While there may be no perfect solution to any engineering challenge, this replacement bridge is about as close as one could get.
The York Bridge had problems. Spanning Washington state’s Sammamish River between King County and the city of Redmond, the 50-year-old, 117-ft bridge was structurally deficient, with cracked concrete and rusting reinforcing steel. Users also had to cope with substandard sight distances and the danger of at-grade crossings with riverbank trails. Citing these conditions, as well as seismic concerns, the two cognizant jurisdictional entities decided to replace the bridge. That is when things got complicated.
The design team determined that the new $12 million York Bridge would have to be 103 ft longer, 16 ft wider and 15 ft higher than the original span so that the trails could continue uninterrupted beneath the bridge. moving the line was too expensive and time consuming, designers elected to raise the road instead, using lightweight material that would not add significant loads to the supporting soil. Enter geofoam.
“Other cities in the area have been using it too,” explained Ron Grant, P.E., construction division manager for the city of Redmond. “It’s readily available, and it’s environmentally sensitive as well.” Weighing only 1-2 lb per cu ft (most soils are almost 100 times heavier, and even lightweight fills can weigh 20 to 30 times as much), geofoam—expanded polystyrene—has been used successfully in at least 20 states, according to the Federal Highway Administration. And because geofoam does not biodegrade, it will not adversely affect soil or groundwater. But geofoam was new to most of the York Bridge team members. In fact, it had never been used before in King County.
“There was a learning curve that kept popping back up as we went along. It never really stopped challenging us during the three months it took to place the geofoam,” said King County Resident Engineer Dee Gilmore.
Packed in peanuts
Placing the geofoam involved first removing enough soil to equal the weight of the geofoam blocks. Then, after a layer of geogrid soil reinforcement had been positioned and covered by 6 in. of compacted sand, geofoam blocks were stacked as much as 9 ft high, stopping approximately 3 ft below the finished grade. Each block averaged 2 1/2 ft high by 4 ft wide by 8 ft long, with the blocks stacked in alternating directions. Once the blocks were in place, a 6-in.-thick reinforced concrete structural load distribution slab was placed on top to distribute live loads evenly over the geofoam mass. The topping slab also protected the geofoam against potentially damaging solvents, and provided lateral support for concrete facing panels fronting the embankment. But before that could be done, the team had to conquer a few geofoam-related hurdles.
Needing to insert utility conduits into the geofoam itself, the team discovered a unique and simple fill cover for the new conduit utility trenches. “We ended up using packing peanuts,” added Gilmore. “Every other material we tried caused the geofoam to displace, and we couldn’t keep it in its alignment.”
The team also had to contend with the geofoam blocks sliding when stacked. Again, Gilmore explained. “We had to run rebar down through some of the edge pieces to make sure that when we were stacking layers we didn’t knock them out of whack. It was like putting together a giant, three-dimensional jigsaw puzzle.”
“Wherever there are settlement issues, you’re going to find a lot of roadway beds supported by geofoam,” said Kiva Lints, P.E., bridge project engineer from the Bellevue, Wash., office of project design consultant DMJM Harris. “Contractors are going to have to learn to deal with it and figure out how to bid it. And it’s going to take a while for the engineering side to come up with the best way to show details in the plans; we need to give contractors some flexibility in doing what they need and want to do, without requiring a lot of extra changes.”
In total, the project used almost 6,000 cu yd of geofoam. According to Lints, building the west approach with geofoam not only eliminated potentially destructive settlement, it also saved almost one year of construction time. “It was about the only option we had. I would highly recommend geofoam for similar situations where there are settlement issues—but not if you have other alternatives.”
In fact, the project team elected not to use geofoam on the east approach because the soft soils were not as deep, and there was no sewer line complicating the issue. The east approach was excavated and filled with conventional material, using structural earth and retaining walls.
While they posed a considerable challenge, the approaches were by no means the only difficulties faced by the project team. The new four-span precast concrete girder bridge presented its own complications. To stay true to the context of other Sammamish River bridges, the structure had to be arch-supported. A four-span precast concrete girder bridge with a shallow cast-in-place arch and inclined columns supporting the two center spans was chosen. But aesthetic elements further complicated the choice.
Percent of progress
Because King County and the city of Redmond included the bridge in their One Percent for Arts program, artistic elements had to be incorporated into the design. After a nationwide search, an artist was selected to help the engineers refine the geometry of the arch so as to complement nearby bridges. Though some elements could simply be added after construction, others had to be embedded structurally. For example, while decorative railings were just bolted in place, large horizontal deck bulbouts had to be integrated into the design. In fact, the changes made the bridge asymmetrical, complicating the bridge’s structural analysis. And it affected construction. According to Lints, instead of building the entire bridge and then releasing all of the falsework, it was necessary to construct the arch and then gradually load it with weight before completing the deck across it.
In addition to its other functions, the bridge also serves as a critical element in a 27-mile regional trail system that extends from Redmond to Seattle. The York Bridge setting hosts a paved trail on the east riverbank; it is used by pedestrians, skaters and bicyclists. On the west bank, the unpaved trail provides a haven for equestrians and runners.
With the old York Bridge, the trails rose to meet the bridge at grade. Conflicts arose often, since the road averages 6,000 vehicles per day. And three nearby facilities—a major golf course, a robotic airplane field and a 16-field soccer complex—attracted a significant number of road and trail users.
Speer addressed the issue from her firsthand perspective: “Being a regular user of the area trails, it’s exciting to see that the trails are now going under the bridge, as opposed to hitting the roadway right at the crest of where the bridge was. We used to have to scurry across to avoid being hit.”
The new bridge also improved trail access through the use of paved ramps from the road. River access was enhanced by adding a paved parking lot and a kayak launch point. But the benefits were not only on land.
Originally a meandering stream, the Sammamish River was transformed in the mid-1960s. To control flooding and conserve agricultural land, the U.S. Army Corps of Engineers made it into a straight, riprap-lined canal. Unfortunately, that approach resulted in a slow-moving river lacking vegetation, which in turn produced oxygen-poor waters that are detrimental to salmon and other native fish. And the potential mitigation solutions envisioned as part of the bridge project faced serious hurdles of their own.
In addition to dealing with Corps requirements, the project team had to address the concerns of stakeholders in city, county and state agencies—as well as the Muckleshoot Indian tribe. In all, 22 permits were ultimately required for what seemed at first to be a simple yet effective solution: The team just placed large woody debris in the water. According to Redmond’s Ron Grant, “Old trees and their root wads provide pools for the fish and also provide higher ground for ducks to stand on.” Like much else on the project, this somewhat unconventional solution worked just fine.
The York Bridge had an array of problems: soft soils, an irregular arch design, environmental concerns and a host of stakeholder concerns and requirements. This bridge solution had to be all things to all people. And in effect it was. Completed and opened to traffic in November 2006, the bridge stands as a lucid example of how a comprehensive design solution can greatly enhance an environment, even while achieving a multitude of disparate goals. While there may be no such thing as a perfect solution, this replacement bridge is about as close as one gets.