Study of energy transfer and stress wave propagation during SPT through energy measurement and PIV

Abstract

The blow counts in standard penetration tests (SPT) require suitable corrections to account for energy losses. Over 4,000 energy measurement data collected from four test sites across Hong Kong have been collected and analysed in this study. On average for all instrumented hammer blows, approximately 62% of the theoretical hammer potential energy is transferred to the drill rod assembly. Using high-speed camera imagery and particle image velocimetry (PIV), the detailed mechanism of stress wave propagation has been visualised and assessed. This has revealed new details and allowed nuances in the results of routine energy measurements to be interpreted in new light. It has been confirmed that the initial compressive wave is reflected as a tensile wave for low end-restraint at the SPT sampler and as a second compressive wave for high end-restraint. The commonly observed discrepancy between the theoretical and measured speeds of wave propagation is highlighted and explored. Energy loss arises from friction during free-fall of the hammer, at the anvil, and from other sources such as the horizontal motion of the hammer-anvil system. The proportion of each major source of energy loss is quantified.