Measurement of Pile Set and Impact Velocity for Driven Pile Acceptance

Traditionally driven pile control has been based on pile driving formulas. These formulas are typically based on an estimation of the potential energy of the hammer, an assumed hammer efficiency (representing energy input) and measurement of the pile set or blow count (representing pile response). In some instances, measurement of the transient movement of the pile during impact (rebound) is also required. Some formulas also include adjustments for losses in the hammer impact process. The advent of dynamic pile testing allowed the energy actually delivered to the pile to be measured, from which an evidence-based hammer transfer efficiency could be computed, and adopted for project control on the 90% or more of untested piles using formulas or simple set or blow count measurements. For diesel hammers, stroke can be inferred from blow rate, and some hammers measure ram impact velocity. However, it is asserted that measurements of stroke or ram velocity are insufficient to predict delivered energy to the pile primarily due to the effect of the helmet and cushion materials between the hammer and pile. This paper discusses why effective project control therefore relies on direct measurements of hammer input and pile response on the pile itself.