Correct Answer - Option 1 : 18
Concept;
Dilatancy correction:
It is to be applied when No obtained after overburden correction, exceeds 15 in saturated fine sands and silts. IS: 2131-1981 incorporates the Terzaghi and Peck recommended dilatancy correction (when No > 15) using the equation
\(\rm N= 15 + \frac{1}{2}\left( {N_0-\;15} \right)\)
N0 - SPT value after overburden correction
Calculation:
Given: N0 = 21
\(\rm N= 15 + \frac{1}{2}\left( {21-\;15} \right)\)
⇒ N = 18
