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Publication Number:  FHWA-HRT-13-101    Date:  November 2013
Publication Number: FHWA-HRT-13-101
Date: November 2013

 

Characterization of Bridge Foundations Workshop Report

APPENDIX C—WHITE PAPER ON THE UNKNOWN FOUNDATION PROBLEM

By: FRANK JALINOOS

Federal Highway Administration, Office of Infrastructure R&D

frank.jalinoos@dot.gov

SUMMARY

Bridges with unknown foundations potentially pose a significant scour safety problem to the bridge owners. Presented herein is a summary of the unknown foundation problem relating to the hydraulics vulnerability for bridge scour, discussion of other engineering risks with this population of bridges, and the new Federal Highway Administration (FHWA) research initiative to address this national problem.

BACKGROUND

Following the catastrophic collapse of the Schoharie Creek Bridge on the New York State Thruway in April 1987, national attention has been focused on the bridge scour problem. Foundation characteristics are needed for accurate scour analysis—which was nonexistent for the Schoharie Creek Bridge. Therefore, as a result of addressing scour vulnerability of bridges, the unknown foundation also became a national priority.

The loss of support from scour damage can result in increased movement and deformations, which can lead to subsequent failure of the entire structure. To determine the susceptibility of a bridge to scour, information on the foundation type and depth is needed, along with the hydraulic conditions at the site, to perform an accurate scour evaluation of each bridge. To mitigate scouring problems, hydraulics engineers consider numerous scour countermeasure designs. The aim is to control channel instability and to mitigate scour at foundations of abutments and piers. Proper scour prediction is essential for safe design of bridges over rivers, streams, and in coastal areas, both on the National Highway System (NHS) and the non-NHS (NNHS) bridges.

As of December 2012, the National Bridge Inventory (NBI) included 607,380 structures (bridges and culverts) with a span greater than 20 ft (6 m).(1) Of those structures, 504,893 (83 percent) have the service under the bridge coded as waterway, or a combination that includes a waterway. 36,076 bridges over waterways (riverine and tidal) in the NBI database were identified as having unknown foundation characteristics.

The number of bridges in the NBI database that are coded “U” under Item 113 (for “unknown foundations”) have been reduced considerably over the years. In 1996, 104,000 were quoted in the NCHRP Research Results Digest 213, in 2001, 89,000 in the FHWA HEC No. 18, in 2005, over 80,000 bridges were quoted at the Denver “Unknown foundation Summit,” and in 2007, the NBI database identified 67,002 bridges. (See references 3 and 20–22.) The main reasons for this reduction can be attributed to efforts by State departments of transportation in finding the lost plans, conducting field evaluations, and performing risk-based assessments such as that recommended in NCHRP Document 107 or the recent 2009 FHWA memorandum, as described next.(10)

FHWA GUIDANCE

The National Bridge Inspection Standards (NBIS) regulation 23 CFR 650.313 requires that bridge owners identify bridges that are scour critical (coded 0, 1, 2, or 3 in Item 113) and to prepare a Plan of Action (POA) to monitor known and potential deficiencies. Bridges coded “U” for Item 113 represent a unique subset of bridges that were exempted from being evaluated for scour vulnerability due to the lack of a process and guidance that would have allowed owners to determine the necessary foundation characteristics.

On October 23, 2009, the FHWA issued series of memoranda providing technical guidance for conducting scour evaluation of bridges over waterways with unknown foundations.(4) The FHWA provided example guidance for conducting risk-based assessment noting that other methods exist and the guidance is meant to support ongoing efforts and not to supersede them. FHWA also provided guidance on developing a POA for bridges that will remain coded “U” in Item 113.

Using the example provided in the guidance, FHWA recommended grouping bridges into categories corresponding to risk. For those bridges categorized as having the highest risk, it is recommended that owners establish a means for positively identifying the foundation type, location and depth such that a scour evaluation can be conducted. Positive discovery may include field testing, drilling of borings with field testing, or the use of test pits.

Referencing the example, FHWA recommends high-risk bridges that lack as-built plans to undergo positive discovery of foundation characteristics. For moderate risk bridges, FHWA recommends inferring or assuming necessary foundation characteristics. However, if the degree of confidence for inference or assumption is not high enough, the owner can choose between positive discovery of the necessary foundation characteristics or develop and implement an appropriate POA. For low risk bridges, a risk-based inference method is allowed. However, if the degree of confidence for inference or assumption is not high enough to warrant recoding, a POA would need to be developed and implemented by the owner.

