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An investigation of standard penetration test hammer efficiency through measurement of impact velocity and stress wave energy Miller, Kurtis Henry

Abstract

This thesis presents a study of SPT hammer efficiency measurement methods in a set of innovative underground test chambers. The standard stress wave method of hammer efficiency measurement was used alongside hammer impact velocity measurement to create a new database of 1,263 automatic hammer SPT blows. These data were collected with drill rod lengths of 8.5, 16.2 and 32 m. Previous researchers have concluded that estimating SPT hammer efficiency by measuring impact velocity gives equal or slightly better uncertainty in N₆₀ at a lower cost than direct stress wave measurement. The collected dataset confirms this conclusion. Impact velocity is shown to be a reliable indicator of stress wave energy. Measurement of impact velocity is shown to be an attractive complement (if not full replacement) to direct stress wave measurement due to lower measurement uncertainty and the method’s relative simplicity. The hammer impact velocity is measured with a string potentiometer, a high-speed camera, and proprietary sensors integral to a commercially available automatic SPT hammer (the eSPT from MARL Technologies). The stress wave energy is measured using two different devices, the SPT Analyzer (PDI Inc.) and another from Northwoods Instruments. Using two different stress wave devices shows the inherent uncertainty in SPT stress wave measurement. This uncertainty is best seen with the 16.2 m drill rod, where the two devices produced a 5.2% difference in average ETR. The entire dataset has an average post-impact efficiency (e2, stress wave energy divided by impact kinetic energy) of 96.4% (1,263 blows, standard deviation = 4.0%). With this hammer, applying a 96.4% correction to the measured impact velocity will provide a better estimate of ETR than direct measurement. The same logic should apply to all automatic hammers if a tool for impact velocity measurement is available. The innovative test site used to collect these data has several benefits to traditional research methods. Three closed, cased boreholes were installed to different depths of interest, and a basket of energy-dissipating media was placed at the bottom of each borehole. Without further drilling, the test holes can now be used for testing with different hammers, drill rods, or LPT-type tests. Supplementary material(s) available at: http://hdl.handle.net/2429/81499

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Attribution 4.0 International