Georgia red clay and your foundation
You have probably been told that Georgia red clay swells when it rains, shrinks when it dries, and tears houses apart. Before you buy a repair based on that, it is worth reading what the USDA soil survey actually records for the ground under this part of the state.
What the soil survey records for Cobb County
The NRCS soil survey update for Cobb County maps 221,310 acres. Its largest single mapped units are Madison and Pacolet soils, 10 to 15 percent slopes, severely eroded at 14,555 acres, or 6.6 percent of the county, followed by Madison clay loam, 6 to 10 percent slopes, severely eroded at 12,815 acres and Gwinnett clay loam, 6 to 10 percent slopes, severely eroded at 12,805 acres, each 5.8 percent. USDA-NRCS, Update for the Soil Survey of Cobb County, Georgia (1996) Cecil is mapped here too, at a smaller share.
Those are four different series. The USDA classifies every one of them the same way on the two properties that matter for a foundation:
| Series | Taxonomic class | Shrink-swell | Drainage |
|---|---|---|---|
| Madison | Fine, kaolinitic, thermic Typic Kanhapludults | Low | Well drained |
| Pacolet | Fine, kaolinitic, thermic Typic Kanhapludults | LOW | Well drained |
| Gwinnett | Fine, kaolinitic, thermic Rhodic Kanhapludults | LOW | Well drained |
| Cecil | Fine, kaolinitic, thermic Typic Kanhapludults | Low | Well drained |
Ratings quoted from each official series description: Madison, Pacolet, Gwinnett, Cecil.
Kaolinitic is the word that matters
Kaolinite is a non-expanding clay mineral. It does not take water into its crystal structure the way smectite does. That is the reason the USDA rates all four of these series low for shrink-swell, and rates them well drained, with bedrock more than 60 inches down in the case of Cecil. USDA-NRCS: Cecil, shrink-swell Low, well drained
The genuinely expansive soils in the United States are smectite-rich and concentrated elsewhere. The national reference is the USGS Swelling clays map of the conterminous United States, Olive and others, 1989. USGS Map I-1940 (1989)
What this does and does not tell you
It does not tell you your house is fine. Foundations in Cobb County crack, settle and need real repair. What it tells you is that a shrink-swell explanation does not follow from the mapped soil, so if that is the explanation you were given, it is fair to ask what it is based on.
It also does not tell you what is under your particular lot. The county survey maps the natural soil. If your lot was graded when the subdivision was built, what your footing bears on may not be the mapped soil at all. Ask a geotechnical or structural engineer whether your footing sits on cut, fill, or both. That is a question for someone standing at your house, not for a web page.
You can at least see what is mapped for your own parcel, free, before anyone tells you what is down there. USDA-NRCS Web Soil Survey
Moisture does swing here
Whatever the mechanism at your house, the moisture environment is not gentle. NOAA's Georgia climate summary records that the number of 3-inch extreme precipitation events reached its highest level during the 2015 to 2020 period, and projects that higher temperatures will increase the rate of soil moisture loss during dry spells, which could lead to more intense droughts. NOAA / NCICS Georgia State Climate Summary 2022
For reference, the Cecil description lists Index Surface Runoff: Medium. USDA-NRCS: Cecil, surface runoff What that means for the water around your particular footing depends on your grading, and is worth raising with whoever inspects it.
What to ask about your quote
If a contractor explains your cracks with expansive clay, ask which soil series they mean and what rating they are relying on. If they recommend piers, ask what they think caused the loss of bearing, and whether drainage and grading have been assessed first. The answers tell you a lot.