Lesson 4: Why does clay behave like plastic?
Water content and the Atterberg limits · about 40 minutesYou will learn
Same clay, different behaviour
Dry clay can be as hard as brick. Add a little water and you can mould it like plasticine. Add more and it slumps and flows like thick paint. The clay has not changed. Only the water has. In this lesson you will learn how engineers measure this, using a set of simple tests called the Atterberg limits.
- Explain how water changes a fine soil from solid to liquid, and what the Atterberg limits are.
- Work out the liquid limit, plastic limit, plasticity index and liquidity index from test results, and use them to judge a soil.
You will need a calculator. A ruler helps for reading the graphs.
Why it matters
Grain size is not the whole story
In Lesson 3, grain size told us almost everything about sands and gravels. Clay is different. Its grains are tiny, flat plates, and their surfaces carry a small negative electrical charge. That charge pulls water molecules towards the plates, so each plate is wrapped in a thin layer of tightly held water, called adsorbed water.
Because clay plates are so small, there is an enormous amount of surface for this water to cling to. That is why clay is sticky, why it can be moulded, and why its behaviour changes so much as its water content changes. In sands, the effect is too small to matter.
The point
For fine soils, the key question is not just “how big are the grains?” but “how does this soil respond to water?” The Atterberg limits answer that question.
The idea, in plain words
Measuring how wet a soil is
Everything in this lesson is measured as a water content (also called moisture content). In soil mechanics, it compares the mass of water with the mass of the dry soil, not the total mass:
To measure it, you weigh a wet sample, dry it in an oven (usually at 105 to 110 °C) until its mass stops changing, then weigh it again. The mass lost is the water.
| Example | Mass (g) |
|---|---|
| Wet soil | 65.5 |
| Dry soil (after the oven) | 50.0 |
| Water (65.5 − 50.0) | 15.5 |
w = 15.5 ÷ 50.0 × 100% = 31%
Watch out
Dividing by the wet mass is the most common mistake. It would give 15.5 ÷ 65.5 = 24%, which is wrong. Because we divide by the dry mass, a very wet clay or peat can have a water content above 100%.
The idea, in plain words
From solid to liquid
Around 1911, the Swedish scientist Albert Atterberg described how a fine soil passes through four states as its water content rises. The water contents where it changes from one state to the next are the Atterberg limits, also called the consistency limits.
| Limit | What it means |
|---|---|
| Shrinkage limit (SL) | Below this water content, drying the soil further does not make it shrink any more. |
| Plastic limit (PL) | The lowest water content at which the soil can be moulded without cracking. |
| Liquid limit (LL) | The lowest water content at which the soil flows like a liquid under a small, standard disturbance. |
Engineers use the plastic and liquid limits most. The tests are done on the part of the soil that passes a 0.425 mm sieve (BS EN ISO 17892-12).
Measuring it
The fall cone test
“Flows like a liquid” sounds vague, so the test fixes exactly how the soil is disturbed. In the UK the usual method is the fall cone test:
The wetter the soil, the further the cone sinks. The tests are spread to cover penetrations of roughly 15 to 25 mm. The results are plotted as penetration against water content, and a best-fit straight line is drawn.
Wherever you are
Many countries use the Casagrande cup instead: a brass cup is repeatedly dropped onto a hard base, and the liquid limit is the water content at which a groove cut in the soil closes after 25 drops. Both methods are recognised in international standards. They can give slightly different answers, so always report which method you used.
Worked example
Finding the liquid limit from the graph
Four fall cone tests on a clay gave these results:
| Test | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Water content (%) | 38.4 | 42.1 | 46.9 | 51.2 |
| Cone penetration (mm) | 15.0 | 17.6 | 21.0 | 24.0 |
Read across from 20 mm to the line, then down to the water content axis. The liquid limit is about 45.5%, which we round to 46%.
Check the points, not just the answer
When I review lab results, I look at the plot as well as the number. Points that do not sit close to a straight line, or that all bunch on one side of 20 mm, mean the liquid limit has been stretched from too little evidence. A good test brackets 20 mm with points either side.
Measuring it
Rolling threads
The plastic limit test is simple and surprisingly reliable. A small ball of soil, a little drier than for the cone test, is rolled by hand on a glass plate into a thread.
- Roll the thread down to 3 mm across. A 3 mm rod is kept beside the plate for comparison.
- If it rolls to 3 mm without cracking, it is still too wet. Fold it up, knead it to dry it a little, and roll again.
- The end point is when the thread just starts to crack and crumble at about 3 mm.
- Take the water content of the crumbled threads. Do this on two portions and average them.
In our example, the two portions gave 22.8% and 23.4%. The average is 23.1%, so the plastic limit is 23%.
