What do lab tests show about Oklahoma clay soils?
In short: Lab tests on Oklahoma clays show plasticity index values mostly in the high 20s to low 40s in the subsoil, with the strongest shrink-swell found below the surface layer. Some western soils with modest plasticity carry very high sulfate, which changes how they react to lime treatment. These measured values give a firmer picture than general labels like "high PI."
What do the key lab terms mean?
- Liquid limit (LL): the water content at which clay starts to flow like a liquid. Higher LL usually means more clay activity.
- Plasticity index (PI): the range of water content over which the soil stays moldable. Higher PI generally means more swelling and shrinking.
- COLE (coefficient of linear extensibility): how much a soil clod shrinks in length from moist to dry. A COLE of 0.09 means about 9% shrinkage.
- Swell: how much a sample grows when soaked, reported as a percentage.
- Sulfate (ppm): soluble sulfate, which matters if lime or cement is added.
More background is in soil testing and lab values and clay mineralogy.
What do statewide soil tests show?
An Oklahoma Geological Survey geologist reported that over 75% of Oklahoma has bedrock units that can produce expansive soils, and that montmorillonite clay can swell to roughly 1.5 to 2 times its dry volume when wet 7.
What do soil tests show in central Oklahoma?
| Soil and location | LL | PI | Clay | Other measured values |
|---|---|---|---|---|
| Kirkland subsoil (B horizons), Canadian Co. | 52 to 53 | 30 to 32 | about 38 to 40% | COLE 0.076 to 0.090 1 |
| Kirkland surface (Ap) | 26 | 4 | about 17% | COLE 0.016 1 |
| Bethany subsoil (B21t), Canadian Co. | 47 | 27 | 31 to 38% in B horizons | COLE 0.095 1 |
| Renfrow clay, Logan Co. | 59.4 | 33.2 (27.2 after 14-day cure) | 35% | Sulfate 262 ppm 2 |
| Kingfisher soil (lean clay) | about 39 | about 21 | not given | Swell 6.1%, no soluble sulfate detected 3 |
| Clays at I-35 and SH-59, McClain Co. | about 46 to 52 | 29 to 36 in logs, up to 41 | not given | Sulfate 38 to 1,300 ppm 4 |
What the numbers show
- The swell is in the subsoil. Kirkland's surface layer tested at PI 4, while its subsoil tested at PI 30 to 32 1. The study described Kirkland as an "inherently unstable soil" because of its high, montmorillonite-dominated clay content 1.
- Shrink-swell is measurable. Kirkland and Bethany subsoils had COLE values up to 0.090 and 0.095 1.
- Predicted heave. The McClain County report estimated a potential vertical rise of 2.8 inches, against a commonly tolerated maximum of 0.75 inch 4. Potential vertical rise is an estimate of how far the ground surface could heave if the clay became fully wet.
The rock vs the soil
The USGS cored Hennessey shale at Lake Hefner after drilling through about 6.4 meters (21 feet) of soil and surface clay 6. The principal clay minerals in the shale were illite and kaolinite, both with low shrink-swell potential 6. Thin gypsum lenses appeared at about 19 to 24 meters 6. By contrast, the soils studied in Canadian County had fine clays dominated by montmorillonite 1. This contrast is why the Hennessey shale page treats the weathered soil zone and the rock below separately.
What do soil tests show in the Tulsa area?
City of Tulsa boring reports provide measured values on weathered shale ground:
- E 91st St between Memorial and Mingo: LL about 27 to 56 and PI about 9 to 39 across the borings 5.
- The highest single sample tested at LL 56 and PI 39 5.
For more Tulsa borings, see depth to rock from borings.
What do soil tests show in southern and western Oklahoma?
- Durant (south-central): the NRCS series description says upper subsoil horizons are 40 to 60% clay and that cracks more than 1 cm wide and 30 inches deep form during dry periods 8. See Durant.
- Vernon soil (Major County): lean clay with PI about 11 to 15 but soluble sulfate of 15,400 ppm 3. Adding 3% lime increased its swell by 1,237%, which the researchers attributed to formation of the expansive mineral ettringite 3. See Vernon.
- Carnasaw (southeast): fat clay with LL about 58 and PI 29 3.
The Vernon result shows why sulfate, not just PI, matters when chemical treatment is considered. More on that in stabilization research.
Related
- Soil testing and lab values
- Shrink-swell and active zone
- Clay mineralogy
- Stabilization research
- Kirkland soil
- Renfrow soil
Common questions
What is a typical plasticity index for Oklahoma clay?
Measured subsoil values in central Oklahoma commonly fall around 27 to 41. For example, Kirkland subsoil in Canadian County tested at PI 30 to 32 and Renfrow clay in Logan County at about 33 before curing.
Is Oklahoma shale itself expansive?
Not always. The U.S. Geological Survey found that the main clay minerals in Hennessey shale cored at Lake Hefner were illite and kaolinite, which have low shrink-swell potential. The soils formed on top of such rock can be far more expansive.
How much of Oklahoma has expansive soil?
The Oklahoma Geological Survey reported that over 75% of Oklahoma has bedrock units that can produce expansive soils.
Why does sulfate in soil matter?
When lime or similar calcium-based treatment is added to sulfate-rich soil, an expansive mineral called ettringite can form. In one Oklahoma test, 3% lime increased the swell of a high-sulfate soil by 1,237%.
Sources
- Shrink-Swell and Other Characteristics of Five Benchmark Soils of Western Oklahoma (Technical Bulletin T-150) Oklahoma Agricultural Experiment Station, Oklahoma State University
- Report FHWA-OK-13-06 (ODOT SP&R Item 2242), University of Oklahoma Oklahoma Department of Transportation / U.S. DOT ROSA P
- Engineering Properties of Stabilized Subgrade Soils for Implementation of the AASHTO 2002 Pavement Design Guide (FHWA-OK-08-10) Oklahoma Department of Transportation
- Subsurface Exploration, New ODOT Maintenance and Residency Facilities Oklahoma Department of Transportation
- Geotechnical Engineering Report, Retaining Walls A and B Along 91st Street South from South Memorial Drive to South Mingo Road City of Tulsa
- Geologic and Hydraulic Characteristics of Selected Shaly Geologic Units in Oklahoma (WRI 96-4303) U.S. Geological Survey
- Abstract: Geologic Hazards in Oklahoma Geological Society of America (Oklahoma Geological Survey author)
- Official Series Description - DURANT Series USDA Natural Resources Conservation Service
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