The Free Swell Index (FSI) is a laboratory test used to estimate the expansion potential of cohesive soil when immersed in water. It identifies the presence of active clay minerals like Montmorillonite, providing a simple yet effective measure of how much a soil will "heave" upon saturation.
Test Objectives
- Determine the differential swelling characteristics of fine soils
- Identify expansive clay minerals that cause structural heave
- Support soil classification based on the degree of expansion
Site Application
- Evaluates the risk of upward pressure on foundations and slabs
- Assists in design decisions for expansive clay regions
- Supports planning for stabilization methods (e.g., lime treatment)
Safety & Durability
- Prevents cracking in light structures due to soil swelling
- Assesses the suitability of soil for use in earth embankments
- Guides the selection of deep foundations vs. stable fill replacement
Technical Result
- Formula: $FSI = \frac{V_w - V_k}{V_k} \times 100$
- $V_w$: Volume in Water | $V_k$: Volume in Kerosene
- Direct categorization (Low to Very High expansivity)
The test relies on comparing the volume of soil in two different liquids: water and kerosene. While water causes expansive clay minerals to hydrate and swell, kerosene acts as a non-polar liquid that does not induce any volumetric change.
By measuring the difference in final settled volume between the two cylinders after 24 hours, our laboratory calculates the Free Swell Index. This is particularly critical for projects in areas with Black Cotton Soil, where FSI values can exceed 100%, indicating a severe risk to foundation integrity if not properly addressed.
Our Methodology
- 1 Preparation of two 10g oven-dried soil samples (passing 425μm)
- 2 Immersion in graduated cylinders containing water and kerosene
- 3 24-hour observation period to allow for complete settlement
- 4 Precision calculation of FSI and classification reporting