Sustainability of Bridge Foundations Using Electrical Resistivity Imaging and Induced Polarization to Support Transportation Safety
Stefan Hurlebaus, Jean-Louis Briaud, and Stacey Tucker, January 2014
As of September 2007, there were 67,240 U.S. bridges in the National Bridge Inventory classified as having unknown foundations (FHWA 2008). The bridges spanning rivers are of critical importance due to the risks of potential scour. In fact, it is estimated that 60 percent of all bridge collapses are due to scour (Parola et al. 1997). Not only are these failures costly, they can be deadly for the traveling public. On April 5, 1987, 10 people were killed in New York when a pier collapsed on the Schoharie Creek Bridge causing two spans of the deck to fall into the creek. Several other fatal collapses have occurred since the Schoharie Creek Bridge failure. Detecting scour is only part of the assessment that must take place to determine risk of failure and knowing the foundation depth is a critical component of the assessment.
This research explored the feasibility and effectiveness of induced polarization (IP) and electrical resistivity imaging (ERI), near surface geophysical methods, for determining the depth of unknown foundations. With budget cuts and deteriorating infrastructure, there is a need to seek alternative solutions for nondestructive structural integrity testing that are more robust to limit bridge failures that hinder transportation safety. The existing methods for unknown bridge foundations in the literature are often hindered by the type of foundation or require the use of a borehole, making them very costly. As a result, only a few states are working to identify the depth of unknown bridge foundations around them. In order to solve this national problem, a new and effective method needs to be investigated with full scale bridge testing and disseminated nationwide. In this work, an experimental study was conducted at a National Geotechnical Experimentation Site (NGES) to identify key parameters for the testing design and setup in order to obtain optimal surveys of bridge foundations. The conclusions of the NGES investigations were used to plan the field surveys on four bridges with known foundations. The outcomes of the four bridges showed that IP and ERI can be used in concert with one another to estimate the type and depth of bridge foundations. The results of the field surveys were used to create a probability of non-exceedance curve for future predictions of unknown bridge foundations using the methods described in this research. Finally, the probability of exceedance curve was used to validate the method with testing on a foundation unknown at the time of testing, and the use of IP and ERI were extended to other subsurface infrastructure when a gas line was imaged.
Keywords: Unknown Bridge Foundations, Scour, Electrical Resistivity, Induced Polarization, Probability of Exceedance
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