What is St. Paul soil and how does it affect Oklahoma foundations?

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

In short: St. Paul is a deep, well-drained soil on broad old terraces in western Oklahoma, with dark silt loam over a silty clay loam subsoil. Its clay content is moderate, about 27 to 40 percent in the subsoil, and water moves through it moderately slowly. Foundation questions on St. Paul center on moisture control, with the mapped shrink-swell rating checked for each site.

What is St. Paul soil?

St. Paul is an official USDA soil series classified as "Fine-silty, mixed, superactive, thermic Pachic Argiustolls" ¹. It formed in a thin layer of Pleistocene (Ice Age) silty alluvium over residuum (rock weathered in place) from Permian siltstone and sandstone ¹. It lies on broad, nearly level to moderately sloping paleoterraces, old terrace remnants of an alluvial plain ¹. Slopes are mainly under 3 percent, ranging up to 8 percent ¹.

The series occurs in the Central Rolling Red Plains (MLRAs 78B and 78C) of Oklahoma, Texas and Kansas and is rated as extensive ¹. The type location is in Roger Mills County, about 2 miles south and 4 miles west of Leedey; the series was established in Kiowa County in 1931 ¹. Laboratory-sampled profiles come from Dewey, Woodward, Woods and Harper counties ¹. Mean annual precipitation is about 26 inches (range 21-28) ¹. Most areas are cropped to wheat and other small grains ¹.

In Oklahoma, St. Paul falls in the Southwest (Lawton and Red Plains) and Northwest (Gypsum Hills) regions, on Western Permian red beds and Quaternary alluvium and terraces.

What is the typical soil profile?

Source for the table: ¹. Other properties from the official description ¹:

• Clay: 23 to 38 percent in the control section; upper Bt 27 to 35 percent, up to 40 percent in the lower part; BC or BCk 20 to 40 percent ¹.
• Dark topsoil: the mollic layer extends to about 45 inches in the typical profile ¹.
• Drainage: well drained; permeability moderately slow ¹.
• Runoff: low on 0-1 percent slopes, medium on 1-5 percent, high on 5-8 percent ¹.
• Depth to bedrock and water table: not stated ¹.

What does St. Paul soil mean for foundations?

Shrink-swell

The USDA rates shrink-swell as linear extensibility, a linear expression of the change in soil volume between moist and oven-dry states: under 3 percent low, 3 to 6 percent moderate, 6 to 9 percent high, 9 percent or more very high ³. St. Paul's subsoil is silty clay loam with clay up to about 40 percent ¹. The rated value for each mapped unit is in Web Soil Survey ².

Clayey soils swell as they gain water and shrink as they dry, and uneven moisture change around a shallow foundation is what drives damaging movement ⁴. See shrink-swell and the active zone and heave vs settlement.

Moisture control

Moderately slow permeability ¹ means water that soaks in near a house drains away gradually. The ASCE Geo-Institute guide lists these moisture sources and responses ⁴:

• Roof runoff: discharge downspouts at least 3 feet from the foundation ⁴.
• Grading: slope soil away at least 3 inches in 5 feet ⁴.
• Irrigation: keep lines and heads at least 5 feet away and water evenly ⁴.
• Trees: plant farther from the house than the tree's mature height ⁴.
• Plumbing leaks: watch water bills and repair leaks ⁴.

See surface drainage and grading and trees and roots.

Runoff

Runoff on St. Paul ranges from low on nearly level ground to high on 5 to 8 percent slopes ¹.

Related

• Quinlan soil
• Pond Creek soil
• Woodward soil
• Soil testing and lab values
• Soil series index