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

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70 chapter five ConClusions Foundation reuse is a promising tool for transportation agencies managing large inventories of aging bridges. Foundation reuse has economic and environmental benefits, and is consistent with emerging societal aspirations for sustainability. However, the challenges for agencies considering reuse are significant. This Synthesis documents current practices and guidelines for the reuse of bridge foundations and identifies challenges and gaps in information impeding foundation reuse. The Synthesis consists of a literature review, a survey of transportation agencies, and case examples of select agencies. This chapter summarizes the most notable findings from each before presenting conclusions and recommendations for future research. summary of major findings Experience reusing bridge foundations is common among U.S. agencies; however, agency experience levels vary widely. All but one of the 45 U.S. agencies surveyed have experience with foundation reuse when reuse is defined as any agency action that changes the design load of the existing founda- tion. As reported in chapter three, 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 agen- cies reported rates of reuse that were low—one or zero instances in an average year for most applica- tions. The vast majority of the bridge foundation reuse applications reported in the survey was either superstructure replacement or bridge widening. The survey and agency interviews indicated that foundation reuse is generally motivated by economic issues. Survey data disclosed that accelerated construction is another common motivator. Case example agencies reported that at a project level, reuse is sometimes required by unique con- straints related to constructability or environmental factors. FHWA’s Foundation Characterization Program identified four major challenges for reusing bridge foundations: condition assessment, load capacity, remaining service life, and design codes. These challenges are consistent with the experiences documented by the case example agencies. Although applications of foundation reuse are fairly widespread, documented policies and pro- cedures are rare. Seven agencies reported having documented policies and procedures, and five of those agencies shared their policy documents. Three of the documents were found to include relatively thorough policies related to reuse, whereas the other two acknowledged reuse as an option, but did not establish specific policies or procedures regarding reuse. Investigation practices for existing foundations vary widely among agencies and within agencies on a project-level basis. Case example agency reports indicated that the scope of the investigation generally depends on project-specific considerations such as the significance of the structure, site geology, and foundation type. The survey revealed that nearly all agencies rely on historical records, approximately half have excavated to expose portions of the foundation for observations, and approximately half have performed core drilling foundations. Use of geophysical methods was less common, with five different geophysical methods having been used by at least three and as many as eight agencies. The literature review revealed that many of the geophysical methods are useful only for certain types of structures, foundations, and geologic conditions. Case example agencies

71 and survey respondents noted that shallow foundations are easier to reuse than deep foundations, primarily because of investigation difficulty. All case example agencies emphasized challenges associated with predicting the capacity of existing foundations, particularly with respect to justifying increased capacities necessary to sat- isfy 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. The RuFUS manual provides a basis for increasing capacity values from the original design values based on consideration of the conservatism often included in new design estimates. Information presented in the literature review indicated 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 (2014). Two case example agencies mentioned that consulting engineers are generally reluctant to design foundation reuse projects because of legal risks. In contrast, one case example agency reported no hesitation on behalf of 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 unclear; however, it appears likely that the uncertain standard of care makes at least some consulting engineers hesitant to design reuse projects. Two-thirds of survey respondents reported that they did not know how their agencies evalu- ated remaining service life, and short answer responses from the other one-third suggested that remaining service life is typically considered implicitly in reuse decisions rather than explicitly evaluated. This is consistent with the literature review findings, which indicated that specific information regarding how to predict foundation service life is limited. An exception is AASHTO Standard R 27-01 (2015), which includes a method for predicting service life for steel piles in nonmarine environments; however, the method requires observations of pile condition and rate of corrosion. Scour complicates foundation reuse. Case example agencies reported that they were less likely to reuse foundations for bridges over water compared with land crossings because of hydraulic issues, including scour risk. The survey revealed that approximately half of the agencies will consider foundation reuse when scour countermeasures are required, and half will not. Agency experience with post-reuse monitoring of foundation systems is limited. Approximately half of the agencies reported always or sometimes monitoring new or reused foundations, and most of that monitoring was limited to observations required by bridge inspection programs. Four agen- cies reported having collected survey or other deflection data for reused foundations. ConClusions Compared with a new foundation project, the scope of site investigation for a foundation reuse project is fundamentally different, equally important, and in many ways more challenging. For a new foundation project, the primary objective of the site investigation is to identify and characterize subsurface materials. For a reuse project, the site investigation seeks not only subsurface material information but also information about the existing foundation, including the type, geometry, condi- tion, and load capacity. The finding that practices for investigating existing foundations vary widely among agencies and within agencies by project is likely related to the finding that only three agencies have detailed poli- cies regarding foundation reuse. On one hand, it is possible the lack of foundation reuse guidance prevents reuse of some foundations that would be strong candidates for reuse, and also contributes to

