Characterization of Gravelly Alluvium

Characterizing project sites where gravelly alluvium is present poses particular challenges. Conventional field and laboratory techniques are significantly limited when large gravel particles either prevent their operation or adversely influence their measurements. The deposition of gravelly alluvium often involves complex, energy and sediment load dependent processes that can result in highly spatially variable deposits comprised of gravel to clay sized particles. This pervasive variability warrants an integrated site characterization approach anchored in an understanding of the geological formation processes and the expected controlling deformation/failure mechanisms, confirmed by the site investigation, simplified as part of site idealization, and verified during construction and long term monitoring. Systematic incorporation and handling of the site variability during the site idealization process is particularly important. Recent work at several gravelly alluvial sites has allowed the development of new approaches and techniques for improved site characterization. In particular, the benefit of continuous sonic sampling is demonstrated and a systematic method for evaluating gravel influence on SPT N values is outlined. Finally, an overview of the instrumented Becker Penetration Test (iBPT), a closed-ended, large diameter penetrometer with new direct energy measurement at the drill string tip is presented.