Free Swell Index of Soil

Free Swell Index compares the volume a soil occupies when poured loosely into water against the volume it occupies in kerosene (a non-polar liquid that causes no swelling). The percentage difference indicates how reactive the clay minerals in the soil are to water.

This is a quick, low-cost screening test used to flag expansive (black cotton) soils early in a site investigation, before committing to the more detailed swell-pressure or differential free swell testing that foundation design on such soils requires.