United States Department of Transportation
i
Application of non-destructive testing to evaluate unknown foundations for Pennsylvania bridges.
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2013-08-01
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[PDF-19.59 MB]
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Abstract: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.
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