Reusing bridge foundations.
Some owners are building on existing elements when widening and replacing structures. Here is how this strategy can help expedite construction, minimize site impacts, and address sustainability needs.
Motorists cross bridges every day, sometimes multiple times a day, without giving it a thought, or even realizing that they are crossing a bridge, especially when it spans dry land. Bridges are such an integral part of today's transportation network that it is easy to take them for granted. However, bridge owners must periodically assess these structures to ensure the safety of the traveling public, and to make decisions regarding the maintenance and operation of their bridges.
A critical component of bridge owners' decisionmaking is the ability to characterize the foundations of bridges over both dry land and waterways. They then can use this information as part of the process for determining whether they should rely on the existing foundations to continue to carry increasingly heavy loads and to be resilient to geotechnical and hydraulic hazards, or whether they should consider major rehabilitation, replacement, or widening of a bridge.
But the characterization of bridge foundations is complex. Bridges may be supported by shallow or deep foundations of varying geometries and materials. The foundation elements range in size, shape, material, and construction. In addition, design or as-built drawings of foundations are not always available, resulting in a population of bridges with missing plans (or "unknown foundation" bridges). Plus, technologies for determining foundation condition are expensive and sometimes unreliable. The Federal Highway Administration is researching methods to clarify and mitigate these issues.
In 2013, FHWA approved a new multiyear strategic research program for the characterization of bridge foundations. To narrow the focus of the program and to solicit input from key stakeholders, FHWA held a workshop in Arlington, VA, from April 30 to May 1, 2013. The workshop identified the engineering problems associated with characterizing foundations, including assessing foundation type, pile type, embedment depth, geometry and material, foundation integrity, and load-carrying capacity. The workshop also focused on changes related to structural loading demands and foundation reuse issues from the perspective of FHWA and State departments of transportation. For more information, download the Characterization of Bridge Foundations Workshop Report (FHWA-HRT-13-101) at www .fhwa.dot.gov/publications/research /infrastructure/structures/bridge /13101/13101.pdf.
FHWA used the knowledge garnered from the workshop to establish the roadmap of its new Foundation Characterization Research Program. This research program's objectives are to develop and evaluate new and existing methodologies for characterizing bridge foundations to determine unknown geometry, material properties, integrity, and load-carrying capacity.
Engineering Application Areas
The Foundation Characterization Research Program supports identified needs in three engineering application areas: geotechnical and hydraulic hazards, changes in current loads, and foundation condition assessments.
In the area of geotechnical and hydraulic hazards, a foundation's vulnerability to scour and seismic events are of particular concern for the population of bridges with unknown foundations. Another concern is the post-event assessment, particularly evaluation of foundations after flooding, hurricane, seismic, or impact forces.
The Foundation Characterization Research Program is also looking into the impacts that current loadings have on foundations. Over time, changes in design code and regional and local needs can result in increased loading conditions that are different from the original design intent. This is particularly important when bridge owners are determining whether to reuse existing foundations in bridge replacement, widening, or rehabilitation projects. Further, characterization is imperative for making decisions about whether oversized, particularly overweight and routinely permitted, truck loads can cross bridges safely.
The third area of research under this program, foundation condition assessments, addresses the concerns related to degradation, decay, and long-term or inservice performance of substructure materials. Substructure materials can include timber, masonry, steel, and concrete.
Of all the engineering application areas, foundation reuse is of particular interest to State DOTs and bridge owners. Reuse of bridge foundations often is needed when the replacement, rehabilitation, or modification of the existing bridge superstructure is required.
In support of recommendations from the foundations workshop in 2013, several follow-on workshops and sessions on the topic of foundation reuse were held at the Transportation Research Board's annual meetings in 2014 and 2015. More related sessions are planned for the TRB 2016 meeting.
"Given the high inventory of deficient bridges and budget deficit, reusing existing bridge foundations would save considerable costs and shorten delivery times significantly," says Mohammed Mulla, geotechnical engineer with the North Carolina Department of Transportation and chair of the TRB Soil and Rock Properties Committee, a key sponsor of the TRB sessions on foundation reuse.
Reuse applications include replacement of the superstructure of an existing bridge because of structural deterioration, widening of an existing bridge, repurposing and reuse with strengthened foundations, and accelerated bridge construction (ABC) technologies, including prefabricated bridge elements and systems (PBES).
"The reuse of bridge foundations and substructures is also an important consideration for the ABC/ PBES community," says Ben Beerman, senior structural engineer with the FHWA Resource Center. "When compared to other alternatives, it can be an effective strategy to meet the objectives of accelerated construction in the areas of time savings, cost reductions, improved safety, and the need to minimize site impacts."
