What is Tillman soil and how does it affect Oklahoma foundations?

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

In short: Tillman is a very deep clayey soil of southwestern Oklahoma that formed in alluvium washed from Permian red-bed clays. Its official description records cracks more than 48 inches deep and a linear extensibility above 6 percent, which is in the high shrink-swell class. In a dry climate with about 26 inches of rain a year, seasonal drying and wetting of this clay drives foundation movement.

Where does Tillman soil occur?

The USDA Natural Resources Conservation Service (NRCS) keeps an Official Series Description (OSD) for each named soil. Tillman was established in Tillman County, Oklahoma, in 1932, and its current type location is in Wilbarger County, Texas, near Lockett ¹. The OSD places it in the Rolling Limestone Prairie (MLRA 78D) and the eastern Central Rolling Red Plains (MLRA 78C) of northwest Texas and western Oklahoma, and its extent is large ¹.

Tillman forms on paleoterraces, which are old alluvial terraces left above today's streams, on nearly level to gently sloping plains ¹. It developed in loamy and clayey alluvium from Permian red-bed clays and claystone, mainly of the Clear Fork Group ¹. That ties it to the Western Permian Red Beds and Quaternary terraces of the Southwest (Lawton and Red Plains). Most Tillman land grows small grains, cotton and sorghum ¹.

What are the key properties of Tillman soil?

"Vertic" in the classification means the soil shows strong shrink-swell features such as deep cracks; Tillman was reclassified from Typic to Vertic Paleustolls in 2000 ¹. Some profiles also show a few slickensides, polished faces formed when swelling clay shears against itself ¹. The "k" in Btk marks a layer with lime (calcium carbonate) accumulation ¹.

Associated soils include Hollister, on slightly lower landscape positions, and Vernon ¹.

What do these properties mean for foundations?

High shrink-swell

The coefficient of linear extensibility (COLE) is the change in length of a soil clod from moist to oven-dry; times 100 it is linear extensibility percent ³. NRCS classes 6 to 9 percent as high and notes that values above 3 percent can damage buildings and commonly need special design ⁴. Tillman's COLE above 0.06 meets the high class ¹. The rated value for a specific map unit is in Web Soil Survey ².

Expansive clays are easiest to recognize in the dry season by their deep cracks, and seasonal moisture change can reach depths of 3 to 40 feet ⁵. Areas with pronounced wet and dry seasons carry the greatest hazard ⁵. Typical movement patterns include perimeter drying settlement in drought and perimeter wetting heave or center heave after wet spells.

Trees and watering in a dry climate

Tree roots can dry clay unevenly and cause settlement, and the ASCE guide suggests keeping trees farther from the house than their mature height ⁶. It also warns that overwatering can cause heave, and recommends even moisture on all sides of the foundation ⁶. Lawn and garden watering creates a moist zone along the outside of a foundation ⁵. See trees and roots and drought and wet cycles.

Related

• Shrink-swell and the active zone
• Hollister soil
• Foard soil
• Tree-root desiccation settlement
• Soil Series Index