Why does some Oklahoma clay swell more than others?

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

In short: How much a clay soil swells depends mostly on which clay minerals it contains ¹. Montmorillonite (a smectite) takes water between its layers and swells the most; kaolinite swells much less ¹². Much of Oklahoma's soil is rich in these swelling clays, which is why so many Oklahoma foundations move with the seasons ¹³.

What are clay minerals?

Clay minerals are microscopic, plate-shaped crystals built from layers of silica and alumina. Soil scientists group them by how those layers stack:

• 2:1 clays (two silica layers around one alumina layer), which include montmorillonite and illite.
• 1:1 clays (one silica layer and one alumina layer), such as kaolinite.

An Oklahoma State University bulletin identifies water absorbed into the interlayers of 2:1 clays as the main mechanism of shrink and swell, and notes that kaolinite-type 1:1 clays swell much less ¹. Forensic engineers describe montmorillonite's structure as loosely bound, letting water into the spaces between its layers and giving it more expansion capacity than illite ².

What are the main clay minerals?

What have Oklahoma studies found?

• Soils on Permian red beds. In central and western Oklahoma soils formed from Permian shales, the fine clay was dominantly montmorillonite, with a mix of illite and montmorillonite, plus small amounts of kaolinite, in the coarser clay ¹. Kirkland soil, with montmorillonite-dominated fine clay, had high shrink-swell and was judged unstable for most engineering uses ¹.
• Shrink-swell numbers. The study measured COLE (coefficient of linear extensibility, how much a soil clod shrinks from wet to dry). Values above 0.03 indicated substantial smectite; Bethany, Kirkland and Norge soils reached 0.095, 0.090 and 0.066 ¹.
• Rock vs soil. Hennessey shale cored near Oklahoma City was mainly illite and kaolinite with low shrink-swell potential, as were shale cores from two other Oklahoma sites; other studies have reported montmorillonite and mixed-layer clays in the same units elsewhere ⁴.
• Statewide. More than 75% of Oklahoma's bedrock can produce expansive soils ³.

Can soil color show how much a clay will swell?

High-plasticity clays occur in both red and gray soils. Red Permian soils such as Kirkland are montmorillonite-rich ¹, and a south Tulsa report on a site underlain by Nowata Unit shales found olive, brown and gray fat clay with plasticity indices up to about 39 ⁷. Lab tests, not color, are used to judge swell.

Which lab tests stand in for mineralogy?

Mineral analysis is rarely run for a house. Engineers instead use index tests. The residential code treats soil as expansive when it has a plasticity index of 15 or more, more than 10% passing a No. 200 sieve, more than 10% finer than 5 micrometers, and an expansion index above 20; if the expansion index is run, the other three are not required ⁶. See soil testing and lab values and soil lab values data.

How does mineralogy relate to chemical treatment?

Chemical treatments work on the clay itself. Forensic engineers list lime, cement, potassium and other chemicals among soil injection options ². Oklahoma pavement research found that stabilizers react with soil particles to form cementing compounds that add strength ⁵. Sulfate is a known exception: in a soil with about 15,400 ppm soluble sulfate, adding 3% lime increased swell by more than 1,200% because it formed ettringite, an expansive mineral ⁵. See chemical soil stabilization and stabilization research.

Related

• Shrink-swell and the active zone
• Soil testing and lab values
• Hennessey shale
• Kirkland soil
• Bethany soil