Quick answer
The soil plasticity index, or PI, is the difference between a soil’s liquid limit and its plastic limit, which is the range of water content over which the soil can be molded without crumbling or flowing.
A soil with a wide range is more plastic and usually holds more clay. Engineers use PI, with other tests, to classify soil and to flag clay that may swell and shrink under a foundation.
Quick facts
The key facts about Soil Plasticity Index, with sources:
- What it is
- Liquid limit minus plastic limit, in percent water content 1
- Also in the code test
- Fine particles passing a No. 200 sieve and clay-size particles, or an expansion index above 20 23
- Where it appears
- NRCS soil survey engineering tables list liquid limit and plasticity index 1
- Companion rating
- NRCS linear extensibility rates shrink-swell directly 1
- What PI cannot do
- Predict movement of one house on its own 4
Key takeaways
- The plasticity index is a lab number that shows how much water a soil can take while staying moldable. More range usually means more clay.
- It is calculated as the liquid limit minus the plastic limit, both found by a standard test.
- The model residential code treats a PI of 15 or greater as one test for expansive soil, together with grain-size tests, or an expansion index above 20 instead.
- PI is a clue to shrink-swell behavior, not a measure of how far a house will move.
- Only a site test and an engineer’s reading give a value that applies to your lot.
What the soil plasticity index measures
Clay changes character with water. Dry, it is hard and crumbly. With more water it becomes moldable, like modeling clay. With still more water it flows like a thick liquid. The plasticity index is the width of that moldable stage.
General information, not a diagnosis of your house. NRCS describes liquid limit and plasticity index, also called Atterberg limits, as the numbers that indicate the plasticity characteristics of a soil 1. Soil scientists and engineers use them to name a soil and to guess how it will behave when moisture changes.
Why does this matter for a foundation? Plastic clays usually carry more of the clay minerals that take up water and swell. A wide moldable range often goes with a large swell and shrink. Read expansive clay soil for the movement that can follow.
How the plasticity index is found in a lab
The test steps below are described in general terms. The code names ASTM D4318 as the method 23:
- Prepare the sample: A technician sieves the soil to the fine fraction and mixes it with water.
- Find the liquid limit: The water content is found at which the soil just begins to flow under a standard test.
- Find the plastic limit: The water content is found at which the soil can no longer be rolled into a thin thread without crumbling.
- Subtract: The plastic limit is subtracted from the liquid limit. The result is the plasticity index.
- Classify: The values feed a classification chart that names the soil, along with grain-size results.
- Prepare soilFine fraction, mixed with water
- Liquid limitWater content where the soil starts to flow
- Plastic limitWater content where it crumbles when rolled
- SubtractLiquid limit minus plastic limit
- ClassifyUse with grain size to name the soil
A general outline of the Atterberg limits tests named in the code as ASTM D4318. Lab procedure details belong to the standard. Source: [2] [3]
PI = LL − PL
PI- Plasticity index, in percent water content
LL- Liquid limit
PL- Plastic limit
The index is a difference of two water contents, so it is a range, not a single moisture level.
The Atterberg limits and what each one means
| Term | Plain meaning | Why it matters for a foundation |
|---|---|---|
| Liquid limit | Water content where soil starts to flow | Higher values go with clay that holds more water 1 |
| Plastic limit | Water content where soil can no longer be rolled thin without crumbling | Marks the dry edge of moldable clay |
| Plasticity index | Liquid limit minus plastic limit | Wider range often goes with more clay and more shrink-swell 1 |
| Linear extensibility | Length change of a soil clod from moist to dry | NRCS uses it directly to rate shrink-swell 1 |
| Expansion index | Measured swell in a standard test | The code lets it replace three other tests 2 |
Plasticity vs. swell
Plasticity describes how a soil behaves as water content changes. Swell describes how much its volume changes. They often move together, but the code lists them as separate tests, and a soil can be tested on both 2.
