Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is the plastic limit? The plastic limit (PL) is the lowest water content at which soil can still be moulded. In the lab, it is the water content at which a soil thread just starts to crumble when rolled to 3 mm thick.
Plasticity index (PI) = Liquid limit − Plastic limit. It is the range of water content over which the soil stays plastic (mouldable).
Why it matters: PI tells you how “clayey” and water-sensitive a soil is. It decides whether soil can be used for GSB, WMM, subgrade or embankment. This guide follows IS 2720 (Part 5).
The Thread Test in Pictures
How to Judge the Thread
💧 Too wet
Thread rolls thinner than 3 mm without breaking. Knead it in your hands to dry it a little and roll again.
✅ At plastic limit
Thread crumbles into pieces just at 3 mm. Collect the pieces for water content.
🏜️ Too dry
Thread cracks before reaching 3 mm. Add a little water, mix and try again.
Apparatus Required
Glass plate (10 mm thick)
Smooth surface on which the soil thread is rolled.
3 mm rod
Reference to judge when the thread is 3 mm thick.
Dish & spatula
For mixing about 20 g of soil with distilled water.
Balance (0.01 g) & oven
For the water content of the crumbled threads.
Airtight containers
Hold crumbled threads so no moisture is lost.
425 µm sieve
Only soil passing 425 µm is used.
Procedure
- Take about 20 g of soil passing the 425 µm sieve. Mix it with distilled water until it is plastic enough to mould into a ball.
- Take about 8 g of the paste and make a ball.
- Roll it between your fingers and the glass plate at 80–90 strokes per minute (one stroke = one full forward-and-back movement) to make a thread of even thickness.
- When the thread reaches 3 mm, if it has not crumbled, gather it up, knead it into a ball and roll again. The soil slowly dries as you handle it.
- Repeat until the thread crumbles just at 3 mm diameter.
- Collect the crumbled pieces in an airtight container and find the water content (oven 105–110°C, 24 h).
- Do at least two tests; the average water content is the plastic limit, reported to two decimal places.
Calculations
🔸 Non-plastic (NP)
If a thread cannot be rolled at all (e.g. sandy soil), report PL and PI as NP.
🔹 PL ≥ LL
If the plastic limit is equal to or more than the liquid limit, report PI = 0.
🧪 Sandy soils
For sandy soils, do the plastic limit test first; if it is NP, the liquid limit test may not be needed.
Where Does Your Soil Fall? The Plasticity Chart
LL and PI together place a soil on the plasticity chart (IS 1498). Soils above the A-line are clays (C); below it are silts or organic soils (M / O). The letters L, I and H mean low, intermediate and high plasticity.
Try It: Plastic Limit and PI Calculator
Precautions
🚫 Don’t force it
Don’t try to make the thread break at exactly 3 mm by changing the speed or pressure. Keep rolling normally until it crumbles.
💧 Use distilled water
And mix the soil thoroughly so the moisture is even.
🔒 Seal the pieces quickly
Crumbled threads are small and dry fast; put them in the airtight container straight away.
✋ Even pressure
Use just enough pressure to roll the thread evenly; too much pressure flattens it.
Practical Tips for Site Engineers
1. Acceptance Limits Used on Site
| Material | Liquid limit (max) | Plasticity index (max) |
|---|---|---|
| Embankment / subgrade soil | 70% | 45% |
| Granular sub-base (GSB) | 25% | 6% |
| Wet mix macadam (WMM) | – | 6% |
Common MoRTH limits; always follow the limits in your contract specification.
2. What PI Tells You on Site
PI 0–7: low
Silty or sandy soil; drains and compacts more easily, but can lose strength if very silty and wet.
PI 7–17: medium
Clayey soil; control moisture during compaction and protect from rain.
PI above 17: high
Highly plastic, may swell and shrink. Consider lime stabilisation or replacement; check free swell index.
3. Good Site Habits
- Quick hand check: roll moist soil on your palm. If you can easily make a long, strong 3 mm thread, the soil is plastic (clayey). If it keeps crumbling, it has low plasticity.
- Check fines in GSB and WMM: test the material passing 425 µm. Plastic fines (PI above 6) make granular layers soft and rutted in rain.
- Use the liquidity index for natural ground: LI near 0 or below = stiff soil; LI near 1 or above = very soft soil that needs care in foundations and excavations.
- Rough guide for compaction: for many fine-grained soils, the optimum moisture content (OMC) is near the plastic limit. Use the Proctor test for the actual OMC.
- After lime stabilisation, repeat LL and PL tests; a drop in PI shows the lime is working.
Key Takeaways
- PL = water content at which a thread crumbles at 3 mm; use about 20 g passing 425 µm, 8 g balls, 80–90 strokes/min.
- PI = LL − PL. Report NP if no thread can be rolled; PI = 0 if PL ≥ LL.
- Do at least two tests; report the average to two decimals.
- LL and PI place the soil on the plasticity chart (above A-line = clay).
- On site: GSB and WMM need PI ≤ 6; embankment PI ≤ 45; high PI soils may need lime stabilisation.
References: IS 2720 (Part 5): Methods of Test for Soils – Determination of Liquid and Plastic Limit; IS 1498: Classification and Identification of Soils; MoRTH Specifications for Road and Bridge Works.