One additional problem is that the SPT "N-value" is affected by the energy transfer from the SPT hmmer into the rod. This efficiency can be measured by for example a Pile Driving Analyzer. The effect can be quite substantial as efficiencies have been measured as low as about 20% on some hammers and essentially 100% on others. Thus if liquifaction is a concern, then SPT energy should me measured and then the N value can be a more reliable guide to liquifaction potential.
An ASTM procedure for liquifaction has been developed which recognizes the importance of measuring this energy. This specification is ASTM D-6066. It describes a rational method to adjust the N values for differing hammer efficiencies.