What is Free Swell Index (FSI) Test?

The Free Swell Index (FSI) test is a laboratory test used to determine the swelling potential of soil when immersed in water under unconstrained conditions. It helps in identifying expansive soils containing clay minerals such as montmorillonite.

Scope & Applicable Standards

• IS:2720 (Part 40) – Determination of Free Swell Index of Soil
• MoRTH Section 300 – Subgrade and Earthwork
• IRC:75 (2015) – Guidelines for Embankments and Subgrades
• IRC:SP:89 – Soil Testing in Road Works
• NHAI QA/QC Manual

Apparatus Required for FSI Test

• 425 micron IS sieve
• Two 100 ml graduated glass cylinders (IS:878)
• Oven maintained at 110 ± 5 °C
• Electronic balance with 0.01 g accuracy
• Distilled water and kerosene
• Glass rod, spatula and weighing dishes

Free Swell Index Test Procedure (IS 2720 Part 40)

1. Take about 500 g of air-dried soil and sieve it through a 425-micron sieve.
2. Weigh two soil samples of 10 g each.
3. Place one sample in each 100 ml graduated cylinder.
4. Fill one cylinder with kerosene and the other with distilled water up to the 100 ml mark.
5. Stir gently to remove entrapped air bubbles.
6. Allow the samples to stand undisturbed for 24 hours at 27 ± 2 °C.
7. Record the final soil volumes:
   • Vk = Volume of soil in kerosene
   • Vw = Volume of soil in water

Free Swell Index Formula

FSI (%) = [(Vw - Vk) / Vk] × 100

FSI Interpretation and Acceptance Criteria

Precautions During FSI Test

• Ensure glass cylinders are clean and dry.
• Maintain the specified test temperature.
• Do not disturb the samples during the standing period.
• Record volume readings accurately.

Frequently Asked Questions

Is Free Swell Index the same as swell pressure?

No. Free Swell Index measures volume increase without restraint, whereas swell pressure measures pressure developed under confinement.

Can Free Swell Index alone be used for soil design?

No. It is only an indicator test and must be supported by other geotechnical tests.

How can expansive soil be treated?

Common methods include lime stabilization, cement stabilization, blending with non-expansive soil, or soil replacement.