Foundation Characterization Program (FCP)
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.
Purpose of Foundation Characterization Program (FCP)
Foundation characterization is essential in three engineering applications:
- Geotechnical and Hydraulic Hazards
- Hydraulics Vulnerability – Scour evaluation for the population of bridges with unknown foundations.
- Post-hazard (Extreme Events) Assessment
- Post-seismic Assessment – Foundation depth and integrity evaluation of bridges.
- Post-flooding/Hurricane Assessment – Evaluation of foundations after flooding and high-flow events that can cause large lateral and hydrodynamic forces.
- Changes in Service Loads
- Foundation Reuse – Reuse of foundations for bridge replacement, widening, or rehabilitation.
- Truck Size and Weight (TS&W) – Use of heavier truck loads and increase in frequency of trucks.
- Heavy Industrial/Mining/Military Loads – Use of heavier loads and frequency.
- Foundation Condition Assessment
- Assess environmental and age-related degradation, such as decay of timber piles, corrosion of steel piles, and degradation of concrete piles.
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:
- Foundation type (shallow versus deep, or a combination).
- Elevation of the base of the foundation (e.g. pile tip elevation).
- Foundation geometry (width, length, and depth), which includes checking the accuracy of as-built plans.
- Material type (foundation materials, including concrete, masonry, stone, steel, or timber).
- Integrity (foundation condition, such as corroded steel, rotted wood, or cracked or defective concrete).
- Foundation load-bearing capacity.
- Pile distribution – Target avoidance for foundation rehabilitation or underpinning.
- Remaining service life of the foundation.
- Subsurface characterization and imaging of soil and rock.
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:
- Formation of an Expert Task Group.
- Perform extensive literature review.
- Provide stakeholder outreach to refine issues and to research data gaps and needs.
- Conduct special projects to support this task as defined by FHWA.
TASK 2. Analytical and Developmental Research Program
The Analytical and Development Research program includes the following subtasks:
- Technology Evaluation – Evaluation of new or existing methodologies, including:
- Ground imaging and characterizations (such as geophysical methods).
- Foundation testing and evaluation (such as destructive, nondestructive, and wireline logging).
- Geotechnical site investigation.
- Advanced methodologies from other fields.
- Technology Development – Development of system prototypes for field testing and instrumentations.
- Load Testing – Investigation of new ideas for static and dynamic load testing of existing bridge foundations, including rapid load testing methodologies.
- Numerical Modeling/Analysis (two-dimensional (2D) and three-dimensional (3D)) – Development of numerical simulations to model complex soil-structure interaction and to calibrate field test results.
- Risk-based Analysis and Statistical Methods.
TASK 3. Experimental Program
A three-tiered field testing program will be conducted for the evaluation of technology:
- Inservice Bridges – Use of a selective group of inservice bridges mainly for the determination of foundation type, geometry, material, load, and integrity.
- Decommissioned Bridges – Use of decommissioned bridges mainly for the development of new load testing methodologies for existing bridge foundations.
- Small Testbed with Engineered Defects – Used mainly for the validation of technology under controlled conditions.
TASK 4. Deliverables – Disseminate the Research Findings
- Develop guidance and synthesis documents.
- Federal Highway Administration publications, such as TechBriefs, TechNotes, and research reports.
- Journal articles.
- Web-based documents.
TASK 5. Outreach Program
- Create and maintain a project Web site.
- Outreach to communicate research findings at workshops, panel discussions, conferences, etc.
- Organize training and webinars in cooperation with FHWA headquarters, resource center, and field offices.
Progress To Date
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:
- Technology Evaluation: The focus of this subtask was to perform evaluation of existing technologies for unknown foundations, foundation reuse, and condition assessment applications. Evaluations have been performed at three field sites: Lake Mary Bridge site in Arizona, the 11th street viaduct in New York City, and at a National Geotechnical Experimentation Site (NGES) near the campus of Texas A&M University. The report on these evaluations is undergoing final preparation.
- Technology Development: The focus of this task was on the development of new technologies for the characterization of bridge foundations. Based on the findings of task 1 and subtask 2a, we focused on the development of two technologies: (1) total waveform inversion of ultra-seismic field data and (2) a robotic crawler for foundation investigation.
- Load Testing: The focus of this task was to develop an approach for foundation characterization that could be used for the FHWA Best Practice Technical Manual for foundation reuse. The work on this approach has been completed and will be included as a chapter in the manual as well as a technical paper in the American Society of Civil Engineers (ASCE) Journal of Bridge Engineering.
- Numerical Modeling/Analysis: The focus of this task was to develop a detailed numerical approach to analyze effects of foundation movements on the behavior and load capacity of bridges. A technical journal article on this work has been completed and is being submitted to the ASCE Journal of Bridge Engineering.
- Risk-based Analysis and Statistical Methods: The focus of this task was to develop a risk-based approach for the reuse of bridge foundations. Based on the needs and gaps study on foundation characterization, the research team expanded this task to develop an FHWA best practice technical manual on the reuse of bridge foundations. This manual covers all issues identified in the National Cooperative Highway Research Program (NCHRP) Synthesis Report on foundation reuse that was published in January 2017.
Task 3: This task consists of the following subtasks:
- Inservice Bridge Sites: As a part of this task, detailed investigation on foundation reuse was conducted at the Lake Mary Bridge Site in Arizona and 11th street viaduct in New York City. The research team also carried out indepth investigations of more than 10 bridge sites where foundation reuse has been considered.
- Decommissioned Bridge Sites: This task was integrated with task 3c as a decommissioned engineering test site.
- Small Testbed with Engineered Defects: We have carried out extensive investigation of drilled shafts in the NGES Test site near the campus of Texas A&M. This site is a decommissioned engineering test site and has nine drilled shafts with engineered defects. The focus of the work on this test site was on drilling coreholes in nine drilled shafts and then performing wireline logging, surface geophysical, and ultra-seismic measurements at these drilled shafts.
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.