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Home » Geotechnical Tests » Free Swell Index (FSI) Free Swell Index (FSI) Test — Procedure, Calculation & Acceptance Criteria Method: IS:2720 (Part 40) • Standards: MoRTH, IRC:75 (2015), IRC:SP:89 Overview The Free Swell Index (FSI) test evaluates the expansive nature of soil by determining its swelling in water under unconstrained conditions. A high FSI indicates the presence of expansive clay minerals (e.g., montmorillonite) which may cause heaving, cracking, and pavement instability. This test is essential for embankment and subgrade suitability assessments. Quick fact: Soils with FSI > 50% are generally unsuitable for direct use in subgrade or embankment without stabilization. Scope & Applicable Standards IS:2720 (Part 40) — Determination of Free Swell Index of Soils MoRTH Section 300 — Subgrade and Earthwork Quality Requirements IRC:75 (2015) — Guidelines for Design and Construction of Embankments & Subgrades IRC:SP:89 — Guidelines for Soil Testing in Road Works NHAI QA/QC Manual — Frequency & acceptance criteria for geotechnical testing Apparatus Required 425-micron sieve Two 100 ml glass graduated cylinders (as per IS:878-1956) Oven capable of 110 °C Balance (500 g capacity, accuracy 0.01 g) Distilled water and kerosene Spatula, weighing dishes and glass rod Testing Procedure (Step-by-Step) Take 500 g of oven-dried or air-dried soil and sieve through a 425-micron sieve. Divide 20 g of the sieved soil into two equal portions (10 g + 10 g). Place each portion into separate 100 ml glass graduated cylinders. Fill one cylinder to the 100 ml mark with kerosene and the other to the 100 ml mark with distilled water. Stir gently with a glass rod to remove trapped air bubbles. Allow samples to stand undisturbed for 24 hours at (27 ± 2)°C until volume stabilizes. Record the final volume readings of soil in both cylinders (Vk for kerosene, Vw for water). Calculation Use the following formula to compute the Free Swell Index: FSI (%) = ((Vw – Vk) / Vk) × 100 Where: Vw = volume of soil in water (ml) and Vk = volume of soil in kerosene (ml). Technical Interpretation & Acceptance Criteria Interpretation of FSI based on MoRTH and IRC guidance: Free Swell Index (%) Swelling Nature Suitability for Subgrade 0 – 20 Low Good — suitable for direct use 20 – 50 Moderate Usable with control measures (moisture regulation, compaction) > 50 High / Very High Requires stabilization (lime/cement) or replacement Recommendation: For moderate to high FSI soils, always run complementary tests (Atterberg limits, swell pressure) and design appropriate stabilization. Precautions Use clean, dry glass cylinders and ensure removal of trapped air bubbles. Maintain the test temperature at 27 ± 2 °C during the 24-hour period. Avoid disturbing the samples until volumes stabilize. Record all readings carefully and document trial-to-trial variation. Quick Facts Test Method IS:2720 (Part 40) Sample Size 10 g (per cylinder) Test Duration 24 hours (stand period) Key Application Assessing expansive soil for embankment/subgrade Conclusion The Free Swell Index test is a simple, reliable indicator of soil expansivity and is a key part of highway geotechnical investigations. Soils with FSI > 50% should not be used directly in embankment or subgrade without suitable treatment. Proper testing and stabilization help ensure pavement performance and reduce long-term maintenance costs. Download Lab Test Format (Excel) FAQ Is FSI the same as swell pressure? No. FSI is a measure of volume increase in unconstrained conditions. Swell pressure measures the pressure developed when soil is allowed to swell under constrained conditions. Can we rely on FSI alone to design stabilization? FSI is an indicator. For design you should combine FSI with Atterberg limits, swell pressure, and field investigations before finalizing stabilization measures. What stabilization methods are commonly used? Lime stabilization, cement stabilization, blending with non-expansive materials, and undercutting/replacement are common methods depending on project needs. References: IS:2720 (Part 40); MoRTH Section 300; IRC:75 (2015); IRC:SP:89; NHAI QA/QC Manual. © 2025 Highway Quality Test     Frequently Asked Questions Why Black Cotton Soil is Expansive in Nature Ans: The Black cotton soil, widely found in India, is known for its challenging behavior in civil  engineering projects. What makes this soil so problematic? Its ability to swell during wet seasons and shrink during dry periods can severely affect pavements, embankments, and building foundations. In this post, we explore the science behind its expansive nature and why         understanding it is crucial for highway and geotechnical engineers. Black cotton soil, commonly found in central and western India, is known for its expansive behavior — it swells when wet and shrinks when dry. This property poses serious challenges in highway and foundation engineering. What Makes Black Cotton Soil Expansive? Ans: The key reason lies in its mineral composition, specifically the presence of montmorillonite, a clay mineral with a high capacity for moisture absorption. Montmorillonite has a layered crystal structure that allows water molecules to enter, causing the soil to expand during wet seasons (like the monsoon) and contract during dry weather. This moisture-driven volume change leads to: Pavement heaving and surface distortions Longitudinal and transverse cracks in roads Loss of ride quality and pavement durability Uneven settlements in embankments and building foundations Structural failure in subgrade and base layers Bentonite, a commercially known form of montmorillonite clay, exhibits similar swelling and shrinkage behavior — often used in lab studies as a reference for expansive soils. Why It Matters in Highway Engineering Ans: To understand the swelling behavior of black cotton soil  becuase it is essential for effective highway design. Without proper soil stabilization, these soils can cause premature pavement failure and unsafe road conditions. Hence, it is mandatory to perform this test prior to use the borrow area materials. What to do if FSI limit exceeds the accetable Limit?Recommended stabilization methodologies (when FSI > acceptable limit) Recommended Stabilization Methodologies — When FSI Exceeds Acceptable Limits Guidance for highway engineers and site teams on choosing and validating stabilisation treatments for expansive soils (Free Swell Index > acceptable limit). 1) Lime Treatment (Modification / Stabilization) When to use: Highly plastic clays and soils with significant clay fraction where