Beerman continues, "A unique and important outcome of this effort is to develop an AASHTO LRFD [American Association of State Highway and Transportation Officials Load and Resistance Factor Design] approach for foundation reuse. This is a paradigm shift [from] our current and past practices that will help advance our industry in a more unified and systematic manner."
The drivers for foundation reuse include asset management; reduced cost and time savings; efficiency; past performance (does not show any sign of excessive deterioration or movement); improved highway safety; reduced site, traffic, and environmental impacts; sustainability issues; and historic preservation. However, many challenges exist in the reuse of foundations. The primary concerns are the ability to accurately assess the integrity and capacity (structural and geotechnical) of existing foundations and the effects of their reuse when using current design codes. Determining the feasibility of foundation reuse requires multidisciplinary collaboration among staff with structural, hydraulic, geotechnical, and construction expertise.
Four options exist for new developments on existing bridge sites.
Build a new foundation adjacent to an existing one. This option is perhaps the simplest from an engineering perspective. However, this option can be costly and may have the largest impact on the site from the environmental and permitting perspectives. If a new alignment is required, it may entail impacts related to property and right-of-way acquisition, in addition to utility accommodations. In river crossings, additional scour analysis is required to account for flow interference of the existing bridge piers with the proposed new piers during and after construction. (See "Hydraulics Testing of [Woodrow] Wilson Bridge Designs" in the March/ April 2000 issue of PUBLIC ROADS.)
If the existing piers need to be removed, doing so requires additional time onsite. Often, pier foundations must be removed at or below the mudline to address the needs of partner permitting agencies.
Build a new in-place foundation. With this option, the existing foundation is demolished and replaced with a new one. When the new structure must be in the same place as the existing structure, removal and replacement of the existing foundation could be costly.
Reuse the existing foundation. This option requires no new foundation/substructure repair or rehabilitation work. The existing foundation is reused "as is" after a detailed assessment and analysis of the existing foundation's load-carrying capacity and condition to meet the service life of the design.
Reuse the existing foundation and strengthen and enhance the capacity. Examples of foundation enhancement include widening the pier stem; extending the pile cap and adding piles or shafts; soil improvement measures such as jet, compaction, or chemical grouting; and foundation underpinning using micropiles. The enhancement of the existing foundation reduces the risk associated with simply reusing the foundation "as is."
Foundation Reuse In Virginia
In one example, the Virginia Department of Transportation (VDOT) made the decision to reuse existing foundations for 10 bridges along the highly trafficked I-95 corridor near Richmond. The 4-year project began in September 2010. VDOT completed the project more than 3 months ahead of schedule and at a savings of nearly $16 million compared to fully replacing the bridges.
How did they do it? First, VDOT reduced the weight of the bridges by about 7 percent with the use of lightweight concrete for the new deck superstructures. The department analyzed the original foundations with this weight reduction and the proposed loading, and determined that the existing foundations were adequate to support the new superstructures with their reduced loads. Reusing the existing foundations saved the department considerable time and money.
When deciding whether to reuse the foundations, VDOT also evaluated the cost and remaining service life of reusing the repaired substructures with additional corrosion-protection countermeasures compared to the cost of entirely new substructures. The department found reuse to be the most cost-effective strategy.
In addition, VDOT did not want to disrupt traffic for the length of time that would have been required to replace the foundations. Because the existing foundations were adequate for the proposed loads, the department made the decision to reuse them after performing repairs and adding corrosion protection. The decision meant less disruption to traffic on the interstate. (For more on these projects, see "Keeping Virginia Moving" in the July/August 2015 issue of PUBLIC ROADS.)
Strengthening The Foundation: The Milton-Madison Bridge
When the aging Milton-Madison Bridge needed to be replaced, the Indiana Department of Transportation and the Kentucky Transportation Cabinet chose to reuse the existing foundations, while strengthening and enhancing their capacity. The agencies also employed innovative ABC/PBES techniques for the project, which broke ground in 2010, enabling them to shorten the delivery time considerably.
The Milton-Madison Bridge spans the Ohio River between Milton, KY, and Madison, IN. The project involved reusing and strengthening four of the five main piers, widening the pier stem, and replacing the superstructure using PBES. Project engineers evaluated soil-structure interaction for the existing foundations, which they ultimately strengthened by coring and adding supplemental reinforcement to the existing pneumatic caisson foundations (foundations built underwater). Considering the new design of the widened pier stem, hydraulics engineers performed extensive scour analysis, including design of scour countermeasures. The construction team then moved a preassembled steel truss superstructure from adjacent temporary piers 55 feet (17 meters) laterally onto the refurbished piers. This project is one of the world's largest river crossings to use the lateral slide method on existing foundations. The resulting effect on the traveling public was reduced from years to just a few days.
