Question

During the surface investigations for design of foundation, a standard penetration test was conducted at 4.5 m below the ground surface. The record of number of blows is given below

$\begin{array}{cc}\text{Penetration depth(cm)}& \text{Number of blows}\\ 0-7.5& 3\\ 7.5-15& 3\\ 15-22.5& 6\\ 22.5-30& 6\\ 30-37.5& 8\\ 37.5-45& 7\end{array}$

Assuming the water table at ground level, soil as fine sand and correction factor for overburden as 1.0, the corrected N value for the soil would be

• A. 18
• B. 19
• C. 21
• D. 33

Answer 1

The correct option is C 21

$\begin{array}{cc}\text{Penetration depth(cm)}& \text{Number of blows}\\ \begin{array}{c}0-7.5\\ 7.5-15\end{array}& \begin{array}{c}3\\ 3\end{array}\}\text{Discarded}\\ \begin{array}{c}15-2.5\\ 22.5-30\end{array}& \begin{array}{c}6\\ 6\end{array}\}{N}_2=12\\ \begin{array}{c}30-37.5\\ 37.5-45\end{array}& \begin{array}{c}8\\ 7\end{array}\}{N}_3=15\end{array}$

For calculation of SPT number N, we discard the value of blows for first 15 cm and count number of blows for next 30 cm.
Uncorrected SPT number,
$N=N_2+N_3$
=12 + 15 = 27
N > 15 and soil is saturated fine sand

Applying Dilatancy correction
Corrected
$N^{\prime }=15+\frac{1}{2}(N-15)$

$=15+\frac{1}{2}(27-15)$

= 21