Instrumented Becker Penetration Test. II: iBPT-SPT Correlation for Characterization and Liquefaction Assessment of Gravelly Soils

The Becker penetration test (BPT) is a widely used tool for the characterization of gravelly soils, especially for liquefaction assessment. An instrumented Becker penetration test (iBPT) was developed and integrated into the standard, closed-ended Becker drill string. The iBPT produces a continuous profile of energy-normalized blow count values NB30, which are computed using the acceleration and force measurements recorded directly behind the driving shoe. The NB30 profile is repeatable and unaffected by hammer driving energy or accumulated shaft resistance. This paper presents the correlation between iBPT NB30 and standard penetration test (SPT) N60 values, which is necessary for performing liquefaction assessment in gravelly soils. In developing this correlation, field variability was addressed by comparing median iBPT NB30 and SPT N60 values from adjacent soundings over geologically consistent depth intervals. A framework was also developed to assess and, when appropriate, correct for gravel influence on measured SPT blow count values. This framework uses SPT blows per inch as well as physical evidence from SPT and adjacent Sonic samples. The correlation factor between iBPT NB30 and SPT N60 is shown to be a constant value of 1.8 and independent of soil type or penetration resistance magnitude.