PROBLEM SCOPE

The nature of the problem with unknown bridge foundation is complex. Bridges can be supported by shallow (spread footings) or deep foundations. Footings can be square, circular, or rectangular in shape. They may also be pedestal masonry stone footings or massive cofferdam footings. Piles may be present with or without pile caps and may be battered or vertical. Piles can be made of concrete (round, square, or octagonal), steel (H-piles or round pipe sections), or timber. Deep foundations can be pre-cast concrete piles, drilled shafts, and auger-cast concrete piles. The top of footings or pile caps may be buried underneath riprap, backfill mud and/or channel soils.

The NCHRP Project 21-5 report stated that bridges with unknown foundation are considered to lack the following information:(20)

In this report, we expand this definition to include other concerns with a bridge foundation—known or unknown—including the following concerns:

For scour evaluation, the base of the foundation elevation and the foundation type are considered to be the two most critical items. For other types of evaluations, foundation integrity and bearing capacity can be of prime importance as discussed in the next section.

ENGINEERING RISK WITH UNKNOWN FOUNDATION

As discussed, so far the unknown foundation has been associated only with the population of existing bridges over waterways that cannot be evaluated against the hydraulic vulnerability related to scour. However, many bridges built over land are also expected to have unknown foundations and there are other engineering risks beyond scour that should be evaluated. Many practitioners report that substantial projects involving unknown foundations now involve more than just scour studies and are beginning to focus on structural and geotechnical assessments.

The risk with unknown foundation can be divided into three categories, as described below:

  1. Geo/Hydraulic Hazards.
    • Hydraulics/Seismic Vulnerability—Scour evaluation, thus far the primary driver.
    • Post-Hazard (Extreme Events) Assessment.
      • Post-Seismic—Post-earthquake assessment of bridges with unknown foundation is a concern as foundation depth and integrity is very important for bridges with visible indication of superstructure damage or movement.
      • Post-Flooding /Hurricane Assessment—Evaluate foundation events after flooding and high water events that can cause large lateral forces.
  2. Changes in Service-Loads—Currently, there is a lack of guidance for load rating of bridges with unknown foundations. Engineering risk of unknown foundations involving changes in service loads includes:
    • Foundation Reuse—Reuse of foundations for bridge replacement, widening, or rehabilitation projects.
    • Truck Size and Weight (TS&W)—Proposed use of higher truck loads by the trucking industries and changes in truck routes.
    • Heavy Industrial/Mining/Military Loads—Use of the highway infrastructure.
  3. Foundation Condition Assessment—Age-related decay such as visible degradation or rotted and broken timber piles.

More detail is provided in the next section regarding some of the engineering concerns listed above.

SCOUR VULNERABILITY (PRINCIPAL DRIVER)

As previously discussed, the unknown foundation is considered principally a scour vulnerability issue. Therefore, the term unknown foundation has been traditionally associated only with the population of existing bridges over waterways that cannot be evaluated against the hydraulic hazards related to scour.

At the present, the NBI database does not track bridges built over land with unknown foundations, as this is considered as low risk issue. However, many of these bridges built over land are expected to have unknown foundations and many others lack complete documentation of their as-constructed details (e.g., material properties, continuity and reinforcing details of structural elements) that are needed for engineering analysis, such as load rating and other considerations, as described next.

POST-SEISMIC (EXTREME-EVENT) ASSESSMENT

A survey of available statistics reveals that more than one-third of U.S. highway bridges may be vulnerable to damage and/or failure due to earthquakes.(23) Some of these structures are very old and typically possess non-ductile structural details. Many have either unknown foundations or incorporate lightly reinforced concrete substructures. Bridge foundations, columns, and pier caps are critical when addressing seismic loading because earthquake forces are generated from ground up. Preventing bridge collapse is often accomplished by averting unseating of the superstructure or shear failure of the columns. Foundation integrity is an issue when an earthquake triggers geotechnical hazard such as liquefaction and settlement, slope failure, surface fault rupture, and flooding, which can in damage to the foundations. Soil liquefaction—a loss of shear strength in loose, fully saturated cohesionless (sandy) soil—can cause a loss of bearing capacity resulting in foundation failure, settlement, or tilting of abutments and piers. Examples of foundation failure are shear failure of the pile cap, anchorage failure, pile pullout, and pile shear failure. Therefore, the knowledge of foundation type, geometry, and material are of concern to seismic engineers.

Post-earthquake integrity assessment of substructure elements, such as column, abutment and bearings is a concern to seismic engineers as well as reduced load carrying capacity of the foundation elements. Bridges with unknown foundation have other critical information missing, such as reinforcement detail and material properties.

Post-flooding/hurricane force of bridges with unknown foundation is a concern to hydraulics engineers as foundation depth and integrity is very important for bridges with visible indication of superstructure damage or movement.

Foundation characteristics are also needed to assess bridge vulnerabilities to other extreme event such as ship/barge/truck impact, as well as intentional or unintentional blast events.