72 the uncertain standard of care for consultants designing reuse projects. On the other hand, the vari- able nature of reuse projects suggests that guidance that is overly restrictive would likely discourage reuse. The three detailed guidance documents that were reviewed for this Synthesis all focused on recommended rather than required practices. Agencies generally appear more concerned with load capacity of existing foundations than with condition assessment. The difference in attention to load capacity compared with condition assessment can be observed in documented project examples as well as in agency policies regarding founda- tion reuse. Agencies evaluate the load capacity of existing foundations frequently, explicitly, and quantitatively 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 impedes consideration of remaining service life. The need to evaluate above- and below-ground portions of the substructure concurrently calls for close collaboration among many agency personnel, 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. suggestions for future researCh The results of this Synthesis demonstrate that foundation reuse offers clear, primarily economic benefits, but also societal and environmental benefits, for transportation agencies responsible for large bridge inventories that have outlived their intended service life. The results also disclosed that many agencies are hesitant to reuse foundations because of the significant challenges associ- ated with foundation reuse, which are primarily associated with condition assessment and load capacity. Research efforts could help reduce agency hesitations and advance the state of founda- tion reuse in the United States. Specific research ideas are presented here: • Methods for investigating existing foundations: This Synthesis found that agency practices for investigating existing foundations vary widely, not only by agency but also within agencies on a project-by-project basis. Research evaluating the numerous investigation techniques discussed in this synthesis in the context of various project-level constraints would help engineers design the scope of investigations for existing foundations. The objective of the research would be to identify 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: Advances in geophysical and electrochemical methods will proceed with or without founda- tion reuse. At a minimum, research similar to the first research idea is needed to capture how ongoing developments in geophysical methods can be used to improve existing foundation investigations. A more proactive approach would involve tailoring developments in geophysi- cal methods to meet the investigation needs related to foundation reuse. There are many poten- tial research projects that fall into this category. In general, the objectives of the geophysics research projects would involve improving the accuracy and precision of geophysical methods for identifying foundation characteristics and condition and/or improving the applicability of the geophysical methods so that they can be used for a larger set of foundation types. An example of improving applicability would be research to improve surface methods so they can be applied to small diameter deep foundation elements (e.g., driven piles) instead of just large diameter ele- ments (e.g., drilled shafts). • Methods for estimating the load capacity of existing foundations: Research to improve pre- dictions of load capacity of existing foundations is needed, particularly considering this was the primary foundation reuse concern for most case example agencies. The research is also needed because the most common method of load capacity prediction is to use the original

73 design value. The objective of the proposed research would be to improve load capacity pre- dictions 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 subsur- face 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. – Unload tests: Demolition of old structures above foundations to be reused is an opportu- nity to collect valuable performance data without the need for costly loading and reaction systems. The work by Bell et al. (2013) from chapter two shows that the method is viable. Research to evaluate the concept for bridge foundations could provide another tool for engineers to estimate load capacity of existing foundations. Methods for performing more conventional load tests of existing foundations are likely worthy of research effort as well. The RuFUS methodology for identifying capacity improvements (“reserves”) is a useful framework for the proposed load capacity research. If the proposed research considers the new information sources listed previously in the context of the RuFUS methodology, load capacity predictions would likely be increased from the original design value. Finally, consideration of foundation retrofitting techniques (e.g., additional foundation elements) in the load capacity prediction research is needed for reuse applications where existing foundation capacity is insuf- ficient. Consideration of foundation retrofitting improvements from the perspective of strain compatibility would be valuable. • Reliability aspects of foundation reuse: The Synthesis identified consideration of foundation reuse in the context of design codes as a major challenge for foundation reuse. Consideration of reuse of existing foundations from a reliability-based design perspective is difficult because the sources of uncertainty for an existing foundation are different from a new foundation. Research is needed to develop reliability-based design methods for foundation reuse that consider uncer- tainties in existing foundation condition and load capacity. Ideally, the reliability-based design methods would include resistance factors that would be consistent and compatible with current LRFD methods for bridge design. The reliability research idea is closely tied with the research idea presented earlier regarding methods for predicting load capacity. • Use of innovative materials and technologies to facilitate foundation reuse: Developments in bridge construction technology could help facilitate foundation reuse. For instance, use of lightweight high-performance concrete can result in replacement superstructures that are lighter than their original counterparts, which reduces the demand for foundation load capac- ity. Similarly, developments in ground improvement technologies can be used to increase the load capacity of existing foundations. 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: The Synthesis found that most agencies are not explicitly considering remaining service life, likely because methods for doing so are lim- ited. Foundation service life is a topic of interest beyond just reuse applications, and prelimi- nary research into the topic is currently underway. The research proposed here would develop methods to predict remaining service life of existing foundations by adapting emerging meth- ods from ongoing research efforts related to condition assessment, performance monitoring (including monitoring for corrosion), and prediction of foundation service life. The proposed research would also evaluate innovative technologies for improving the collection of founda- tion 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. • Decision methods for foundation reuse: Research to develop a preliminary decision methodology for foundation reuse would help agencies evaluate reuse in early stages of bridge rehabilitation

74 projects. The work of Laefer and Farrell (2015) presented in chapter two is a useful framework because it considers both technical and socio-economic factors. The proposed research would adapt the methods presented in chapter two for transportation applications. The adapted methods could be calibrated using reuse project examples from U.S. bridge 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.