Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is the liquid limit? As you add water to clay, it changes from hard, to soft and mouldable, to a thick liquid. The liquid limit (LL) is the water content at which the soil just starts to behave like a liquid.
In the lab, it is the water content at which a groove cut in soil in the Casagrande cup closes over 12 mm after 25 blows.
Why it matters: LL and plastic limit (PL) together classify fine soils and show how much they will swell, shrink, and lose strength when wet. This guide follows IS 2720 (Part 5).
The Idea: Soil Changes State with Water
Apparatus Required
Casagrande apparatus
Brass cup that drops 10 mm on a hard rubber base (IS 9259).
Grooving tool
Cuts a standard V-groove; its handle also checks the 10 mm drop.
Dish, spatula, knives
For mixing soil with distilled water into a smooth paste.
Balance (0.01 g) & oven
For water content of each trial (105–110°C, 24 h).
425 µm sieve
Only soil passing 425 µm is used (about 120 g).
Airtight containers
Hold the soil slice from each trial for water content.
The Casagrande Apparatus
Procedure
- Take about 120 g of air-dried soil passing the 425 µm sieve. Mix it with distilled water into a smooth, uniform paste. (Clayey soils should be left to stand, often overnight, so the water spreads evenly.)
- Check the apparatus: the cup must drop exactly 10 mm (use the gauge on the grooving tool handle).
- Put some paste in the cup, level it, and trim it to 1 cm deep at the thickest point.
- Cut a clean groove through the centre with the grooving tool, holding it upright.
- Turn the crank at 2 turns per second and count the blows until the two halves of soil touch for about 12 mm along the bottom of the groove (see Fig. 3).
- Take a slice of soil across the closed groove and find its water content (oven 105–110°C, 24 h).
- Return the rest of the soil to the dish, add a little more water, mix again and repeat. Do at least 4 trials, with blows between 15 and 35, going from drier (more blows) to wetter (fewer blows).
Flow Curve, Plasticity Index and Classification
Plot each trial’s water content (normal scale) against the number of blows (log scale) and draw the best straight line: the flow curve. The water content where it crosses 25 blows is the liquid limit.
Plasticity Index
PI = LL − PL
The range of water content in which the soil is plastic.
Flow Index
If = (w1 − w2) ÷ log(N2 ÷ N1)
Slope of the flow curve: how fast the soil loses strength as water increases.
Compressibility (IS 1498)
L (low): LL < 35
I (intermediate): 35–50
H (high): LL > 50
Try It: Liquid Limit Calculator
Enter the blows and water content of each trial (and the plastic limit, if known). The values filled in are a worked example.
Precautions
💧 Dry to wet
Start with the driest paste (most blows) and add water step by step. If you must go wetter to drier, dry it by kneading, never by adding dry soil.
📏 Check the 10 mm drop
Check the drop height before every test; a wrong drop gives a wrong LL.
⏱️ Steady speed
Turn the crank at a steady 2 turns per second.
🧽 Clean cup and base
Wipe the cup and tool after each trial; dried soil on the cup or base changes the blows.
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. Reading the Results
✅ Low LL (below 35)
Low plasticity; generally easier to compact and less sensitive to water.
⚠️ Medium (35–50)
Intermediate; control moisture carefully during compaction.
🚫 High LL (above 50)
High plasticity, often expansive (e.g. black cotton soil). May need replacement or lime stabilisation; check free swell index too.
3. Good Site Habits
- Quick field idea: if moist soil rolls into a long, strong 3 mm thread and feels sticky and soapy, expect a high LL clay. Confirm in the lab.
- Test every new borrow area and whenever the soil colour or feel changes; one result does not cover the whole area.
- Always test LL and PL together; PI is what decides suitability for GSB and WMM.
- Watch GSB and WMM fines: plastic fines (PI above 6) make granular layers soft in rain. Reject or rework such material.
- Use distilled water in the test; tap water with salts can change the result.
Key Takeaways
- LL = water content at which the groove closes over 12 mm in 25 blows.
- About 120 g of soil passing 425 µm; cup drop 10 mm; 2 turns per second; 4 trials between 15 and 35 blows.
- Plot water content against blows (log scale); read LL at 25 blows.
- PI = LL − PL; LL < 35 low, 35–50 intermediate, > 50 high plasticity.
- On site: embankment LL ≤ 70 and PI ≤ 45; GSB LL ≤ 25 and PI ≤ 6; WMM PI ≤ 6 (as per specification).
References: IS 2720 (Part 5): Methods of Test for Soils – Determination of Liquid and Plastic Limit; IS 9259: Liquid Limit Apparatus for Soils; IS 1498: Classification and Identification of Soils; MoRTH Specifications for Road and Bridge Works.