CHANGES IN SERVICE-LOADS

As described below, several overload hazard events can result in catastrophic failure or collapse of bridges. In recent years, increased legal load limits on the Nation’s highway have resulted in questions concerning the load carrying capacity of bridges, especially older bridges. Bridge and maintenance engineers are increasingly faced with decisions on accommodating the significantly increased truck weights on their structures. Bridges with unknown foundations carry higher risk as these have unknown/uncertain structural (e.g., reinforcement) details and material properties vitally needed for objective structural evaluation (load rating) determinations.

The overall structural appraisal rating for a bridge is determined based primarily on the safe load carrying capacity. Currently, there is a lack of guidance for load rating of bridges with unknown foundations. The AASHTO “Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges” states that “bridges which cannot be load rated by computations because of insufficient information on their internal details and configuration need proof testing to determine a realistic live-load capacity. Bridges that are difficult to model analytically because of uncertainties associated with their construction and the effectiveness of repairs are also potential candidates and beneficiaries of proof load testing.”(24)Neither the AASHTO Manual, nor NCHRP reports—such as NCHRP Digest, “Manual for Bridge Rating through Load Testing”—offer any guidance for performing safe proof-load testing of a bridge with unknown characteristics.(25)

Concerns under this category include: (1) increases in service loads such as structural upgrade projects, such as bridge widening or lane increases, (2) proposed use of higher truck loads by the trucking industries and changes in truck routes, and (3) use of the highway infrastructure by heavy mining/military equipment.

Bridge Rehabilitation

Understanding the type and condition of existing foundation are essential for decisions associated with bridge rehabilitation or widening. Without a complete foundation evaluation, it is not possible to assure a rehabilitated bridge will perform adequately for any design life.

Use of Heavy Truck Loads

In the United States, half the shipped freight tonnage moves more than 100 mi (161 km), and 18 percent moves more than 500 mi (805 km).(26) Vehicles on Interstate highways must conform to the Federal Bridge Formula (FBF), which is designed to protect bridges from catastrophic overloads. FBF restricts vehicle’s axle groupings and vehicle weight to 80,000 lb. The proposals for TS&W liberalization are requesting a switch from the dominant heavy truck—5-axle tractor semitrailer—to trucks that have higher payloads and additional axles. These proposals are requesting the elimination of the FBF’s 80,000 lb cap on gross vehicle weight with minimal or no increase in the gross weight of a five-axle tractor semitrailer; but allowing vehicles with additional axles to operate substantially above 80,000 lb. For typical short-twin trailers, for example, the FBF allows 99,500 lb with 7 axles, 104,500 lb with 8 axles, and 110,000 lb with 9 axles.

In evaluating the effects of changes in TS&W limits on bridges, both overstress and fatigue should be considered. Overstress creates the possibility of severe damage and possible collapse caused by a single extreme loading event. Fatigue produces the cumulative damage caused by thousands to millions of load passages. For overstress, as the heavy truck traffic increases, there is a higher likelihood of a "critical" load event in which several heavy vehicles are on the bridge simultaneously.(27)

Use of Heavy Mining and Military Equipment

In the recent years, the U.S. Department of Defense has undergone mission changes that have involved relocating more than 123,000 military and civilian personnel.(28) One of the major initiatives includes the Base Realignment and Closure Act (BRAC) of 2005, which involved base realignment and closure. When personnel from closed bases relocate or commute to another base, the increased defense traffic increases burden on the State and local infrastructure.

The same consideration applies to the use of heavy mining equipment of the State and local infrastructure. As a recent example, in March 2003, the West Virginia legislature passed Senate Bill 583, which established the Coal Resource Transportation System (CRTS) in fifteen southern West Virginia counties.(29) On these designated routes, coal haulers may purchase a permit that will allow for a Gross Vehicle Weight (GVW) of up to120,000 lb (54,430 kg) which are much heavier than regular legal truck loads. The CRTS includes over 600 short to medium span bridge structures, with about 100 posted for live loads less than the allowable CRTS truck loads. Many of the typical reinforced concrete (RC) bridges structures on the CRTS were constructed in the 1920s. Several CRTS bridges have unknown foundation characteristics and many more lack critical information pertaining to material properties, continuity and reinforcing details of the superstructure and substructures, and soil properties critical for proper load testing.

An FHWA-sponsored research team initially selected several West Virginia bridges and conducted detailed inspection and material sampling followed by diagnostic level load tests. However, due to the extremely small levels of response captured, proof-level load tests had to be conducted in order to calibrate the analytical models and forecast capacity. Given the high expense associated with this type of bridge investigation involving unknown foundations (around $100,000 per bridge), the researchers recommended the development of rapid load testing systems.(30)

Bridge Replacement Involving Foundation Reuse

In recent years, the reuse of bridge foundation is being considered for a large number of bridge replacement projects. Foundation characteristics are needed to assess vulnerabilities to site conditions such as settlement or other types of movements. The ASCE Geo-Institute has held several panel discussions and technical session on the topic of foundation reuse. (See references 31–34.)