Example: reading the code’s expansive soil test
An example with invented lab results, checked against the IRC R403.1.8.1 provisions as printed in Seattle’s 2021 code 2. A soil is considered expansive when it meets all four items, except the first three need not be shown if the expansion index test is run.
| Test (item) | Code threshold | Example lab result | Meets threshold? |
|---|---|---|---|
| 1. Plasticity index (ASTM D4318) | 15 or greater | 22 | Yes |
| 2. Passing No. 200 sieve (ASTM D422) | More than 10 percent | 60 percent | Yes |
| 3. Particles smaller than 5 micrometers (ASTM D422) | More than 10 percent | 30 percent | Yes |
| 4. Expansion index (ASTM D4829) | Greater than 20 | Not run in this example | The code lets this test stand in for items 1 to 3; an engineer decides how it is applied |
The thresholds are the code’s. The lab results are invented to show the method and do not describe any real soil. A different example, such as a PI of 8, would fall short of item 1 on this list.
Where you will see a plasticity index
You will find PI in three places. The first is a geotechnical report, where a table lists liquid limit and plasticity index for each soil layer.
The second is the NRCS soil survey, whose engineering properties tables list liquid limit and plasticity index by layer for each soil 1.
The third is a code or plan note, where an engineer cites the PI to show that a soil is or is not expansive.
NRCS also says the Unified classification groups soils by grain size, plasticity index, liquid limit and organic matter content 1. That is why a soil report can say a soil is a fat clay or a lean clay: the name comes from where its limits fall on a plasticity chart.
Texas regulators point to NRCS soil survey reports, with engineering data downloadable by county, as a minimum way to learn whether expansive soil is present 3.
What PI is good for, and where it falls short
What the index gives you
Where it falls short
- It is measured on a remolded sample, not on the soil as it sits in the ground.
- It does not say how much moisture will change at your house, which is what drives movement 3.
- A survey value covers a map unit, not your lot, and fill on a lot can differ 1.
- One number cannot replace the expansion index or a full geotechnical review.
What the plasticity index does not tell you
PI describes the soil, not the house. A high-PI clay under a slab with steady moisture may stay quiet for years. A moderate clay with a leaking pipe can move. Moisture history, drainage, tree roots and foundation design decide the outcome 3.
NRCS says its soil survey tables rest on test data from the survey area and nearby areas, plus field examination 1. That is a reminder that a number is only as good as the sample behind it, and fill or imported soil on your lot can differ from the map.
No PI value, high or low, is proof that a crack is harmless or harmful. Never read a foundation diagnosis from a soil number. Read geotechnical soil investigation for how site testing is planned.
What drives the cost of getting a plasticity index
We print no prices, because fair numbers need dated local quotes. The cost drivers are the number of samples, the depth of borings or pits, whether other lab tests are ordered at the same time, site access and whether a full geotechnical report is written.
Many homeowners never pay for a PI test, because the NRCS survey lists it for free. The foundation cost guide explains where soil testing fits in a repair budget.
How to look up and use a plasticity index step by step
- Open the NRCS Web Soil Survey: Mark your address as an area of interest, then open the soil map.
- Read the engineering properties: Look for liquid limit, plasticity index and the shrink-swell rating for each layer 1.
- Note the map unit limits: Remember that the data cover an area and that fill may change what is under your house.
- Ask for the soil report: If the house was built with one, it may list measured values for the lot.
- Bring both to an engineer: A licensed engineer decides whether further testing is needed.
Survey values are a screen
NRCS bases these tables on test data from the survey area and nearby areas, plus field examination 1. Treat them as a first look, not a site test.
What a homeowner can do about plastic clay soil
You cannot change a soil’s plasticity, but you can control the water that acts on it:
- Keep grade sloping away from the house and downspout extensions in place 4.
- Keep plants and sprinklers back from the wall, as local extension guidance recommends 4.
