SPT N60 Correction

Reference

Raw SPT blow counts depend on the energy the hammer actually delivers, and N60 normalizes every rig to 60 percent of theoretical energy.

The equation

N60 = N (ER / 60)
  • N is the field blow count
  • ER is the measured energy ratio of the rig in percent
  • N60 is the count corrected to the standard energy

Worked example

A field count of 15 from an automatic hammer delivering 80 percent energy becomes N60 = 15 × 80/60 = 20. The same soil tested with a rope and cathead rig at 45 percent would have read about 27 raw.

When to use it

Apply before any correlation, since nearly all published relationships expect N60. Overburden correction to N1(60) follows for sands. The scatter in SPT data is legendary, so lean on trends across many tests.

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