Instrumented Becker Penetration Test. I: Equipment, Operation, and Performance

Abstract

The Becker penetration test (BPT) is a widely used tool for characterizing gravelly soils, especially for liquefaction assessment. Interpretation of BPT data is complicated by the variable energy transferred from the hammer to the drill and by the shaft resistance that develops along the drill string which generally increases with penetration depth. Existing BPT interpretation methods that use above-ground measurements have had limited success in accurately separating the shaft and tip resistance. Therefore, penetration resistance with depth cannot be reliably predicted. An instrumented Becker penetration test (iBPT) that measures acceleration and force directly behind the driving shoe in order to compute the energy delivered to the driving tip was developed and integrated into the standard closed-ended Becker drill system. The equipment and data acquisition system are described in this paper. The analysis procedure used to compute energy-normalized blow count values and produce continuous penetration resistance profiles is outlined. The energy-normalized blow count profiles generated are shown to be independent of penetration depth, predrilling depth, shaft resistance magnitude, and hammer operating conditions. In particular, the efficacy of a residual energy-based normalization scheme is demonstrated. Finally, a discussion regarding energy measurements, locked-in stress at the drill string tip, and the use of the pull-back and redrive procedure is presented.