Characterization of Bridge Foundations Workshop Report

SUMMARY OF THE SECOND PLENARY SESSION: GEOTECHNICAL AND HYDRAULIC HAZARDS AND IMPACTS

INTRODUCTION TO THE PLENARY SESSION

Session 2 of the workshop consisted of presentations on the hazards and impacts of geotechnical and hydraulic features from the perspective of FHWA and State transportation department personnel. The session was moderated by Dr. Phil Yen, Principal Bridge Engineer at the FHWA Office of Bridge Technology, who began the session with a discussion of the importance of seismic hazards by posing the question: What is the situation after an earthquake? Dr. Yen emphasized the need to quickly estimate the capacity and integrity after an extreme event, with post-hazard evaluation being a key issue.

NATIONAL BRIDGE INSPECTION PROGRAM: THE UNKNOWN FOUNDATION AND HYDRAULIC SCOUR QUESTION

Mr. Dave Henderson, Senior Bridge Engineer (Scour) at the FHWA Office of Bridge Technology, provided an overview of the national bridge inspection program. He began the presentation with a graphic showing the relationship between foundation characterization, unknown foundations, and hydraulic scour (see figure 7), then asked the question: How does it all fit?

He described the components of the national bridge inspection program and the FHWA scour program. He explained that the national bridge inspection program has three fundamental components: the NBI, the National Bridge Inspection Standards (NBIS), and the National Bridge Inspection Program (NBIP). The inventory, standards, and program provide the database of information available for over 600,000 bridges nationwide. The performance of bridges is measured by 23 metrics.⁽⁹⁾ Metric 18 measures scour.

The FHWA Scour Program consists of the following key elements: scour evaluations, scour critical bridges, unknown foundation bridges, plan of action bridges, and scour countermeasures. In 2011, FHWA implemented the risk-based and data-driven NBIP oversight process. The risk-based component provides a strategy of prioritizing vulnerable bridges based on bridge importance and consequences of failure. The data-driven component provides the key operational characteristics of the facility. These two strategies were further elaborated as they relate to unknown foundation elements. Mr. Henderson offered the following three important takeaways:

1. The bridge owner must develop prioritization and decision making strategy, which is consistently applied and easily replicated.
2. Low risk bridges Coded "U" (unknown foundation) in Item 113 can be "low hanging fruit" and owner´s resources may be focused on bridges with highest risk.
3. The term "Unknown Foundation" for bridge owners is a performance measurement of compliance with NBIP.

Figure 7. Diagram. Interrelationship of Characterization, Unknown Foundations and Hydraulic Scour.

UNKNOWN FOUNDATION INVESTIGATION PROGRAM IN NORTH CAROLINA

Mr. Mohammed Mulla, Assistant State Geotechnical Engineer at the North Carolina Department of Transportation (NCDOT), provided an overview of the unknown bridge foundation program in North Carolina. Early efforts focused on records searches and field testing to identify the foundation type, with estimates of minimum pile embedment or footing size and depth, and an evaluation of the foundation with respect to scour using soundings. By 2005, a rigorous unknown foundation process had been developed (see figure 8). Mr. Mulla detailed the process used for unknown foundations in the bridge management system, including the sorting of microfilms. The non-destructive testing (NDT) was conducted by consultants and in-house staff. The testing procedures were reviewed in detail, with examples shown of their use on bridges and foundations in the State. In 2010, the use of risk-based management guidelines for scour was suggested to evaluate remaining unknown foundation low risk bridges.⁽¹⁰⁾ By November 2012, review of all unknown foundation bridges had been completed. Mr. Mulla concluded by asking a series of questions regarding NDT and asset management, with an exhortation to think outside the box and communicate.

