Rational Alternative to Positive Discovery of Pile-Supported Bridges with Unknown Foundation Depth

Bridges with unknown foundations have been a burden on the transportation agencies across the United States because of the lack of funding and availability of rational methods to determine the risk of failure of these bridges from scour. The embedment of unknown foundation bridges is a key to proceed further with any risk-based management approach and/or the widely used Federal Highway Administration (FHWA) guidelines on the methods of evaluation of unknown foundations. The need for a practical, cost-effective, and rational approach to positive discovery of bridge foundations and to reclassify bridges with unknown foundations is greater than ever. This technical note presents an assessment of the predictive capability of various methods currently in use to determine the embedment of unknown foundation bridges. The methods discussed in this paper include sonic echo method (or pile integrity tester) as non-destructive testing (NDT), inference (IM) and/or back calculation (BC) [i.e., reverse engineering (RE)], and static/back calculation (S/B-C). The well-documented case histories presented in the paper illustrate the usefulness of using the S/B-C method in determining the embedment of unknown foundations. The S/B-C also can be used to assess/validate NDT results and embedment estimates made by IM and/or BC (RE) approaches. For high priority unknown foundation bridges, the S/B-C reasonable minimum embedment and the embedment required for stability considering the scour can be used to plan and efficiently implement any future NDT for these bridges. It can significantly reduce the cost of treating unknown foundation bridges by optimizing the use of NDT, if warranted. The three case histories presented in the paper also suggest that, in general, the sonic echo (i.e., pile integrity tester)/NDT tends to be conservative whereas the IM and/or BC (RE) methods tend to be unconservative. This would lead to high cost or high risk, respectively, if such estimates are used to assess the stability of unknown foundation bridges impacted by scour.