Red Clay Under Charlotte: What the Soil Survey Says About Foundations

Cecil soils cover about 37% of Mecklenburg County's soil acreage. They have a firm red clay subsoil, drain well and have low shrink-swell (USDA NRCS). About 18% of the county's acreage is in high shrink-swell soils such as Iredell. For houses, the NRCS names shrink-swell as the most common limitation, followed by slope and a shallow wet zone.

Why is Charlotte's soil red?

Mecklenburg County lies in the Piedmont, where soil formed in place as old igneous and metamorphic rock weathered, leaving a deep layer of soft, rotted rock called saprolite between the soil and solid bedrock. Granite, gneiss and schist weather into clay, and in Cecil soils that clay is brick red: the official series description lists red clay from about 8 to 42 inches down (USDA NRCS). Cecil is very deep and well drained, and its shrink-swell potential is rated low.

The soils that swell

Over darker rocks such as diabase, diorite and gabbro, the ground is different. Iredell soils, which formed in material from those rocks, are very slowly permeable and carry a brown clay subsoil with slickensides, polished faces made as the clay swells and shrinks (USDA NRCS). In the county's soil data, Iredell, Enon and Helena have the highest shrink-swell values. Together, soils rated high or very high for shrink-swell cover about 18% of the acreage with soil data, and another 20% rate moderate.

How the survey rates house sites

The NRCS rates every soil as a site for houses with and without basements. For houses without basements, which covers slab and crawl space homes, the county's rated acreage breaks down this way:

• Very limited: about 34%
• Somewhat limited: about 28%
• Not limited: about 37%

The reasons it gives most often are shrink-swell, slope, depth to a saturated zone, depth to soft bedrock and flooding, in that order (USDA NRCS, Mecklenburg County soil survey). About 12% of the county is not rated at all, mostly urban land and water where the original soil has been disturbed or covered.

Storms that test the drainage

Heavy rain finds the weak spots. On August 26–28, 1995, the remnants of Tropical Storm Jerry flooded the Briar, McMullen and McAlpine Creek basins in southeast Charlotte. On July 22–24, 1997, the remnants of Hurricane Danny brought up to 13.11 inches, caused as much as $60 million in property damage and took three lives (USGS Fact Sheet FS-036-98). Most houses never see creek water, but storms like these saturate the clay around every foundation.

Using this for your own house

Look up your lot in the USDA Web Soil Survey to see which soil series lies under it. Whatever it shows, the habits are the same: move roof water away, keep soil sloping off the foundation, and watch cracks and doors through a full year of seasons. The guide to foundation warning signs explains what to watch for, and house leveling on piers covers a corner that has already dropped. Typical prices are on the Charlotte cost page.