Figure 8. Diagram. Flowchart for North Carolina Unknown Bridge Foundation Process.⁽¹¹⁾

FLORIDA DEPARTMENT OF TRANSPORTATION’S APPROACH TO RESOLVE UNKNOWN FOUNDATIONS

Mr. Larry Jones, Assistant State Structures Design Engineer and State Geotechnical Engineer at the Florida Department of Transportation (FDOT), provided an overview of FDOT´s unknown foundation program. The FDOT unknown foundations process is summarized in figure 9. Based on 2010 statistics, Florida has determined that the majority of unknown foundation bridges are on local roads, with only nine percent on principal arterials. FDOT has developed an assessment plan to sequence the effort into phases. The unknown foundations process involves data gathering, risk assessment, embedment prediction, and Phases 2 through 4 scour evaluations. The National Cooperative Highway Research Program (NCHRP) Web Only Document 107 procedure is followed with some modifications for Florida costs, failure rates and tidal bridges.⁽⁷⁾ Risk thresholds are based on lifetime risks tied to specific dollar amounts. Embedment predictions are based on artificial neural network or geotechnical analysis methods. The results of the Florida processes were detailed with comparisons of predicted versus measured embedment depths by the various methods. An extensive summary for the statistics of Florida’s program was presented. Of some 2,500 bridges, only 160 were determined to be scour-critical, with about 400 not reported. Mr. Jones closed by characterizing some issues for thought regarding MSE walls and service limit versus strength limit states, particularly that proof loading of unknown foundations addresses the service limit state, but not the strength limit state.

Figure 9. Diagram. FDOT´s Unknown Foundation Process.⁽¹²⁾

WASHINGTON DEPARTMENT OF TRANSPORTATION FOUNDATION EVALUATIONS FOR GEO/HYDRAULIC HAZARDS AND DESIGN PURPOSES

Mr. Jim Cuthbertson, Chief Foundation Engineer at the Washington State Department of Transportation (WSDOT), provided an overview of WSDOT´s history of emergency bridge issues over the past century. Seventy bridges (out of 3,500 State bridges) have been damaged beyond repair or collapsed in that time, for a 2.0-percent failure rate, 43 of which did so under flood conditions (1.23 percent) and only 2 with unknown reasons (0.06 percent).⁽¹³⁾ Thus, of those that failed, slightly more than 60 percent did so under flood conditions. Earthquakes and landslides have not yet caused collapse or complete replacement. Except for flood/scour causes, foundation issues have not been a primary cause of structure failure or replacement.

Mr. Cuthbertson showed a slide on Geotech Emergency Response that highlighted the key issues to be addressed under time and money constraints (see table 4). Foundation evaluation procedures for scour, flood, and seismic causes were presented. Earthquakes have mainly caused structural damage and the primary response thus far has been by the Structures Preservation Unit. Mr. Cuthbertson closed with thoughts on reuse of foundations in widening efforts.

Table 4. Geotechnical Emergency Response (Regardless of the Event).⁽¹³⁾

EVALUATION OF EXISTING FOUNDATIONS WITH NON-DESTRUCTIVE METHODS

Mr. Khamis Haramy, Senior Geotechnical Engineer, FHWA Central Federal Lands, provided a brief overview of existing NDE methods used for foundation characterization and foundation material integrity evaluation. At the outset, Mr. Haramy stipulated that the objective of nondestructive evaluation of bridge foundations was twofold: (1) determine unknown bridge foundation characteristics for scour vulnerability concerns, and (2) assess conditions and integrity of unknown and known bridge foundations for increasing bridge structure design life and foundation reuse. A brief description of the existing NDE methods, their applicability and limitations was provided. Mr. Haramy indicated that the FHWA manual "Application of Geophysical Methods for Highway Related Problems" and the associated searchable, web-based e-manual contain a summary of the methods and their limitations. Mr. Haramy demonstrated the use of the e-manual for determining the most reliable methods for a certain application (figure 10). He indicated that, in his opinion, these methods provide a useful way to characterize bridge foundations; however, a combination of methods may be required to best characterize some sites. He also indicated that advanced technologies used in medicine and oil exploration—3D full waveform tomography—may significantly improve foundation characterization if adapted by the transportation field. He recommended that funds be allocated for the development of advanced methods and by utilizing newly developed algorithms for improving image clarity.

Figure 10. Picture. Screen capture of Geophysical "Webmanual."⁽¹⁴⁾