MEASUREMENT OF PILE SET AND IMPACT VELOCITY FOR DRIVEN PILE ACCEPTANCE-Paper

Paper

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Traditional driven-pile acceptance relies on pile-driving formulas that estimate hammer energy and compare it with pile set or blow count. These methods commonly assume consistent hammer performance and energy transfer. The paper argues that these assumptions are unreliable: hammer stroke or ram impact velocity does not reveal how much energy reaches the pile, because transfer also varies with ground conditions and the helmet and cushion system.<br /><br />Three case studies demonstrate substantial variability. For hydraulic hammers driving steel piles, delivered energy ranged from 45% to 100% of measured ram kinetic energy, with further variation between blows at the same stroke. For a diesel hammer, overall efficiency varied from 17% at low stroke to 43% at high stroke. For concrete piles driven at a nominally constant stroke, changes in pile-cushion properties produced delivered-energy efficiencies of roughly 50% to over 100%.<br /><br />Dynamic pile testing can directly assess capacity and delivered energy, but is generally performed on only a minority of piles. The authors therefore propose using high-speed, remote, non-contact measurements of pile displacement to measure pile set and calculate peak pile velocity. Because pile velocity is related to impact force through pile impedance, it provides an indicator of hammer input at the pile itself.<br /><br />After dynamic testing establishes capacity, target set and minimum impact velocity can be specified for subsequent piles. This dual criterion accounts for changes in the hammer, helmet, and cushions more reliably than set alone. The approach could improve confidence in foundation performance and enable more efficient, less conservative designs, with potential savings in time, cost, and environmental impact.

Keywords

driven-pile acceptance

pile-driving formulas

hammer energy transfer

pile set

blow count

dynamic pile testing

high-speed displacement measurement

pile velocity

pile impedance

foundation performance