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.
Why it matters: High Free Swell Index values may lead to pavement heaving, cracking, and loss of subgrade stability if the soil is used without treatment.
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)
Take about 500 g of air-dried soil and sieve it through a 425-micron sieve.
Weigh two soil samples of 10 g each.
Place one sample in each 100 ml graduated cylinder.
Fill one cylinder with kerosene and the other with distilled water up to the 100 ml mark.
Stir gently to remove entrapped air bubbles.
Allow the samples to stand undisturbed for 24 hours at 27 ± 2 °C.
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
Free Swell Index (%)
Swelling Nature
Suitability for Subgrade
0 – 20
Low
Suitable for direct use
20 – 50
Moderate
Use with control measures
Above 50
High to Very High
Stabilization or replacement required
Note: Free Swell Index is an index test and should be used along with Atterberg limits and swell pressure tests for design decisions.
