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The Free Swell Index (FSI) Test is used to determine the swelling potential of expansive soils, especially clayey soils like black cotton soil.
Principle
Soil is added in two cylinders:
One with water
One with kerosene
Clay swells in water but not in kerosene
Difference in volume gives swelling index
Applications
Identification of expansive soils (black cotton soil)
Foundation design (avoid swelling damage)
Road & pavement design
Embankment construction
Soil stabilization planning
Important Points
Simple and quick test
Used for preliminary identification only
Not suitable for coarse soils
High FSI → danger for structures (cracks, heaving)