Unknown bridge foundations present a unique challenge to Departments of Transportation (DOT) across the country since foundation characteristics are a necessary input to assess scour vulnerability and to develop appropriate scour countermeasures. A number of non-destructive testing techniques have evolved over time to address this challenge and evaluate unknown foundation type and geometry. The most comprehensive study of NDT methods for unknown foundations was performed as part of National Cooperative Highway Research Program (NCHRP) 21-05 in 2001. Since NCHRP 21-05 other case studies have expanded the literature and further advancements in computer software and hardware have improved NDT capabilities and reliability for unknown foundation evaluation. In particular, P-wave reflection imaging has the potential to provide higher resolution data and significantly more detail regarding the subsurface in both the laboratory and the field.
An ultrasonic P-wave reflection imaging system was investigated in this study as a viable NDT method to assess unknown foundations. The P-wave system was originally developed to image large scale soil models in the laboratory and to image the profile of vertically embedded structural elements such as foundations. The P-wave system was utilized in a field testing program to image the vertical profile of foundations at two bridge sites in southeastern Pennsylvania. Site selection, characteristics of the foundations tested, and the site soil conditions are summarized. The final foundation depths were unknown to the research team at the sites, which resulted in a true blind study and simulated the unknown foundation problem. The ultrasound probe was lowered in a PVC-cased borehole alongside the foundation to generate and record P-wave signals. The resulting data was plotted to construct an image and predict foundation characteristics. Multiple NDT techniques including Parallel Seismic, Borehole Magnetometer, and Borehole Radar were also performed at the sites in conjunction with the P-wave testing. The resulting data and interpretation of foundation characteristics are compared between all methods.