Wake County holds three soils, and one is bedrock at eleven inches
The county has a soil series named after it, and it is the one that stops a grinder. The Wake series, a Lithic Udipsamment type-located in Wake County, weathers from granite and gneiss, with hard unweathered rock at 11 to 20 inches and a C horizon about 18 percent gravel. Cutting to the city's six-inch standard there means working within a foot of rock. Cecil, the state soil, is the Piedmont default: 35 to 70 percent clay in the Bt horizon over saprolite from about 127 centimetres, its typical pedon in Franklin County, inside the Raleigh-Cary MSA.
Part of Wake County also sits in a Triassic basin. The NC Geological Survey names Wake among eleven counties over the Deep River Basin without saying which part, and reports those soils "do not perform adequately during percolation tests for septic drain fields." The matching White Store series carries 45 to 70 percent clay and a COLE of 0.09 or more from 51 to 127 centimetres, with slickensides in the subsoil. NCGS links that shrink-swell to heave, so a backfilled grinding void there moves with the seasons.