SPT N60 correction standard

Equations

$$N_{60} = N \cdot C_E \cdot C_B \cdot C_R \cdot C_S$$

Document each correction factor and its source table or measured basis rather than treating the product as a black-box adjustment.

Variables and Units

• $N$ = recorded field blow count for the reported penetration interval.
• $C_E$ = hammer energy correction factor relative to the 60 percent reference energy.
• $C_B$ = borehole diameter correction factor.
• $C_R$ = rod length correction factor.
• $C_S$ = sampler correction factor.

Required Assumptions

• Hammer energy ratio is measured or conservatively assumed before correction.
• Borehole diameter, rod length, and sampler configuration are documented.
• The corrected blow count is used only for comparisons within the method's stated range.

Validity Ranges

• Intended for conventional split-spoon SPT workflows where the field record supports the applied correction factors.
• Use engineering judgement before extending the correction outside documented hammer, rod, or sampler conditions.

Implementation Checks

• Confirm hammer type, measured or assumed energy ratio, borehole diameter, sampler configuration, and rod length before applying the correction.
• State whether additional normalization, such as overburden correction, is handled in a separate step outside the base N60 method.
• Check that the field log resolution and interval notation match the blow count being corrected.

Known Limitations

• Missing field metadata can make one or more correction factors non-defensible.
• N60 does not remove broader geological uncertainty or sampling disturbance.

References

• Skempton, A. W. (1986). Standard penetration test procedures and the effects in sands of overburden pressure, relative density, particle size, ageing and overconsolidation.
• Youd, T. L., et al. (2001). Liquefaction resistance of soils: summary report from the 1996 NCEER and 1998 NCEER/NSF workshops.

Evidence to Attach

• Field boring log showing blow counts, sampler details, hammer system, and rod lengths.
• Calibration or manufacturer basis for the hammer energy ratio and any assumed default factors.
• The corrected calculation sheet, script, or notebook used to derive N60 from the raw blows.

Worked Example

Start from the recorded blow count, confirm hammer energy, borehole diameter, rod length, and sampler factors, then apply the N60 correction before comparing borings or liquefaction triggering charts.