Free Swell Index Test
(IS 2720 Part 40 — Expansive Soil)
Laboratory determination of the free swell index of fine-grained soils — quantifying expansive potential and swelling behaviour of black cotton soils and other expansive clays for foundation and pavement design.
What is the Free Swell Index Test?
The Free Swell Index (FSI) measures the increase in volume of a soil when subjected to submergence in distilled water, without any external constraint. A dried soil specimen is poured into a 100ml graduated cylinder containing distilled water and another into kerosene. The FSI is calculated from the difference in settled volumes in water vs kerosene, expressed as a percentage.
Conducted as per IS 2720 Part 40, the FSI classifies soils as low (<50%), medium (50–100%), high (100–200%) or very high (>200%) swelling potential. It is a mandatory test for foundation design in black cotton soil (BCS) zones, expansive clay regions and all highway subgrade assessment under MORTH specifications. Global Lab tests FSI in our NABL-accredited geotechnical laboratory.
Free Swell Index Test — Site Images
How the Free Swell Index Test is Conducted
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1Sample PreparationSoil dried in oven at 60°C. Clods broken and sieved through 425μm sieve. 10g test portions taken.
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2Cylinder PreparationTwo 100ml graduated cylinders — one filled with distilled water, one with kerosene to 100ml mark.
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3Soil Addition10g oven-dried soil slowly added to each cylinder. Cylinders left undisturbed for 24 hours.
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4Volume ReadingSettled volume of soil in water (Vw) and in kerosene (Vk) read from graduated cylinders.
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5FSI Calculation & ReportFSI (%) = (Vw - Vk)/Vk × 100. Swelling classification per IS 1498. NABL report within 3 days.
Applicable Test Standards
| Standard | Title | Scope | Type |
|---|---|---|---|
| IS 2720 Part 40 | Methods of Test for Soils — Determination of Free Swell Index of Soils | Free Swell Index | Indian Standard |
| IS 1498 | Classification and Identification of Soils for General Engineering Purposes | Soil Classification | Indian Standard |
| MORTH | Specifications for Road and Bridge Works — Subgrade Requirements | Highway Subgrade | MORTH |
| IS 1904 | Code of Practice for Design and Construction of Foundations in Soils | Foundation Design | Indian Standard |