Current Practices and Guidelines for the Reuse of Bridge Foundations (2017)

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Current PraCtiCes and Guidelines for the reuse of BridGe foundations Managing aging bridges safely and cost-effectively is a central challenge for an aging U.S. infra- structure. One appealing, sustainable option for many bridge replacement and rehabilitation efforts is reusing bridge foundations, which can result in time and cost savings as well as reduced mobil- ity impacts and environmental benefits. However, foundation reuse presents significant challenges including uncertainties in existing foundation condition, remaining service life, and capacity. The objectives of this Synthesis are twofold: (1) to document current practices and guidelines used by transportation agencies for the reuse of bridge foundations, and (2) to identify challenges and gaps in information that impede foundation reuse. Information in this Synthesis is derived from a literature review, a survey of transportation agencies, and case examples from state agencies with significant foundation reuse experience. The Synthesis addresses: • Extent and frequency of foundation reuse; • Existing specifications, policies, and guidelines related to foundation reuse; • Motivations for foundation reuse; • Challenges for foundation reuse; • Investigation methods and condition assessment of existing foundations; • Methods for predicting capacity of existing foundations; • Construction techniques used to facilitate foundation reuse; and • Post-reuse monitoring and analysis of performance. The survey was distributed electronically to 62 agencies, including state transportation agencies for all 50 states, Puerto Rico, and the District of Columbia, as well as ten Canadian provinces and territories. Forty-five of the 50 state departments of transportation (DOTs) responded to the survey, a response rate of 90%. Eight Canadian provinces also responded to the survey. The survey was distrib- uted to agency geotechnical engineers; however, the survey instructions encouraged these engineers to share the survey with bridge or structures colleagues. The first survey question asked if the respondent’s agency had experience with foundation reuse, which the survey described as any agency action that changes the design load of an exist- ing foundation. Respondents who reported that their agency does not have experience reusing foundations were then asked three follow-up questions. Respondents who indicated their agency does have experience with foundation reuse were asked 22 follow-up questions. Survey responses were reviewed to identify agencies with unique policies regarding foundation reuse, agencies that reuse foundations frequently, and agencies with experience on noteworthy foundation reuse proj- ects. Additional investigation into the practices and experiences of the agencies selected as case examples was performed by conducting interviews with agency personnel and reviewing available agency documents. Experience with reusing bridge foundations is common among U.S. agencies; however, experience level varies widely. All but one of the 45 U.S. agencies surveyed have experience with foundation reuse. However, there are fewer than ten agencies that reuse bridge foundations for various applications at least five times in an average year. The majority of the remaining survey agencies reported rates of summary

2 reuse that were considered low—one or zero instances in an average year for most applications. The vast majority of the bridge foundation reuse applications reported in the survey was either for bridge or superstructure replacement projects or bridge or superstructure widening projects. The survey and agency interviews revealed that foundation reuse is generally motivated by economic considerations, although accelerated construction and constructability were also commonly cited. Case example agen- cies reported that they were less likely to reuse foundations for bridges over water compared with land crossings because of hydraulic considerations, including scour risk. Although applications of foundation reuse are fairly widespread, documented policies and proce- dures are rare. Only three agencies were found to have relatively thorough policies related to reuse, and only one of the three policies includes guidance regarding investigation methods for existing foundations. Investigation of existing foundations is considered a major challenge for foundation reuse. Compared with a new foundation project, the scope of a site investigation for a foundation reuse project is fundamentally different, equally important, and in many ways more challeng- ing. For a reuse project, the site investigation seeks not only subsurface material information but also information about the existing foundation, including the type, geometry, condition, and load capacity. Existing foundation investigation practices vary widely among agencies, as well as within agencies on a project-by-project basis. The survey revealed that nearly all agencies rely on historical records, approximately half have excavated to expose portions of the foundation for visual observation, and approximately half have performed core drilling of foundations. Use of geophysical methods was less common, but not uncommon. Case example agency reports indicated that the scope of the inves- tigation generally depends on project-specific considerations such as the significance of the structure, site geology, and foundation type. Regarding analysis of existing foundations for reuse, agencies generally appear more concerned with the load capacity of existing foundations than with condition assessment and remaining service life. Agencies evaluate the load capacity of existing foundations frequently, explicitly, and quantita- tively while remaining service life evaluations of existing foundations are less frequent and typically either implicit or qualitative. The emphasis on capacity is perhaps attributable to agency practices that are driven by codes focused on capacity; however, it is also likely that the difficulty of condition assessment prevents agencies from focusing on it. All case example agencies emphasized challenges associated with predicting capacity of existing foundations, particularly with respect to justifying increased capacities necessary to satisfy updated live load requirements. Based on the survey, the most common method of predicting load capacity is to identify values from the original project documents. Case example agencies expressed some degree of willingness to rely on observed, historical performance as justification for reuse even when current design criteria could not be satisfied using original capacity values. Although agencies gen- erally reported experience with multiple methods of predicting load capacity, two-thirds of survey respondents reported that they did not know how their agencies evaluated remaining service life. This is consistent with the literature, which provides specific information regarding how to predict that foundation service life is limited even though some deterioration mechanisms; for example, corrosion of steel piling, are relatively well understood. Information presented in the literature review revealed that uncertainties involved in reuse proj- ects, specifically the reliability of historical records, investigation methods for existing foundations, and performance risks associated with reused foundations, result in an uncertain standard of care for consultants designing foundation reuse projects. Two case example agencies noted that consulting engineers are generally reluctant to design foundation reuse projects because of legal risks. In con- trast, one case example agency reported no hesitation on behalf of consultants recommending reuse, and three case example agencies provided example projects that were designed by consultants. The resulting picture of consulting practice for foundation reuse projects is not clear, but it appears likely the uncertain standard of care noted in the literature review makes at least some consulting engineers hesitate to design reuse projects.

