Cecil clay, the state soil of North Carolina, has a dense subsoil horizon with very low permeability. Rainfall infiltrates the sandy topsoil, stalls at the clay, and then moves sideways until it meets a foundation footing, which is why Wake County crawl spaces stay damp long after the storm passes.
The Cecil series profile
Cecil soils weather from granite and gneiss and consist of a thin sandy loam surface over a red clay subsoil that can exceed 40% clay content. Below that lies saprolite, then bedrock. Permeability drops by an order of magnitude at the clay boundary.
Wake County receives about 46 inches of precipitation each year. When infiltration halts at the clay horizon, that water becomes lateral flow toward the lowest point on the lot, which is frequently the crawl space.
Triassic basin clays are worse
Western Wake, Apex, Morrisville, and Durham sit in the Triassic basin, where Creedmoor and White Store clays add shrink-swell behavior. These soils change volume by roughly 15% between saturation and drought, exerting cyclical lateral pressure on block foundation walls.
That movement opens mortar joints and stair-step cracks, which then admit the very water that caused the swelling in the first place.
What actually fixes it
Grading and gutter extensions come first, because surface water is cheap to redirect. Where the water table or perched water is the source, interior drainage plus a sealed 20-mil liner and a dehumidifier is the durable answer.
Foundation vents do not work in this climate. Humid Triangle summer air entering a cool crawl space condenses on framing and adds moisture rather than removing it.