What do soil test numbers for expansive clay mean?
In short: Lab tests show how much a clay soil can shrink and swell. The plasticity index from the Atterberg limits test is a common quick indicator, while swell tests measure movement and pressure directly. Results only describe the samples tested, so the number and depth of borings matter as much as the lab values.
What are the main soil tests?
| Test | What it measures | Notes |
|---|---|---|
| Atterberg limits | Liquid limit and plastic limit; the difference is the plasticity index (PI) | One of the listed methods for identifying expansive clay 1 |
| Activity | PI compared with clay content | Used to identify swelling clays 1 |
| Free swell, swell pressure, expansion index | How much a sample swells and how hard it pushes | Swell pressure tests are seldom run; designers often use correlations instead 1 |
| Coefficient of linear extensibility (COLE) | Change in length between wet and dry states | Used in soil surveys and research 13 |
| Potential vertical rise (PVR) | Estimated inches of heave at the surface | A method used in Texas practice 26 |
| Borings with lab samples | Soil layers, depth to rock, samples for testing | Basis of a geotechnical report 4 |
Plain-English definitions
- Liquid limit: the moisture content at which a soil starts to flow like a thick liquid.
- Plastic limit: the moisture content below which the soil crumbles instead of rolling into a thread.
- Plasticity index (PI): liquid limit minus plastic limit. A higher PI means the clay can hold more water while still moldable, which generally means more swell.
How do you read the plasticity index?
A commonly cited structural engineering rule of thumb 2:
- PI below about 20: only marginally expansive
- PI above about 40: highly expansive
Regional correlations link PI to swell pressure. In TRB material, expected swell pressure rises from about 2.0 to 5.0 kips per square foot as PI goes from 30 to 60 for the Houston area, and from 3.0 to 6.0 for Dallas, San Antonio and Austin 1. Oklahoma-specific correlations should come from local engineering data.
What do Oklahoma lab results show?
An Oklahoma State University study of benchmark soils measured liquid limits, plasticity and swell 3:
| Soil | Lab finding | Building-site rating |
|---|---|---|
| Kirkland | Subsoil liquid limit above 50%; very plastic and highly expansive | Severe for low buildings |
| Bethany | About medium liquid limits and plasticity; appreciable but not extreme swelling clays | Moderate |
| Norge | Liquid limit above 30%; relatively low in swelling clays | Moderate |
| Sandy soils (Grandfield, Nobscot) | Little or no plasticity; low shrink-swell | Slight |
The same study found the fine clays were dominantly montmorillonite, a swelling clay, and that calcium carbonate in the lower Bethany horizons may have depressed natural shrink-swell 3. See the Kirkland and Bethany soil pages.
Why does the moisture state matter?
The same clay can show different behavior depending on how dry it is when tested. A study of clay sites during the 2011 drought found the drought increased the soils' potential vertical rise, meaning more possible heave once they rewet 6. Researchers at Oklahoma State University describe newer design methods based on soil suction, a measure of how strongly dry soil pulls in water 5.
How many borings and samples are needed?
How a site is sampled shapes what the lab can report. As one example of a written requirement, a Virginia county policy for expansive soil areas calls for 4:
- at least two borings within the building footprint;
- borings at least 5 feet deep, or to refusal on non-expansive material, and at least 12 inches below planned footing depth;
- a lab sample from the weakest soil within 12 inches below the footing; and
- a signed report with boring logs, a site sketch and lab results.
Oklahoma cities set their own rules; see codes and permits. For how engineers use these results, see working with engineers.
Related
- Clay Mineralogy
- Shrink-Swell and Active Zone
- OK Data - Soil Lab Values
- Chemical Soil Stabilization
- Inspection Protocol
Common questions
What soil test shows whether clay is expansive?
The Atterberg limits test, which gives the liquid limit and plasticity index, is a common quick indicator. Swell tests measure the actual swelling and the pressure it can produce, but they are run less often.
What plasticity index means highly expansive clay?
One engineering rule of thumb treats clays with a plasticity index under about 20 as only marginally expansive and clays above about 40 as highly expansive.
How deep should soil borings go for a house?
Requirements vary by jurisdiction. As one example, a county expansive-soil policy calls for at least two borings in the building footprint, at least 5 feet deep or to refusal on non-expansive material.
Are there lab results for Oklahoma soils?
Yes. A 1978 Oklahoma State University bulletin tested benchmark soils and found Kirkland subsoil had a liquid limit above 50% and was highly expansive, while Bethany and Norge were rated moderate and sandy soils slight for low buildings.
Sources
- TRB Webinar: Foundations in Shrink Swell Soil: Innovation and State of Practice Transportation Research Board, National Academies
- Expansive Soils (The Mystery) STRUCTURE magazine
- Shrink-Swell and Other Characteristics of Five Benchmark Soils of Western Oklahoma (Technical Bulletin T-150) Oklahoma Agricultural Experiment Station, Oklahoma State University
- County Expansive Soils Policy & Procedures (Commercial & Residential Projects) Roanoke County, Virginia, Development Services
- Residential House Foundations on Expansive Soils in Changing Climates HUD Office of Policy Development and Research (Cityscape, Vol. 25 No. 1)
- Ground Conditions in the 2011 Drought CIGMAT Conference, University of Houston
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