SPT corrected N-value, dense sand

Method: Terzaghi-Peck corrections. First the overburden correction $N' = C_N N$ with $C_N = 0.77\log_{10}(2000/\sigma_v')$, then the dilatancy correction $N'' = 15 + \tfrac12(N'-15)$ for $N' > 15$.

Observed N: the first 150 mm is the seating drive, so

$$N = 21 + 22 = 43$$

Overburden stress: no water table is given, so

$$\sigma_v' = \gamma z = 17 \times 20 = 340\ \text{kN/m}^2$$

Overburden correction:

$$\begin{aligned} C_N &= 0.77\log_{10}\frac{2000}{340} = 0.593 \\ N' &= 0.593\times 43 = 25.48 \end{aligned}$$

Dilatancy correction ($N' > 15$):

$$N'' = 15 + \tfrac12(25.48-15) = 20.24$$

Answer: corrected N-value $\approx 20.2$, say 20. (If the dilatancy correction is applied first, $15+0.5(43-15)=29$ and $29\times0.593=17.2$; the order recommended by Terzaghi-Peck, overburden then dilatancy, is used here.)