Question

If the actual observed value of standard penetration resistance, N, is greater than 15 in a fine sand layer below water table, then the equivalent penetration resistance will be

• A. $15+\left(\frac{N+15}{2}\right)$
• B. $15-\left(\frac{N+15}{2}\right)$
• C. $15+\left(\frac{N-15}{2}\right)$
• D. $15+\left(\frac{15-N}{2}\right)$

Answer 1

The correct option is C $15+\left(\frac{N-15}{2}\right)$

Saturated fine or silty dense or very dense sand deposits below water table have a tendency to dilate during shear under undrained conditions. Therefore the observed N value is more than actual value and dilatancy correction should be applied as
$=15+\left(\frac{N-15}{2}\right)=\frac{N+15}{2}$

It is based on the assumption that critical void ratio occurs at N= 15.