Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is Free Swell Index (FSI)? It is the increase in volume of a soil when it is put in water, with nothing holding it down, expressed as a percentage.
Why it matters: some clays, like black cotton soil, swell when wet and shrink when dry. This movement lifts and cracks floors, walls, roads and pipes. FSI is a quick way to spot such expansive soils before you build on them or use them in an embankment.
This guide follows IS 2720 (Part 40).
Why Some Soils Swell
The Idea of the Test
Two equal soil samples are put in two cylinders: one with kerosene and one with distilled water. Kerosene doesn’t make clay swell, so it shows the soil’s original volume (Vk). Water makes it swell, giving the swollen volume (Vd).
Apparatus Required
Sieve (425 micron)
Only the fine soil passing it is tested.
Two 100 ml cylinders
Graduated glass cylinders (IS 878): one for water, one for kerosene.
Glass rod
Stirs the soil to release trapped air.
Balance (0.01 g)
500 g capacity; weighs two 10 g samples.
Distilled water & kerosene
Water makes clay swell; kerosene does not.
Oven
Dries the soil before the test.
Procedure
- Oven-dry the soil and sieve it through the 425 micron sieve. Take two samples of 10 g each.
- Pour each sample into a separate 100 ml graduated cylinder.
- Fill one cylinder with kerosene and the other with distilled water, up to the 100 ml mark.
- Remove trapped air by gently shaking and stirring with the glass rod.
- Leave both cylinders undisturbed for at least 24 hours so the soil settles to a steady volume.
- Read the soil level in each cylinder: Vd (water) and Vk (kerosene).
Calculation
Vd = soil volume in distilled water (ml); Vk = soil volume in kerosene (ml). Report each result and the mean, to two decimal places.
Degree of Expansiveness
| Free Swell Index | Degree of expansiveness |
|---|---|
| Below 20% | Low |
| 20–35% | Moderate |
| 35–50% | High |
| Above 50% | Very high |
Classification as per IS 1498. For embankment and subgrade, FSI should not be more than 50% (MoRTH).
Try It: Free Swell Index Calculator
Enter the soil volumes for both tests. The values filled in are a worked example.
Precautions
🫧 Remove air
Stir gently so no air bubbles stay trapped; bubbles make the soil look bigger.
⏳ Wait the full 24 hours
Read only after the soil level has stopped changing. Don’t shake the cylinders before reading.
🧪 Very expansive soil
For soils like bentonite, use a 5 g sample, or a 250 ml cylinder with the 10 g sample, so the swollen soil fits.
🔥 Kerosene safety
Kerosene is flammable. Keep it away from the oven and flames, and dispose of it properly.
Practical Tips for Site Engineers
1. Spot Expansive Soil on Site
⚫ Dark, sticky clay
Black or dark grey soil that is very sticky when wet and rock-hard when dry.
🕸️ Wide summer cracks
Deep, wide cracks in a pattern on the ground during dry months.
🏚️ Cracked buildings nearby
Diagonal cracks in walls, lifted floors or uneven roads in the area.
2. What to Do When FSI Is High
- Don’t use it as fill: soil with FSI above 50% should not be used in embankment or subgrade. Replace it or treat it as specified.
- Stabilise with lime: lime reduces swelling of clay. Check the dose with lab tests (FSI and CBR) before use.
- CNS layer: a cushion of cohesive non-swelling soil under foundations and floors absorbs movement (IS 9451).
- Go deeper: use under-reamed piles (IS 2911 Part 3) or take foundations below the zone where moisture changes.
- Keep moisture steady: provide a wide plinth apron, good drainage and sloping ground away from the building. Avoid leaking pipes and planting large trees close to walls.
- Check other tests too: FSI is a quick indicator. Confirm with liquid limit, plasticity index and swell pressure where needed.
Key Takeaways
- FSI shows how much a soil swells in water; it flags expansive soils like black cotton soil.
- Two 10 g samples (passing 425 micron): one in kerosene, one in distilled water; read after 24 hours.
- FSI (%) = (Vd − Vk) ÷ Vk × 100; report the mean to two decimals.
- Below 20% low, 20–35% moderate, 35–50% high, above 50% very high.
- On site: FSI must not exceed 50% for embankment and subgrade; use CNS, lime, piles or moisture control in expansive soil.
References: IS 2720 (Part 40): Determination of Free Swell Index of Soils; IS 1498: Classification and Identification of Soils; IS 9451: Lining of Canals in Expansive Soils (CNS layer); IS 2911 (Part 3): Under-reamed Piles; MoRTH Specifications for Road and Bridge Works.