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).
| 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 |
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.
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:
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.
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.
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:Â Highly plastic clays and soils with significant clay fraction where FSI is moderate to high. Lime is preferred for high-plasticity clays because it reduces plasticity and swell potential effectively.
What it does
Short-term (immediate): Cation exchange and flocculation/aggregation — reduces plasticity and improves workability.
Long-term: Pozzolanic reactions between lime and soil silica/alumina form cementitious products that increase strength and reduce swell.
Typical design & steps
Laboratory trials:Trial several quicklime/hydrated lime contents (commonly 2–8% by dry weight). Determine OMC, MDD, UCS (7 & 28 days), and residual FSI after treatment. Confirm soil chemistry is suitable.
Field process:Scarify to design depth → spread lime uniformly → mix to specified depth (rotary/pugmill) → condition (12–24 h) → remix, shape and compact to target density at OMC → cure (moist curing) for specified days (commonly 7–14 days).
Acceptance:Laboratory-confirmed UCS / soaked CBR targets and reduced FSI before opening to traffic.
Design tip: use hydrated lime or quicklime depending on availability and handling. Always verify reactivity via lab trials and consult IRC guidance for mix selection.
2) Cement Stabilization (Cement Treated Subgrade / Base)
When to use:Â Low to medium plastic soils or granular soils where early strength and durability are required (e.g., cement-treated base or subbase layers).
What it does
Hydration of cement with soil fines creates a cemented matrix that reduces permeability and swell, increasing UCS and CBR.
Typical design & steps
Laboratory trials:Test cement contents (commonly 3–10% by dry weight) to meet target UCS (e.g., 1.5–3.5 MPa at 7/28 days) or required soaked CBR. Check workability and curing needs.
Construction:Spread and mix cement uniformly (pugmill/rotary), moisture condition, compact to design density and moisture, then cure (moist curing or sealing) for the specified period.
Acceptance:Meet CTB/CTSB targets and residual swell limits as per project specs and MoRTH/IRC guidance.
Cement provides rapid strength gain. For highly plastic clays, cement may be less effective than lime unless blended with other pozzolans.
3) Blended Stabilizers — Lime + Fly Ash / Lime + Cement
Blends combine immediate plasticity reduction (lime) with long-term strength (pozzolanic reaction from fly ash or cement). They can be more sustainable and cost-effective.
Design notes
Run mixture trials to establish optimum proportions (example: lime 2–5% + fly ash 10–20%).
Test for UCS, CBR, residual swell/FSI, and durability (wet–dry cycles).
Confirm compliance with applicable IRC:SP:89 or project-specific guidelines.
4) Mechanical / Geosynthetic Measures and Replacement
If stabilization is impractical (very high FSI) consider removal or mechanical solutions:
Excavate & replacewith non-expansive borrow — best where problematic strata are deep.
Over-excavation + granular fillwith geotextile separator to isolate expansive layer.
Geosynthetics(geogrids, geotextiles) to control differential movement and improve bearing capacity.
Drainage— surface and subsurface drainage to prevent seasonal wetting that triggers swelling.
Often the most reliable long-term solution is a combination: replace the worst material and use geosynthetics/drainage to control moisture.
Design & Laboratory Verification (Mandatory)
Before committing to full-scale field works, perform thorough laboratory and instrumented field trials for each candidate stabilizer:
Optimum binder dose trials (UCS at 7 & 28 days) and soaked/unsoaked CBR.
Residual swell / FSI tests on stabilized specimens.
Durability tests (wet–dry, freeze–thaw where applicable).
Proctor curves, workability and compaction characteristics.
Field trial section
Construct an instrumented trial length or representative area (typically 50–200 m or as project requires). Test in-situ density, moisture content, UCS cores or plate bearing, and monitor performance before approving full-scale adoption.
Typical acceptance criteria (example)
Parameter | Typical Target |
Treated layer UCS (7 days) | ≥ 1.5–2.0 MPa |
Treated layer UCS (28 days) | ≥ 3.0 MPa (project dependent) |
Soaked CBR | ≥ Design target (project dependent) |
Residual FSI / Swell | Reduced below project-specified target (e.g., < 20–25% for subgrade use) |
Specify contract-specific acceptance criteria in the trial report and obtain client/engineer sign-off.
References & Guidance
IS:2720 — Soil testing standards
MoRTH (5th Revision) — Section 300 and relevant specifications for CTB/CTSB
IRC:SP:89 — Guidelines for soil testing and stabilisation mix design
Relevant research: Transportation Research Board reports; technical literature on lime–fly ash stabilization and blended binders.
Prepared for Highway Quality Test — practical guidance for geotechnical engineers. For customization (project-specific mix design tables, lab report templates), contact your lab or request a site-specific trial plan.
Explore detailed test procedures, calculations and acceptance criteria as per IS, MoRTH & IRC specifications:
