Hydraulic pile driving hammers: An assessment of energy transfer ratio

The ability of hydraulic pile driving hammers to overcome energy losses during freefall enables a greater proportion of the potential energy to be transferred to the pile in comparison to diesel hammers. This percentage, termed the energy transfer ratio, is not routinely measured in practice however, and there is an element of uncertainty regarding appropriate energy transfer ratios to assume in drivability analyses. In light of such uncertainties, the energy transfer ratios of four and five tonne hydraulic hammers were assessed during installation of driven cast-in-situ piles at several sites in the United Kingdom. The piling rigs were fitted with instrumentation which enabled measurement of the hammer velocity (and hence kinetic energy) at impact for each blow during installation, with the corresponding magnitude of energy transferred to the closed-ended steel installation tube ascertained using a Pile Driving Analyzer. The results of the study showed that energy transfer ratios were strongly dependent on the hammer drop height, with transfer ratios of 95% advocated by the pile hammer manufacturer only achievable when a drop height in excess of about 600 mm was used. As such, lower energy transfer ratios may need to be considered in drivability predictions for these pile types (i.e. steel or DCIS piles) if lower drop heights are used during driving.