Free Swell Index (FSI) Calculation as Per IS Code

Civil Engicon Team
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Update: May 30, 2024

What is Free Swell Index (FSI) of Soil?

The Free Swell Index (FSI) is a measure of the increase in volume of a soil when it is immersed in water compared to its volume when it is in kerosene. This test is important because it helps identify the swelling potential of soil, which is crucial for engineering and construction projects. Swelling soils can lead to significant structural issues, such as cracking or shifting of foundations and pavements, so understanding the FSI helps in designing appropriate measures to mitigate these problems.

Importance and Applications of Free Swell Index (FSI) of Soil

The Free Swell Index (FSI) of soil is important in geotechnical engineering as it indicates the swelling potential of soil when exposed to water. This property is vital for predicting soil behavior under moisture changes, which can affect the stability and safety of structures like buildings and roads. High FSI values suggest a greater likelihood of soil expansion, leading to potential foundation issues and structural damage. Thus, determining FSI helps in designing appropriate foundations and implementing necessary soil stabilization measures to ensure long-term durability and safety of constructions.

Determination of Free Swell Index (FSI) of Soil and Calculator

The Free Swell Index (FSI) of soil measures the percentage increase in volume of a soil sample when submerged in water compared to kerosene, indicating its swelling potential. It helps in understanding the soil's behavior, particularly its expansiveness, which is crucial for construction and geotechnical engineering projects. Higher FSI values denote higher swelling, which can affect the stability and integrity of structures.

Determination of Free Swell Index (FSI) and Calculation as Per IS Code

To determine Free Swell Index, the following procedure is followed:

To determine the Free Swell Index of soil.

Test Reference
IS: 2720 (Part 40) - 1985

Equipment List
1. Oven (capable of maintaining temperatures between 100°C to 220°C)
2. Balance (500 grams capacity with 0.01 g accuracy)
3. 425-micron IS sieve
4. Two 100 ml glass graduated cylinders

Preparation of Soil Sample
Use soil that passes through a 425-micron IS sieve for this test.

Test Procedure for Free Swell Index
1. Take two 10 g samples of oven-dried soil that have passed through the 425-micron IS sieve.
2. Pour each soil sample into separate 100 ml glass graduated cylinders.
3. Fill one cylinder with kerosene oil and the other with distilled water, up to the 100 ml mark.
4. Remove any entrapped air by stirring with an appropriate tool.
5. Allow the soil samples to reach equilibrium over 24 hours, ensuring no further volume change.
6. After 24 hours, record the final volume of the soil in each cylinder.

Result Calculation
The Free Swell Index (FSI) of the soil is calculated using the formula
FSI (%) = [(Vd - Vk) / Vk] x 100

Vd ​= Volume of soil in distilled water (ml)
Vk​ = Volume of soil in kerosene oil (ml)

Test Report
Report the Free Swell Index to the nearest whole number.

Free Swell Index (FSI) Value Vs Soil Properties

The Free Swell Index (FSI) is an important geotechnical parameter that can indicate certain properties of soil related to its potential for swelling when exposed to water. Here is how FSI relates to soil properties:
Swelling Potential
High FSI: A high FSI indicates that the soil has a significant potential to swell when it absorbs water. This can be problematic for construction projects as it may cause damage to structures.
Low FSI: A low FSI indicates that the soil has minimal potential for swelling.
Soil Type
Clayey Soils: Typically have higher FSIs because clay minerals have a tendency to expand when water is absorbed.
Silty and Sandy Soils: Generally have lower FSIs because they are less prone to significant volume changes upon water absorption.
Highly Permeable Soils: Such as coarse sands and gravels, usually have lower FSIs because they allow water to drain away quickly, reducing the potential for swelling.
Low Permeable Soils: Such as clays and silts, tend to have higher FSIs because water can be retained within the soil, leading to swelling.
Engineering Properties
Foundation Design: High FSIs may require special foundation designs to mitigate the effects of swelling.
Construction Methods: The FSI can influence construction methods and materials used to prevent damage from soil swelling.
Moisture Sensitivity
Soils with higher FSIs are more sensitive to changes in moisture content, which can affect their engineering properties and behavior.
Volume Change
A higher FSI indicates a greater potential for volume change due to water absorption, which can lead to heaving or settlement of structures.

Calculate Free Swell Index (FSI)

Calculate Free Swell Index (FSI)

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