Question

If the actual observed value of standard penetration resistance, N is 21 in a fine sand layer below the water table, then the corrected N value for dilatancy is ............

• A. 21
• B. 18
• C. 15
• D. 11

Hint:

Apply dilatancy correction for fine saturated sands and silts below the water table when \( N>15 \) to avoid overestimation of strength.

Answer 1

The Correct Option is B

When conducting Standard Penetration Tests (SPT) in saturated fine sands or silts below the water table, the observed N-value is often influenced by pore water pressure,
especially at higher N-values. This phenomenon is known as dilatancy and may lead to an overestimation of soil strength.
To correct this, IS 2131:1981 recommends applying a dilatancy correction to the observed N-value if:
- The soil is saturated fine sand or silt, and
- The corrected N-value (after overburden correction) is greater than 15.
The formula used is:
\[ N_{\text{corrected}} = 15 + \frac{1}{2}(N - 15) \] Given: \( N = 21 \)
\[ N_{\text{corrected}} = 15 + \frac{1}{2}(21 - 15) = 15 + \frac{6}{2} = 15 + 3 = 18 \] Therefore, the corrected N-value for dilatancy is \( \boxed{18} \).