"Foundation reuse is one of the tools that needs to be in the owners' project development toolbox," says Aaron Stover, project manager with Michael Baker International. "In the case of the Milton-Madison Bridge, reuse and rehabilitation of the substructure on the existing alignment reduced environmental impacts and avoided right-of-way procurement and utility relocations. This innovative approach saved an estimated $50 million ... compared to other new build alternatives and significantly reduced the overall project delivery timeline compared to typical projects."
Studying Foundation Reuse
In 2015, as part of the research agenda for the Foundation Characterization Research Program, FHWA selected the Willow Valley Bridge as the first field site for its assessment of technologies for foundation characterization and reuse. The bridge is located 40 miles (64 kilometers) southeast of Flagstaff, AZ, on 1-17 along the low-volume Lake Mary Road. The rehabilitation plan proposed by FHWA's Central Federal Lands Highway Division (CFLHD) is to replace and widen the existing bridge superstructure.
The existing three-span bridge is 104 feet (32 meters) long and 34 feet (10 meters) wide. Construction of the bridge occurred in two phases: the original southern portion in 1934 and a widened northern portion in 1968. The bridge widening added three more girder lines. A mass-gravity substructure supports the pin-and-hanger steel girders and concrete deck superstructure. Each stone masonry wall foundation is built on rock, and the wingwalls at each abutment are masonry construction.
To reuse the foundations and ensure a safe and stable structural design, FHWA researchers thoroughly evaluated the existing foundations. Their evaluation included reviewing available information such as as-built drawings, conducting field evaluations, and drilling exploratory borings. The purpose was to demonstrate that the structural and geotechnical capacities and resistance of the foundation elements are sufficient to support design loads specified under the current AASHTO LRFD requirements.
For the exploratory borings, CFLHD staff decided to drill two 3-inch (8-centimeter>diameter core holes at each foundation wall (abutments and piers). The core holes were drilled from the bridge deck through the mass-gravity columns into the underlying bedrock.
FHWA plans to publish a report in 2016 with the extensive data gathered for this study, but a sample of field results is presented below to highlight the capability of the technologies.
Geophysical Logging And Imaging
One useful technology for assessing foundations for reuse is wireline logging from the core holes. In concrete and masonry structures, wireline logging technology can assist in mapping cracks, present shear/ Young's modulus as a function of depth, evaluate steel corrosion in reinforced concrete structures, and provide insights on the durability of the construction materials. The FHWA researchers applied wireline logging and imaging in their study of the Willow Valley Bridge.
They used an optical televiewer tool, which generates a continuous 360-degree image of the core hole wall using an optical imaging system. The tool has sensors that correct for its orientation and create a photograph-like "virtual core" image of the core hole wall.
While wireline logging can provide detailed information on deteriotation of the foundation material around a core hole, other geophysical imaging technologies can produce three-dimensional (3-D) volumetric images for assessing structural integrity. A velocity tomogram, for example, indicates the distribution of acoustic velocity in the wall. Low velocities indicate possible anomalies.
The researchers then used the velocity tomogram to obtain an image of the reflected (echoed) wave arrivals. The wave echoes create an image of the foundation wall and the voided areas. In the case of the Willow Valley Bridge, the imaged waveforms correlated well, not only with the known abutment wall boundaries, but also with the interpreted location of voids.
In addition, the researchers conducted several 3-D seismic and resistivity surveys to assess the condition and structures of the bedrock from the ground level. Each 3-D survey consisted of four lines placed parallel and close to each foundation wall and two lines in an orthogonal direction.
"The results obtained by this study assisted the FHWA Central Federal Lands office in developing a plan to grout the voids that were inherent in the original foundation walls [for the Willow Valley Bridge]," says Khamis Haramy, senior geotechnical engineer with FHWA's CFLHD. "It was also valuable in assessing the structural integrity and capacity of the foundation walls."
The Foundation Characterization Research Program is also developing a methodology for load testing of existing bridge foundations, including instrumentation for monitoring foundation response.
The degree and complexity of the surveys obtained by FHWA's Foundation Characterization Research Program might not be necessary for every foundation reuse project. However, the research program intends to deploy the technology at a reasonable cost and develop technical guidance and best practices for foundation reuse.
Frank Jalinoos is a research engineer at FHWA's Turner-Fairbank Highway Research Center. Jalinoos has nearly 30 years of experience with inservice bridge inspections, structural monitoring, sensor development, and ground imaging. He holds a master's degree in geophysical engineering from the Colorado School of Mines.
For more information, visit www.fhwa.dot.gov/research/tfhrc /programs/infrastructure/structures /fcp, or contact Frank Jalinoos at 202-493-3082 or firstname.lastname@example.org.
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|Date:||Nov 1, 2015|
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