Free Swell Index (FSI) Calculator

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Free Swell Index (FSI)

Made By Mohan Dangi

Test inputs

Initial volume (Vi) — ml

Final volume (Vf) — ml

Notes

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Free Swell Index (%)

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Vi — Vf

Tip: negative value → possible shrinkage or measurement error.

Formula: FSI (%) = ((Vf - Vi) / Vi) × 100

Quick interpretation (guide)

• < 25% — Low swelling potential
• 25–50% — Medium swelling potential
• > 50% — High swelling potential

About the Free Swell Test

Purpose, apparatus, sample prep, procedure, interpretation and precautions — handy lab reference.

Purpose

Measure swelling potential of oven-dry soil on wetting (FSI, %). Useful for preliminary assessment of expansive clays.

Apparatus

• Graduated glass cylinder (25 or 50 ml)
• Oven, balance, distilled water, spatula

Procedure (typical)

1. Record Vi of dry soil in cylinder.
2. Add distilled water, agitate if required and allow to stand (e.g., 24 h).
3. Record Vf; compute FSI.

Precautions

• Use distilled water to avoid ionic effects.
• Do not compress sample when placing in cylinder.
• Report Vi, Vf, mass, duration and SOP used.

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