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

The blow counts in standard penetration tests (SPTs) require suitable corrections to account for energy losses. Over 4000 energy measurements collected from four test sites across Hong Kong were collected and analysed in this study. On average, for all instrumented hammer blows, approximately 62% of the theoretical hammer potential energy was transferred to the drill rod assembly. Using high-speed camera imagery and particle image velocimetry (PIV), the detailed mechanism of stress wave propagation was visualised and assessed. This revealed new details and allowed nuances in the results of routine energy measurements to be interpreted in new light. The results showed 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. Commonly observed discrepancies between theoretical and measured speeds of wave propagation were 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 was quantified.