Answer: b) Increase in volume without any constrains
According to IS: 2720-part 40, soil’s free swell is defined as an increase in ground volume without any submergence in water.
The increase in soil volume without any external constraints, especially on submergence in water, is called the Free Swell Index.
Free Swell Index (%) = (Vd-Vk)/Vk x 100
Vd = The volume of the soil specimen read from the graduated cylinder(containing distilled water).
Vk = The volume of the soil specimen read from the graduated cylinder(containing kerosene).