Non-plastic soils
Some soils, such as many silts and sandy soils, cannot be rolled into a 3 mm thread at any water content. They are reported as non-plastic (NP). That is a useful result, not a failed test.
Using the results
Two numbers from the limits
Plasticity index (PI)
The range of water content over which the soil is plastic:
A bigger PI generally means more clay, or more active clay. Soils with no plastic range have PI = 0.
Liquidity index (LI)
Where the soil’s natural water content, w, sits between its limits. Our clay was found in the ground at w = 31%, the value we worked out in step 3:
| LI | What it tells you about the soil in the ground |
|---|---|
| Below 0 | Drier than its plastic limit. Semi-solid or solid, usually stiff or hard. |
| 0 to 1 | In its plastic range. Our clay, at 0.35, is in the lower part of this range. |
| Above 1 | Wetter than its liquid limit. Very soft, and can behave like a liquid when disturbed. |
LI is a quick first indicator. Strength is measured properly with the tests you will meet later in this course.
Using the results
How active is the clay?
In 1953 the British engineer Alec Skempton noticed that PI rises with the amount of clay-sized particles (finer than 0.002 mm), and that the rate depends on the type of clay mineral. He called this rate the activity:
| Activity | Class | Typical examples |
|---|---|---|
| Below 0.75 | Inactive | Kaolinite (about 0.4) |
| 0.75 to 1.25 | Normal | Illite (about 0.9) |
| Above 1.25 | Active | Montmorillonite (from about 1.5 to 6 or more) |
Active clays hold more water, swell and shrink more, are more compressible and tend to be weaker. That matters for foundations, road pavements and anything built on clay that wets and dries through the seasons.
Example: our clay has PI = 23% and 30% clay-sized particles. A = 23 ÷ 30 ≈ 0.77, so it is a normal clay, just above the inactive range.
Wherever you are
Highly active clays cause serious damage in many parts of the world, especially where long dry seasons are followed by heavy rain. Cracked walls, broken drains and heaving floors are common signs. If your local soils are known to shrink and swell, activity and plasticity are among the first things to check.
Using the results
Clay or silt? The plasticity chart
When PI is plotted against LL for soils from all over the world, clays and silts fall in different areas. The dividing line is called the A-line:
For our clay, the A-line at LL = 46 is at PI = 0.73 × (46 − 20) ≈ 19. Our PI of 23 is above the line, so it is a clay.
UK practice also describes plasticity from the liquid limit, in five bands shown across the top of the chart: low (below 35), intermediate (35 to 50), high (50 to 70), very high (70 to 90) and extremely high (above 90). Our clay, at LL = 46, is a clay of intermediate plasticity. Lesson 5 shows how this becomes a classification symbol.
Your turn
Test results for a new clay
Task (15 minutes)
Fall cone tests: water contents of 52.0, 55.0, 58.4 and 61.0% gave penetrations of 16.0, 18.2, 20.7 and 22.6 mm. The plastic limit threads gave 27.6% and 28.4%. The natural water content is 40% and the soil has 45% clay-sized particles.
- Find the liquid limit. Sketch the graph, or work along the line between the two points either side of 20 mm.
- Find the plastic limit and the plasticity index.
- Work out the liquidity index. What state is the soil in?
- Work out the activity.
- Is it a clay or a silt, and what is its plasticity?
Show the answers
1. 20 mm lies between 55.0% (18.2 mm) and 58.4% (20.7 mm). Along that line: 55.0 + (20 − 18.2) ÷ (20.7 − 18.2) × (58.4 − 55.0) = 55.0 + 0.72 × 3.4 ≈ 57.4%. LL ≈ 57%. (A best-fit line through all four points gives the same.)
2. PL = (27.6 + 28.4) ÷ 2 = 28%. PI = 57 − 28 = 29%.
3. LI = (40 − 28) ÷ 29 ≈ 0.41. Between 0 and 1, so the soil is in its plastic state.
4. A = 29 ÷ 45 ≈ 0.64, an inactive clay.
5. A-line at LL = 57: 0.73 × (57 − 20) ≈ 27. PI = 29 is above it, so it is a clay. LL is between 50 and 70, so it has high plasticity.
Quick check
Five quick questions
This is not a test. It simply shows you whether you are ready for Lesson 5. Choose an answer to see the explanation.
Key points
What to take away
- Clay plates are tiny, flat and charged, and hold a layer of adsorbed water. That is why water changes clay so much.
- Water content is the mass of water divided by the mass of dry soil.
- As water content rises, a fine soil goes from solid to semi-solid to plastic to liquid. The limits between them are SL, PL and LL.
