Table 1: Different types of SPT corrections used to obtain 𝑁 𝑐 values.

Different types of correctionCorrection factor

Correction for hammer

Donut hammer0.5–1.0
Safety hammer0.7–1.2
Automatic-trip donut hammer0.8–1.3

Sampler correction

Without liner1.00
With liner: dense sand, clay0.80
Loose sand0.90

Rod length correction

Length > 10 m1.0
6–100.95
4–60.85
0–40.75

Borehole diameter correction

Hole diameter 60–120 mm1.00
150 mm1.05
200 mm1.15
Correction for overburden pressure, 𝜎 𝑣 0 ( 𝐶 𝑁 ) 2 . 2 / ( 1 . 2 + 𝜎 𝑣 0 / 𝑃 𝑎 ) , where 𝑃 𝑎 = 1 0 0 k P a
Correction for fines content ( 𝐶 n e s ) 1 + 0 . 0 0 4 F C + 0 . 0 5 5 ( F C / 𝑁 6 0 ) , where FC = percent fines content (percent dry weight finer than 0.074 mm) and 𝑁 6 0 = SPT value for 60% energy ratio