Instrumented Becker Penetration Test for Liquefaction Assessment in Gravelly Soils
Characterization of cohesionless soils is an essential part of many geotechnical applications, including liquefaction potential assessment. In gravelly soils found in most alluvial deposits, performance of conventional site characterization tools such as SPT and CPT are adversely affected by the presence of large particles. The Becker Penetration Test (BPT) is a widely used tool for characterization of gravelly soils in North America. The test is performed by driving a closed-ended pile into the ground and reporting the number of blows per-foot of penetration after correcting for hammer energy. Interpretation of the conventional BPT data to obtain soil properties below the driving shoe has significant challenges. The main source of the difficulty is that a large fraction of the energy delivered by the hammer is absorbed by the frictional resistance developed along the pile shaft.
The presentation will introduce the development and interpretation of the instrumented Becker Penetration Test (iBPT). The iBPT addresses the shaft resistance issue by directly measuring the energy transferred from the driving shoe to the underlying soil. The iBPT penetration resistance is represented by per foot blow counts normalized to the energy measured behind the driving shoe, NB30 . The improved NB30 profile obtained by the iBPT offers near perfect repeatability and robustness to driving energy and shaft resistance. In order to enable the application of existing liquefaction assessment methods, a correlation is developed for estimating equivalent SPT N60 values from iBPT NB30. iBPT has been used at a number of sites in California, Washington, and British Columbia, where liquefaction of gravelly soils is a concern. Application of the continuous iBPT measurements provides improved liquefaction assessment in a wide range of materials including those with significant gravel content.