3 The need to evaluate above- and below-ground portions of the substructure concurrently calls for close collaboration among many individuals, including structural engineers, geotechnical engineers, hydraulic engineers, and field technicians. In addition, many of the technical concepts associated with foundation reuse require external consultation, notably from foundation and bridge contractors as well as from material scientists for corrosion evaluation and geophysicists for many investigation techniques. For many agencies, such broad collaboration is likely another challenge for successful implementation of foundation reuse. Research efforts would advance the state of foundation reuse in the United States. Specific research ideas are presented here: • Methods for investigating existing foundations: Such research would help engineers deter- mine the scope of investigations for existing foundations by identifying useful investigation techniques for existing foundations as a function of foundation type, size, depth, geology, and potentially other project-specific factors. • Improvement of geophysical and electrochemical methods for foundation reuse applications: In general, the objectives of the geophysics research projects would include improving the accuracy and precision of geophysical methods for identifying foundation features and condi- tion and/or improving the applicability of the geophysical methods so that they can be used for a larger set of foundation types. • Methods for estimating load capacity of existing foundations: The objective of the proposed research would be to improve load capacity predictions based on the original design value by considering several potential sources of new project information: – Existing foundation investigation information such as foundation depth and new subsurface information could be used to develop updated load capacity predictions based on static analysis methods. – Documented history of bridge loading and foundation performance could be used to provide a lower-bound estimate of load capacity. – Field methods for evaluating load capacity, including conventional load test methods applied to existing foundations as well as the “unload test” presented in chapter two. The RuFUS (Re-use of Foundations for Urban Sites) methodology for identifying capacity improvements (“reserves”) is a useful framework for the proposed load capacity research. • Reliability aspects of foundation reuse: Research is needed to develop reliability-based design methods for foundation reuse that consider uncertainties in existing foundation condition and load capacity. Ideally, the reliability-based design methods would include resistance factors that would be compatible with current Load and Resistance Factor Design methods for bridge design. The reliability methods developed for this research idea would be most useful if they consider the uncertainties associated with the sources of new project information listed for the load capacity research. • Implementation of innovative materials and technologies to facilitate foundation reuse: The first phase of the proposed research involves identifying recent developments in innovative materials and technologies, particularly those related to accelerated bridge construction and ground improvement, and evaluating how the methods might help facilitate foundation reuse. A potential second phase of the research would involve field evaluation of technologies identified in the first phase for foundation reuse applications. • Methods to predict foundation service life: Foundation service life is a topic of interest beyond just reuse applications and preliminary FHWA research into the topic is currently underway. The research proposed here for foundation reuse would involve adapting any emerging methods for condition assessment, performance monitoring (including monitoring for corrosion), and prediction of foundation service life to address predicting remaining service life of existing foundations. The proposed research would also evaluate innovative technologies for improving the collection of foundation performance data: – Use of “smart” foundations that continuously monitor loads, displacement, and corrosion over the lifespan of a foundation; – Interferometric techniques for deflection monitoring; and – Techniques to measure load on existing foundations.

4 • Decision methods for foundation reuse: Research to develop a preliminary decision methodology for foundation reuse could help agencies evaluate reuse in early stages of bridge rehabilitation projects. • Methods to promote future reuse: Research to facilitate future reuse of new foundations is needed to reduce the challenges of reuse for future projects. The proposed research would investigate identifying corrosive and noncorrosive conditions, the use of durable materials and sustainable construction details, innovations such as “self-healing” steel to prevent corrosion and other forms of deterioration, and “smart” foundation elements to collect condition assess- ment information over the life of the foundation.