DEFINITION
Free Swell Index is the increase in volume of a soil, without any external constraints,on submergence in water
It is determined by the following way as per IS: 2720 (Part XL) - 1977.
PRINCIPLE
Free swell or differential free swell, also termed as free swell index, is the increase in volume of soil without any external constraint when subjected to submergence in water.
APPARATUS
i) IS Sieve of size 425μm
ii) Oven
iii) Balance, with an accuracy of 0.01g
iv) Graduated glass cylinder- 2 nos., each of 100ml capacity
PROCEDURE
i) Take two specimens of 10g each of pulverised soil passing through 425μm IS Sieve and oven-dry.
ii) Pour each soil specimen into a graduated glass cylinder of 100ml capacity.
iii) Pour distilled water in one and kerosene oil in the other cylinder upto 100ml mark.
iv) Remove entrapped air by gently shaking or stirring with a glass rod.
v) Allow the suspension to attain the state of equilibrium (for not less than 24hrs.).
vi) Final volume of soil in each of the cylinder should be read out.
REPORTING OF RESULTS 
where, Vd = volume of soil specimen read from the graduated cylinder containing distilled water.
Vk = volume of soil specimen read from the graduated cylinder containing kerosene.
