Comparison of Borehole Ultrasound and borehole radar in evaluating the length of two unknown bridge foundations

Foundation length is an important parameter when assessing bridge vulnerability to failures related to scour hazards. As a result, multiple non-destructive testing methods have been developed to evaluate the geometry of unknown foundations, each with their own advantages and disadvantages. Subsurface methods that rely on measurements from boreholes alongside the foundation are often the most robust when evaluating foundation length, particularly for complex foundations (e.g. footings supported on piles). In this study, such a system was developed to evaluate the length of foundations at two bridge sites in Philadelphia using ultrasound acoustic waves (i.e. P-waves). Characteristics of the foundations and the site conditions are summarised as well as the ultrasound system components. The system was lowered in a borehole alongside each foundation as 100 kHz P-waves were generated to develop a reflection image of the pile–soil interface. Foundation length was evaluated based on discontinuity of the reflected signals with depth. This ultrasound system was developed to address inadequacies with borehole radar testing in similar applications related to unknown foundations. In particular, borehole radar has limited capabilities in clayey soil profiles, where high values of electrical conductivity limit radar signal strength. A commercial borehole radar system (MALÅ 250 MHz ProEx) was therefore utilised at both bridge sites to provide a comparison of results using both systems.