Assessment of Energy Transfer Ratio in SPT Using Automatic Hammers

Although the SPT N-value is used with many empirical correlations for foundation design, inconsistencies in the energy that the SPT hammer transfers to the drill rods have a significant effect on the results of the test. To eliminate such deviations in N-value, quality assurance for licensing new generation nuclear power plants requires that the SPT energy measurements be made for each hammer used. The results of 220 PDA energy measurements on 39 different automatic hammers at four different sites are presented, in soils that range from soft clays to silty sand to partially weathered rock, and at depths ranging from a few meter to over 120 m (8 ft to 400 ft). The results are analyzed and compared to the published data by Florida and Utah DOTs. Computed energy transfer ratios are assessed against the effects of automatic versus safety hammer, drill rig type, drill rod type and length, and frequency of hammer blows.