Energy Efficiency and Rod Length Effect in Standard Penetration Test Hammers

Twenty-eight standard penetration test (SPT) hammers owned by the North Carolina Department of Transportation and private consultants were used to investigate the average energy efficiency and variability of manual versus automatic hammers, as well as the effect of SPT rod length on hammer efficiency. The results agree with published data in several regards. Automatic hammers in the study were found to have an average transferred efficiency of 80.9%. This finding agrees very well with the 80% efficiency assumed in geotechnical engineering practice for automatic hammers. Manual hammers in the study averaged 63.9%, close to the 60% efficiency assumed for manual hammers. Manual hammers were found to be twice as variable as automatic hammers in transferred energy from blow to blow within an SPT blow count. The study demonstrated that the measured transferred energy appeared to be affected by rod length. Lengths shorter than approximately 40 ft caused reduced energy to be transferred into the rod. An empirical formula is presented for correcting short rod length energy losses. The data did not demonstrate a strong dependence on SPT N-value, although the data set lacked observations where the N-value was less than 6 blows per ft.