspect a foundation. It helps determine the method of repair, if needed, for your (and my) situation. Stupid clayey soil.


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