What is Hennessey shale and how does it affect Oklahoma foundations?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: The Hennessey Group is red-brown Permian shale and siltstone that forms the surface across western Oklahoma County and much of north-central Oklahoma. It weathers into red clay soils rich in swelling clay minerals. These soils shrink in drought and swell when wet, which is the classic cause of seasonal foundation movement in the western OKC metro.

What is Hennessey Group rock?

The Hennessey is a group, meaning several related rock formations mapped together. It is early Permian in age and named for exposures at Hennessey in Kingfisher County ¹. Its subunits include the Fairmont Shale, Kingman Siltstone, Salt Plain Formation and Bison Formation, among others ¹.

A USGS study of a site northwest of Oklahoma City described the Hennessey as mainly red-brown shale interbedded with red-brown and orange-brown siltstone, with greenish-gray beds, some sandstone and gypsum lenses, about 90 meters (roughly 300 feet) thick there ². A regional groundwater model notes thin gypsum beds of 1/2 to 2 inches higher in the section and more sand to the south ³.

Where is it at the surface?

Rock layers in central Oklahoma dip west, so the surface of Oklahoma County is mostly Hennessey shale, with the Garber Sandstone appearing on the east side ³. The Hennessey is about 400 feet thick on the west side of the model's project area ³. See the Garber Sandstone page for the sandier ground to the east.

How does the shale behave near the surface?

The unweathered shale is very tight and nearly impermeable ³. Near the surface it is different:

• Weathered zones reach about 10 to 15 feet deep and are fractured by expansion, contraction and leaching ³.
• USGS cores found slickensides at depths from about 6 to 37 meters and thin gypsum lenses at about 19 to 24 meters, and hydraulic conductivity was highest at shallow depth ².
• At one test boring, about 6.4 meters of soil and surface clay overlay the rock ².

Slickensides are polished, grooved surfaces left when clay or shale shears against itself, a common mark of repeated swelling and shrinking.

What red clay soils form on the Hennessey?

An Oklahoma State University study measured the coefficient of linear extensibility (COLE), a lab measure of how much a soil shrinks as it dries. It found values up to about 0.090 in a Kirkland-like soil and 0.095 in Bethany, tied to large amounts of smectite clay ⁶. The authors described the Kirkland soil as inherently unstable and said these values indicate severe limitations for many engineering projects ⁶.

What does this mean for foundations?

Because the clay changes volume with moisture, foundations on Hennessey soils commonly show:

• Edge drop in drought. Clay loses volume as it dries, so the ground settles ⁷. See perimeter drying settlement.
• Edge lift or center heave in wet periods. Clay gains volume as it takes up water ⁷. See perimeter wetting heave and center heave.
• Tree-related drying. Roots pull water from clay and can cause local settlement ⁷. See tree-root desiccation.

The ASCE homeowner guide lists sun-driven drying on south and west sides, tree roots, leaks and poor drainage among the main causes of uneven moisture under shallow foundations on clay ⁷. It recommends sloping soil at least 3 inches in the first 5 feet and placing trees farther away than their mature height ⁷.

Related

• OKC metro
• North-Central (Red Prairies and Wheat Belt)
• Renfrow soil
• Kirkland soil
• Shale weathering and bearing strata
• Drought and wet cycles