The 'dilatancy correction' in SPT for fine sands below water table is applied when N exceeds:
Choose the correct answer
N > 30 for all soil types
N > 15 for fine sands/silts below water table — actual N overestimates true density due to negative pore pressure during driving
N < 5 for soft clays
N > 60 (never needed in practice)
Correct Answer
B. N > 15 for fine sands/silts below water table — actual N overestimates true density due to negative pore pressure during driving
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IS 2131 / Terzaghi correction: in saturated fine sands and silts: high SPT N (>15) → pore water suction develops during driving → apparent stiffness increase. Dilatancy correction: N_corrected = 15 + 0.5(N_actual − 15) when N > 15. But only for fine sands below WT (not coarse sands). Reduces N to account for pore pressure effect. Applied BEFORE overburden correction in some methods.
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