The Ground Under Knoxville: Soluble Rock and Red Clay

Knoxville sits in East Tennessee's Valley and Ridge, where limestone and dolomite develop open channels and thick weathered soil holds groundwater (USGS). Knox County's most extensive soil, Dewey, formed from limestone and has a red clay subsoil (USDA). Sinkholes happen, but most foundation trouble here is clay and water.

Rock that dissolves

Carbonate rocks underlie large areas of East Tennessee. In the Valley and Ridge, limestone such as the Jonesboro and dolomite such as the Knox develop what geologists call conduit porosity: open passages that water dissolves along fractures. Above the rock, thick regolith, the weathered layer, stores and moves large amounts of groundwater (U.S. Geological Survey). That is the definition of karst ground, and it is why the bedrock surface under a Knoxville yard can be uneven.

What the sinkhole record shows

A 1980s inventory of karst activity in the eastern Tennessee Valley and Ridge logged more than 300 subsidence events at over 200 sites. Knox Group dolomites accounted for about two-thirds of the reported sinkholes, most activity was in valleys and flat areas, and the most common cause was altered surface drainage combined with soil disturbance from construction (Newton and Tanner, 1987). The lesson for a homeowner: where you send roof and yard water matters.

Red clay and shrink-swell

Dewey covers more of Knox County than any other soil series in the USDA survey. It is very deep and well drained, formed in limestone residuum, with a dark red to red subsoil of clay or silty clay. Across the county's major soils, shrink-swell rates moderate or low, with none rated high, but the USDA still lists it as a limitation for houses with basements on about 46% of rated acreage, and about 64% of rated acreage is classed very limited for basements, mostly because of slope and shrink-swell.

The 50-foot sinkhole setback

In Knox County subdivision approvals, Knoxville-Knox County Planning has required sinkholes and a 50-foot setback around them to be shown on the final plat. Building inside that setback has been allowed only when a registered engineer's geotechnical study finds it acceptable and Knox County Engineering and Public Works approves it, and engineered footings may be required (Knoxville-Knox County Planning case records). If your plat shows a closed contour near the house, keep a copy for any foundation contractor or engineer.

Practical steps on karst ground

• Never run downspouts or a sump line into a depression or sinkhole.
• Watch for new circular dips, pooling that suddenly drains, or a slab that drops quickly.
• Keep construction fill and debris out of sinkholes.

For movement you can see in the house, read about house leveling and piers in Knoxville or foundation crack repair, and compare the typical foundation repair costs.