Use of P-Wave Reflection Imaging and Other Nondestructive Testing Techniques to Evaluate Unknown Bridge Foundations

Abstract

Unknown bridge foundations present a unique challenge to departments of transportation (DOT) across the country because foundation characteristics are a necessary input to assess scour vulnerability and to develop appropriate scour countermeasures. A number of nondestructive testing (NDT) techniques have evolved over time to address this challenge and evaluate unknown foundation type and geometry. An ultrasonic P-wave reflection imaging system developed to characterize the depth profile of vertically embedded structural elements in soil is presented as a viable NDT method to assess unknown foundations. System components and data acquisition are briefly reviewed, followed by a discussion of issues related to acquisition of P-waves in soils. The P-wave system was utilized in a field-testing program to image the vertical profile of foundations at bridge sites in southeastern Pennsylvania. Characteristics of the foundations tested in the field 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 cased borehole alongside the foundation to generate and record p-wave signals. The resulting data were plotted to construct an image and predict foundation characteristics. Multiple NDT techniques including parallel seismic, borehole magnetic, 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 among all methods.