The free swell index (FSI) test is used to identify:
Choose the correct answer
Permeability of silts
Expansive (swelling) soils such as black cotton soil
Compressibility of sands
Shear strength of gravels
Correct Answer
B. Expansive (swelling) soils such as black cotton soil
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Free Swell Index (IS 2720 Part 40): FSI = (Vd − Vk)/Vk × 100% where Vd = volume in distilled water, Vk = volume in kerosene. FSI < 50%: non-swelling; 50–100%: moderate; > 100%: high; > 200%: very high. Test identifies expansive soils (black cotton soil, montmorillonite-rich clays) that cause heave of foundations and pavements.
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