U.S. Department of Transportation
Federal Highway Administration
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Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
Bridge owners face the potential problem of not being able to characterize the foundations of bridges over dry land and waterways. This information is critical in their decisionmaking process for determining whether they can rely on the existing foundations to continue to carry increasingly heavier loads; and/or to withstand geotechnical and hydraulic hazards for an additional 25, 50, 75, or 100 years of service as they consider a major bridge rehabilitation, replacement, reuse, or widening of a bridge.
In 2013, the Federal Highway Administration (FHWA) initiated the development of a multiyear strategic research program to address the “unknown foundation” problem. The unknown foundation has been associated with the population of existing bridges over waterways that cannot be evaluated against the hydraulic vulnerability related to scour due to missing bridge plans. However, there are other engineering risks associated with unknown foundations besides scour. As such, the scope of the research program was broadened from “unknown foundation” to “foundation characterization”, to include bridge foundations over land as well as waterways and incorporate geo-hydraulic hazard issues, changes in service loads, and foundation reuse.
Foundation characterization is essential in three engineering applications:
The objective of the FHWA’s FCP is to develop and evaluate new and existing technologies and methodologies for characterizing existing bridge foundations to determine unknown geometry, material properties, integrity, and load-carrying capacity. The emphasis is on developing guidance and best practices for foundation reuse. Many foundation characterization decisions will likely be made in the framework of life-cycle cost and risk-management analysis.
The characterization of bridge foundations is complex. Bridges can be supported by shallow or deep foundations of varying footing geometries and materials. Footings can be square, circular, or rectangular. Foundations can be pedestal masonry stone footings or massive cofferdam footings. Piles can be present with or without pile caps and may be battered or vertical. Piles can be made of concrete (round, square, cylinder, or octagonal), steel (H-piles or round pipe sections), or timber. Deep foundations can be precast concrete piles, drilled shafts, or auger-cast concrete piles. The top of footings or pile caps can be buried underneath riprap, backfill mud, or channel soils. Additionally, design or as-built drawings of foundations are sometimes not available and technologies for determining foundation condition are expensive and sometimes unreliable. The goal of the FHWA is to identify methods to quantify and clarify these unknowns.
The FCP attempts to determine one or more of the following:
Figure 1 illustrates the complexity in evaluating foundation condition as well as a multitude of site variables—such as differing soil/rock material, level saturation, and existence of scour countermeasures—that need to be considered in a typical field investigation.
|Figure 1. Diagram. Typical foundation conditions.|
|Used with permission. © Transportation Research Board from Olson, L.D, Jalinoos, F., and Aouad, M.F. (1998). Determination of Unknown Subsurface Bridge Foundations, (NCHRP 21-5 Interim Report Summary). Geotechnical Engineering Notebook Issuance GT-16. Federal Highway Administration, Washington, DC.|
The outcome of this research program will be improved characterization of existing bridge foundations through evaluation and development of technologies and guidance documents. This will provide bridge owners with more accurate data when assessing condition, load capacity, and making decisions to improve bridge safety and reliability.
The program will be executed through 5 main tasks and 23 subtasks:
TASK 1. Defining the Foundation Performance Issues
The main issues that requires foundation characterization include:
TASK 2. Analytical and Developmental Research Program
The Analytical and Development Research program includes the following subtasks:
TASK 3. Experimental Program
A three-tiered field testing program will be conducted for the evaluation of technology:
TASK 4. Deliverables – Disseminate the Research Findings
TASK 5. Outreach Program
The main objectives of FCP are the characterization of bridge foundations using field testing methods and the development of approaches for the load rating of bridge foundations. As of May 15, 2017, the following tasks progress has been made:
Task 1: The focus of this task was to perform data needs and gaps analysis on the topic of foundation characterization which is completed.
Task 2: This task consists of the following subtasks:
Task 3: This task consists of the following subtasks:
Task 4: Disseminate the Research Findings: We have been disseminating research findings through sessions at Transportation Research Board (TRB) Annual Meeting, Structures Congress, seminars, and journal and conference publications.
Task 5: Outreach Program: We have undertaken several outreach activities including organizing workshops/sessions at TRB 2015, 2016, and 2017 and organizing many technical sessions at engineering conferences.
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