Determination of free swell index of soils as per Indian Standard IS 2720 Part 40

In this article, we will learn about how to find the free swell index of soil as per IS 2720 Part-40



This method covers a test for the determination of the free swell index of a soil which helps to identify the potential of a soil to swell which might need further detailed investigation regarding swelling and swelling pressures under different field conditions.


 1. Sieve- 425 micron IS Sieve 2. Glass Graduated Cylinders -Two 100 ml capacity


  1. Take two 10g soil specimens of oven-dry soil passing through 425- micron IS Sieve.
  2. In the case of highly swelling soils, such as sodium bentonites, the sample size may be 5g or alternatively, a cylinder of 250 ml capacity may be used.
  3. Each soil specimen shall be poured in each of the two glass graduated cylinders of 100 ml capacity. 
  4. One cylinder shall then be filled with kerosene oil and the other with distilled water up to the 100 ml mark.
  5. After removal of entrapped air (by gentle shaking or stirring with a glass rod) the soils in both cylinders shall be allowed to settle.
  6. Sufficient time, not less than 24 hours shall be allowed for the soil sample to attain the equilibrium state of volume without any further change in the volume of the soils. 
  7. The final volume of soils in each of the cylinders shall be read out.    


The level of the soil in the kerosene graduated cylinder shall be read as the original volume of the soil sample, kerosene being a nonpolar liquid does not cause swelling of the soil. The level of the soil in the distilled water cylinder shall be read as the free swell level. The free swell index of the soil shall be calculated as follows : 

Free swell index, Percent = [(Vd - Vk)/Vk] x 100

Vd = The volume of soil specimen read from the graduated cylinder containing distilled water, and 
Vk = The Volume of soil specimen read from the graduated cylinder containing kerosene.

Soil Free Swell Index

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