- LL is the water content at 20 mm cone penetration (fall cone) or 25 drops (Casagrande). PL is where a 3 mm thread starts to crumble.
- PI = LL − PL. LI = (w − PL) ÷ PI shows where the natural soil sits between its limits.
- Activity = PI ÷ % clay-sized particles. Active clays swell and shrink the most.
- On the plasticity chart, clays plot above the A-line and silts below it.
Up next: Lesson 5
How do engineers describe and classify soil? You will put grading and plasticity together to give a soil its classification symbol, and learn to write a full engineering description in the standard order.
Assessment (optional)
Test yourself properly
Unlike the practice questions, this assessment has no answers on the page. Attempt it on your own, as if it were an exam.
Instructions
- Total: 40 marks. Suggested time: 60 minutes.
- Answer all questions. The marks for each question are shown in brackets.
- You may refer to the lesson notes, but write your answers in your own words.
- Show your working in calculations. Where a question asks you to justify or explain, marks go to your reasoning, not just the final answer.
Section A: Concepts (12 marks)
- Define water content. Explain why it is calculated using the mass of dry soil, not the mass of wet soil. [2 marks]
- Define the shrinkage limit, the plastic limit and the liquid limit. [3 marks]
- Explain why water changes the behaviour of clay so much more than the behaviour of sand. [3 marks]
- What does a liquidity index above 1 tell you about a soil in the ground? [2 marks]
- What does “non-plastic” mean on a test report, and which soils are often reported this way? [2 marks]
Section B: Calculations and interpretation (16 marks)
Fall cone tests on a clay gave these results:
Test 1 2 3 4 Water content (%) 41.0 44.5 48.2 52.0 Cone penetration (mm) 15.2 17.8 20.6 23.4 Two plastic limit portions gave 24.6% and 25.2%. A sample of the clay from the ground weighed 72.6 g wet and 55.0 g after oven drying. A separate test found 40% clay-sized particles.
(a) Find the liquid limit. Show how you found it. [3 marks]
(b) Find the plastic limit and the plasticity index. [2 marks]
(c) Calculate the natural water content of the clay. [2 marks]
(d) A colleague divided by the wet mass instead. What value would they get, and why is it wrong? [2 marks]
(e) Calculate the liquidity index, and say what it tells you about the clay. [2 marks]
(f) Calculate the activity, and classify it. [2 marks]
(g) Use the A-line, PI = 0.73 × (LL − 20), to decide whether the soil is a clay or a silt, and state its plasticity band. [3 marks]
Section C: Applied case (12 marks)
A small housing site has two fine soils. Mature trees grow close to where the houses will go.
- Clay X: LL = 75%, PI = 50%, 30% clay-sized particles.
- Clay Y: LL = 38%, PI = 16%, 40% clay-sized particles.
Write a short technical note to the developer (maximum 300 words) that:
- works out the activity and plasticity band of each clay, and checks each against the A-line [4 marks]
- explains which clay is more likely to shrink and swell, and why the trees and the seasons matter [3 marks]
- recommends what should be considered before foundations are designed [3 marks]
Clear, professional communication suitable for a non-specialist. [2 marks]
How your work is judged
| Criterion | What a strong answer shows |
|---|---|
| Technical accuracy | Correct terms, methods and calculations |
| Use of evidence | Conclusions backed by specific information in the question (test results, descriptions, site details) |
| Engineering judgement | Results linked to what they mean for design and construction |
| Communication | Clear, concise and well organised, in your own words |
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References
- British Standards Institution (BSI) (2018) BS EN ISO 17892-12:2018 Geotechnical investigation and testing. Laboratory testing of soil. Determination of liquid and plastic limits. London: BSI.
- British Standards Institution (BSI) (2020) BS 5930:2015+A1:2020 Code of practice for ground investigations. London: BSI.
- Casagrande, A. (1948) ‘Classification and identification of soils’, Transactions of the American Society of Civil Engineers, 113, pp. 901–930.
- Knappett, J.A. and Craig, R.F. (2019) Craig’s Soil Mechanics. 9th edn. Boca Raton, FL: CRC Press.
- Skempton, A.W. (1953) ‘The colloidal activity of clays’, Proceedings of the 3rd International Conference on Soil Mechanics and Foundation Engineering, Zurich, 1, pp. 57–61.
Further learning
- Read: Chapter 1 of Craig’s Soil Mechanics covers plasticity and the plasticity chart with more examples.
- See it done: look for videos of the fall cone test and the plastic limit thread test. Watching the thread crumble makes the end point much easier to recognise.
- Try it: roll a thread of damp garden clay or modelling clay. Notice how it changes as it dries in your hands.
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