- Water evenly in a long dry spell rather than in one spot, per NDSU extension advice for dry northern soils to apply water slowly 1 to 2 feet from the wall 5. Local soils and rules differ, so ask a local engineer first.
- Fix leaks soon, since wet pockets under a slab swell clay unevenly.
- Photograph and date any cracks.
Warning signs and when to call a structural engineer
A high PI means the soil may be sensitive to moisture. It does not mean a house has failed. Call a licensed structural or geotechnical engineer when you see:
- Cracks that widen, lengthen or return after patching.
- Doors and windows that stick along with new cracks.
- A wall that bows, leans or separates at a corner.
- Floors that slope or slabs that rise at an edge.
- Any safety doubt.
A pattern is a clue, not a diagnosis. Use the engineer or contractor tool to pick who to call first.
What the model code says about plasticity index
IRC R403.1.8.1 says soils meeting all of four provisions are considered expansive: a plasticity index of 15 or greater by ASTM D4318, more than 10 percent of particles passing a No. 200 sieve by ASTM D422, more than 10 percent smaller than 5 micrometers by ASTM D422, and an expansion index above 20 by ASTM D4829.
Tests for the first three are not required if the expansion index test is run 23.
IRC R403.1.8 then says foundations and floor slabs on expansive soil are designed under Section 1808.6 of the International Building Code, with an exception for systems that have performed adequately in similar soil with the building official’s approval 2.
IRC R401.4 lets the building official require a soil test where expansive soils are likely 2. These sections are quoted from Seattle’s 2021 residential code and from a Texas agency bulletin on the 2015 IRC. Check which edition and amendments your jurisdiction adopted.
What does a PI result mean for expansive-soil risk?
The residential code does not grade clay as low or high risk by PI alone. It uses PI 15 as one of four tests, and the result tells you what to ask next 2. For a first screen of your area, try the expansive soil lookup, then read why foundations move.
| Lab result | What the code test says | Sensible next step |
|---|---|---|
| PI below 15 | Does not meet the PI test for expansive soil 2 | Expansive design is not triggered by PI; drainage still matters |
| PI 15 or greater, grain-size tests not met | Meets one test, not all of them 2 | Ask the engineer which other tests were run |
| PI 15 or greater, plus the No. 200 sieve and clay-size tests | Treated as expansive under the code test 2 | Expect expansive-soil design or soil treatment on new work |
| Expansion index above 20 | Expansive, whatever the other three tests show 2 | A geotechnical engineer sets the design |
Plasticity index vs. other ways to judge clay behavior
| Measure | What it tells you | Limit |
|---|---|---|
| Plasticity index | Range of moldable water content 1 | Lab value on a prepared sample |
| Linear extensibility | Length change moist to dry; used to rate shrink-swell 1 | Survey rating for an area |
| Expansion index | Measured swell in a standard test 2 | Needs a lab test on your soil |
| Field behavior | Clay that swells when wet and shrinks when dry 4 | Shows behavior, not a measured value |
| Geotechnical report | Full site picture with recommendations | Costs more, and answers more |
Common myths about the plasticity index
- Myth A high PI means the foundation will fail.
- Reality It means the soil may be moisture-sensitive. Behavior depends on water, drainage and design 3.
- Myth A low PI means no movement is possible.
- Reality Other causes, such as fill, leaks and erosion, can move a foundation without clay.
- Myth PI is the same as shrink-swell potential.
- Reality They are related but separate. NRCS rates shrink-swell by linear extensibility 1.
- Myth A code threshold of 15 is a safe or unsafe line.
- Reality It is the code’s trigger for treating soil as expansive and designing for it, not a safety limit 2.
- Myth A soil survey is the same as a soil test.
- Reality It covers a map unit, not your lot 1.
Where PI matters most
PI matters most where clay-rich soils and big moisture swings meet. Colorado State reports expansive soils under all of Colorado’s major population centers 4.
