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A King County DOT bridge project illustrates the use of geofoam fill.
Abstract
An interesting project under way in the Seattle area is the reconstruction of the York Bridge on Northeast 116th Street north of Redmond, Wash. After the supports for the new bridge were completed, crews worked on the deck and approach roadways using some innovative materials this summer.
Because the bridge over the Sammamish River is surrounded by deep, compressible peat and clay soils, the King County Road Services Division needed to find a lightweight fill for the approach road on the west side.
Engineers on the project turned to geofoam blocks. The blocks were placed between precast concrete panel walls. On top of the foam is a concrete slab—a “load distribution slab”—that secures the geofoam blocks to keep them in place and to distribute the traffic loads. Next is a layer of soil, then the asphalt layers.
Using this technique made it possible to raise the road elevation. In fact, to raise it enough so that the Sammamish River Trail now runs under the new bridge—instead of on the road as it was before the bridge reconstruction—improving safety for trail users as well as motorists.
The new bridge is scheduled to open in September.
Engineering and design details
The King County Department of Transportation and the City of Redmond own the York Bridge. The jurisdictional boundary is at the middle of the structure. The bridge crosses the Sammamish River at Northeast 116th Street in Redmond. The agencies entered into an agreement to replace the bridge with King County DOT as the lead.
The project includes a new 220-ft. bridge over the Sammamish River and two river trails, access to the river trails from Northeast 116th St., 1,400 ft. of new approach roadways, structural earth retaining walls, wetlands mitigation, river enhancement, a three-legged precast concrete box culvert supported on geopiers, and a major geofoam embankment.
DMJM Harris was the prime consultant for the project and was involved in all stages of the design, from preliminary studies through construction. For the design report, the environmental team assessed biological and cultural resources, completed the wetlands delineation, conducted a special stream study, and prepared a habitat mitigation plan. The final design entailed preparation of the plans specifications and estimate (PS&E) documents for the preferred alternative.
Project features include: the new bridge, ADA-compliant access to the Sammamish River Trail, geofoam embankment construction, artistic bridge railings, a concrete finish using custom-designed form liners, and kayak access to the river.
Challenges
The project is located near a high-use recreational area within a rural setting. Several wetlands, sensitive farmland, and riparian habitat are adjacent to the existing roadway. The old bridge was 117-ft. long with a single lane and a narrow shoulder in each direction and was structurally deficient and functionally obsolete. Poor vertical site distances, combined with at-grade trail crossings, made it necessary to place stop signs at the trail crossings. A deep 72-in. sanitary sewer runs along the west bank of the Sammamish River, a fiber-optic network facility crosses the river on the north side of the roadway and a City of Redmond sanitary sewer runs along the south side of the roadway.
The replacement bridge has a number of unusual features that required extensive analysis and design. The selected bridge type is a 4-span precast, prestressed, concrete girder bridge with cast-in-place arches and inclined columns supporting the intermediate pier. Due to the presence of a very thick layer of soft compressible soils, the entire bridge is founded on driven piles. The bridge deck is curved with overhangs of up to 8 ft. The complexities required innovative analysis methods to accurately model the structure.
Explorations of the soil conditions at the site revealed that the proposed bridge and approaches are underlain by loose sand and gravel, soft clay, silt and peat soils to depths up to 195 ft. The depths of the loose soils varied greatly with the deepest soft soils located on the western end of the project. The new bridge is supported on deep pipe piles that penetrate the soft soils and bear within a dense glacial till that underlies the entire site.
The eastern approach was located over somewhat shallower loose materials and was proposed to be a lightweight fill composed of bottom ash constructed after a pre-load surcharge and 8-week settlement period.
During construction, the contractor requested that the pre-load and lightweight fill be eliminated since it appeared that much of the loose material could be overexcavated or densified in place. After additional analysis, the lightweight fill on the east approach was eliminated and replaced with a conventional fill with structural earth walls. Additional pile driving efforts had to be incorporated at the east abutment to account for down-drag forces caused by consolidation of deep loose-soil layers.
Enter geofoam
The west approach overlies deep unconsolidated peats, clays, and silty sands that cannot be overexcavated and would exhibit long-term excessive settlements, a problem for the city sewer that runs underneath the road. To resolve this potentially expensive and long-term challenge, a geofoam embankment was selected as the preferred alternative. (See sidebar articles on pages 40 and 42.)
To eliminate potential settlements, the embankment was designed such that zero new dead load will be added to the bridge roadway approach. The height of the geofoam varies from 1-ft. minimum to about 9-ft.-high at the face of the bridge abutment.
The entire geofoam mass is underlain by a layer of geogrid soil reinforcement and a 6-in.-thick sand drainage and leveling blanket. The geofoam was placed in alternating layers up to within 3 ft. of finished grade.
An additional sand drainage blanket was placed on top of the geofoam and a 6-in.-thick, reinforced concrete, structural load distribution slab is constructed on top of that. The structural slab serves more than one purpose: it distributes live loads evenly to the geofoam, protects the geofoam from solvents that could cause damage, and provides lateral support for the concrete facing panels at the outsides of the roadway embankment. The roadway subgrade and pavement section are then constructed above the structural load distribution slab.
Mowat Construction Co. worked with King County and the designer to refine the final design and improve constructability. The geofoam embankment is concealed by architectural concrete fascia panels that rest on a concrete leveling pad below ground surface and are attached at their tops to the load distribution slab. To offset the weight of the concrete panels, soil below the panels was removed and replaced with a 2-ft.-deep x 6-ft.-wide layer of geofoam. This layer of geofoam is covered with 2 ft. of granular fill and the fascia wall leveling pad is constructed on top of that. From all appearances, there will be no indications that the roadway approach is different than any other earthen embankment. The use of geofoam eliminated the potential for long-term settlement and maintenance of the roadway approach. By not having deep overexcavation, and the placement and removal of a preload surcharge, impacts to the sewer and adjacent wetlands were minimized and the construction of the bridge could be completed independent of the embankment construction.
What is “geofoam?”
EPS geofoam is a lightweight, rigid, foam plastic that has been used around the world as a fill for more than 30 years. Geofoam is approximately 100 times lighter than most soil and at least 20 to 30 times lighter than other lightweight fill alternatives. This extreme difference in unit weight compared to other materials makes geofoam an attractive fill material. Because it is a soil alternative, geofoam embankments can be covered to look like normal sloped embankments or finished to look like a wall.
What are the advantages of geofoam for highway construction?
EPS geofoam can be used as an embankment fill to reduce loads on underlying soils, or to build highways quickly without staged construction. Geofoam has been used to repair slope failures, reduce lateral load behind retaining structures, accelerate construction on fill for approach embankments, and minimize differential settlement at bridge abutments.
Because geofoam weighs only 16 to 32 kg/m3 (1-2 lbs./ft.3), large earthmoving equipment is not required for construction. After the material is delivered to the site, blocks are easily trimmed to size and placed by hand. In areas where right-of-way is limited, geofoam is constructed vertically and faced, unlike most other lightweight fill alternatives. It also can be used in construction during adverse weather conditions.
Source: U.S. Department of Transportation—Federal Highway Administration Resource Center