In urban settings, the concept of foundation reuse for buildings has been relatively well established.(35,36,37) In October 2006, the Reuse of Foundations for Urban Sites (RuFUS) research project published the best practice handbook, written by a cross-European team of foundation and structural engineers.(37) The handbook provides a sound understanding of the background to foundation reuse and provides advice on investigation, design, and construction issues. One of the big problems foundation engineers face when looking to reuse foundations is that as-built construction records are typically very poor.

THE NEED FOR A FEDERAL RESEARCH PROGRAM

The unknown foundations issue remains one of the most persistent problems facing the bridge engineering community. NCHRP Project 21-5 devoted considerable effort to developing new test methods to address this issue, and some good progress was reported.(20,38) However, there are still concerns on the reliability of the available technologies and associated costs, especially when they require the drilling of a borehole adjacent to the foundation.

In November 2005, FHWA organized the “Unknown Foundation Summit” at Denver, CO, to brainstorm this issue among key stakeholders.(3) After the Summit, FHWA created four internal teams to lead the following initiatives:

  1. Policy (Jorge E. Pagán-Ortiz, Lead).
  2. Technical Guidance (Ben Rivers, Lead).
  3. Research and Development (R&D) (Frank Jalinoos, Lead).
  4. Training (Jerry DiMaggio, Lead).

Later on in 2009, FHWA formed a new FHWA Unknown Foundation (UF) team, which resulted in the 2009 FHWA memorandum by the FHWA Bridge Office for reducing the bridges over waterways with unknown foundation population. However, no new effort has been devoted to the R&D needs and specifically research on “positive discovery” methods. (As of December 2012, 36,076 bridges in the NBI database are still identified as having unknown foundation.)(2)

More recently, SHRP 2 devised a research plan for developing technologies to deal with the most urgent requirements for highway facilities. The findings of this project identified unknown foundations as one of the main issues facing the geotechnical community and discussed a need for a national validation test site for unknown foundation research.(39) SHRP 2 estimated $1.5 million is needed for this tier 1 “unfulfilled need” research.(39)

Likewise, the LTBP has identified unknown foundation as a top tier bridge performance priority issue as a result of their March 2010 geotechnical/hydraulic workshop.(8) The program also identified unknown foundation in their assessment for data needs and data gaps related to the geotechnical performance issues.

The European RuFUS project partially funded a validation site for the German Federal Institute for Materials Research and Testing in support of foundation integrity studies.(40,41) This site is located at Horstwalde, 31 mi (50 km) south of Berlin. The test site includes ten small concrete drilled shafts with 24 inch (62 cm) diameter and lengths between 29.5 ft (9 m) and 39.4 ft (12 m) some containing engineered defects. The site consists mainly of sandy soil and the groundwater depth is about 13 ft (4 m). More recently, a pile secant wall was constructed along with a series of boreholes for subsoil investigations.

The U.S. geotechnical field has also successfully used the concept of building designated experimental test sites for the research community. In 1987, the FHWA´s Geotechnical Research Program teamed up with the National Science Foundation (NSF) to establish a system of National Geotechnical Experimentation Sites (NGES) devoted to geotechnical research.(42)

Five sites were selected as follows:

  1. Treasure Island Naval Station at San Francisco Bay, CA.
  2. Texas A&M University at College Station, TX.
  3. University of Massachusetts at Amherst, MA.
  4. Northwestern University at Evanston, IL.
  5. University of Houston at Houston, TX.

These sites are still being used for ongoing research by the geotechnical community. However, there is no experimental test site solely devoted to unknown foundation characteristics.

RECENT FHWA RESEARCH INITIATIVES

In 2013, FHWA proposed a new research program to address the unknown foundation problem. The objective of this research is to develop and/or evaluate new and existing methodologies for characterizing existing bridge foundations.

Initially, a 14-member task force was formed comprised of FHWA and State transportation department stakeholders. The members of this task force met during the January 2013 TRB annual meeting to brainstorm on the steps needed to move forward with a multi-year strategic plan for characterizing bridge foundations. The consensus was to broaden the scope of research from “unknown foundation” to “characterization of bridge foundations” to include multi-hazard concerns. It was also decided to move forward with a workshop to define the research data needs and gaps.

The report herein presents an overview of the “Characterization of Bridge Foundations Workshop” held in Arlington, VA, from April 30 to May 1, 2013. The cross-discipline workshop involved key staff from the FHWA hydraulics, geotechnical and structural disciplines brainstorming with stakeholders. The participants recommended a broad scope of research on foundation characterization with emphasis on foundation reuse, which encompasses all other risks relating to bridge foundations, known or unknown.

 

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