Texas adopted guidance that sends builders to the IRC expansive soil provisions and NRCS soil survey data 3.
Check your state geological survey for swelling soil maps, and your building department for local amendments to R403.1.8.
When plasticity index matters, and what to do next
The index matters when you buy a house on clay, plan an addition, or see seasonal cracks. A quick lookup in the Web Soil Survey takes minutes and tells you whether to ask more questions.
If the survey shows high plasticity and you see movement, record the cracks with dates and ask a licensed engineer to read them. Use the inspection checklist to organize what to bring.
- If Survey shows a high PI and the house is stable
- Then Keep drainage and watering steady
- If High PI and new or growing cracks
- Then Call a licensed structural or geotechnical engineer
- If Buying or building on clay
- Then Ask for the soil report and the foundation design basis
Questions about Soil Plasticity Index
What is the plasticity index of soil?
It is the liquid limit minus the plastic limit, which is the range of water content over which soil stays moldable. NRCS lists liquid limit and plasticity index in soil survey tables as indicators of a soil’s plasticity. A wider range usually goes with more clay.
What is a high plasticity index?
There is no single national line, but the model residential code treats a PI of 15 or greater as one of its tests for expansive soil, along with grain-size tests. Engineers read PI with other results, and your jurisdiction’s adopted code controls.
How is the plasticity index tested?
A lab finds the liquid limit and the plastic limit on the fine fraction of a soil sample, then subtracts. The model code names ASTM D4318 as the method. The test is done on a prepared sample, so sampling matters.
Does a high plasticity index mean my foundation will crack?
No. It shows the soil may be sensitive to moisture. Whether a foundation moves depends on water changes, drainage, the foundation design and the site. Many houses on plastic clay perform well when moisture stays steady.
Where can I find the plasticity index for my lot?
Open the NRCS Web Soil Survey for your address and read the engineering properties tables, which list liquid limit and plasticity index by layer. Those values cover a map unit, so a soil test is the next step if a decision depends on them.
What is the difference between plasticity index and expansion index?
Plasticity index comes from limits tests on how water content changes soil behavior. Expansion index comes from a swell test. The code lists both, and says the expansion index test can replace the plasticity and grain-size tests.
Are Atterberg limits the same as plasticity index?
Atterberg limits are the liquid limit and the plastic limit, and sometimes the shrinkage limit. The plasticity index is calculated from the first two. NRCS lists them together as the plasticity characteristics of a soil.
Can I lower the plasticity of my soil?
No, not for your own yard in any practical way. What you can change is the water around the house. Designers sometimes remove or treat soil on new builds, but that is engineering work, not a homeowner project.
Who should interpret my soil test results?
A licensed geotechnical or structural engineer. They combine the plasticity index with grain size, moisture, loads and site history. A single lab number should never drive a repair decision on its own.
Sources
- USDA NRCS, Technical Guide Section II: Soil Interpretations, Engineering (May 2008), retrieved .
- City of Seattle, 2021 Seattle Residential Code, Chapter 4 Foundations (2021 IRC text with Seattle amendments), retrieved .
- Texas Dept. of Licensing and Regulation, IHB Technical Bulletin TB 10-01, Foundations on Expansive Soils: Residential (2010, rev. 2018), retrieved .
- Colorado State University Extension, PlantTalk Colorado 1617: Expansive Soils, retrieved .
- NDSU Extension, Ken Hellevang, Dry Soil May Cause Building Problems (June 30, 2017), retrieved .
Expert review
Written by the HyreFoundation Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: The definition and the test method names come from the code text as printed in Seattle’s 2021 residential code and a Texas agency bulletin on the 2015 IRC.
NRCS supplies the role of liquid limit and plasticity index in soil surveys. The plain-language test steps are general geotechnical practice and are not tied to a specific lab. Example lab values are invented and labeled. No prices.
Suggest a correction. We fix errors and say what changed.