Instrumented Becker Penetration Test for Improved Characterization of Gravelly Deposits

摘要

The Becker Penetration Test (BPT) is a widely used tool for characterization of gravelly soils, especially for liquefaction potential assessment. BPT results are affected by the variability of the energy transferred from the hammer to the drill string and the distribution of the forces and deformations along the shaft and at the tip. Interpretation of BPT data is particularly challenging in dam foundations where the shaft resistance of the thick dense shell can be significant compared to the tip resistance of underlying loose foundation layers. Current interpretation methods that utilize above ground PDA-type or bounce chamber pressure measurements have had limited success in accurately separating the shaft and tip resistance and, thus detecting potentially weak problematic layers at depth. A Becker drill string was instrumented to allow measurement of the dynamic force and acceleration at the tip in addition to the conventional above-ground location. This provided direct measurements of the energy transferred to the drill string by the hammer and the actual energy that is delivered to the soil below the driving shoe. Blow counts are corrected using the energy measured at the tip and a correlation between this normalized blow count N_(b30) and SPT N_(60) is explored.