Design-Build Team Makes Headway on New SR 520 Floating Bridge

Construction is moving ahead on the Washington State Department of Transportation’s SR 520 Floating Bridge and Landings (FB&L) project. As part of the Kiewit/General/Manson (KGM) Joint Venture design-build team, GeoEngineers is currently working at two job sites, the east approach replacement bridge transition span and bridge maintenance facility located at Evergreen Point in Medina, and at the Kenmore Industrial Yard, located at the north end of Lake Washington.

This anchored soldier-pile wall will be a permanent part of the maintenance facility.

“Most of the geotechnical design for the new east approach bridge and maintenance facility is done,” reported David Phelps, PE, GeoEngineers’ principal geotechnical engineer-in-charge. “Our design and construction team, led by engineer Whitney Trent, PE, has completed anchored soldier-pile and soil-nail retaining wall design, which includes the 70-foot-high, tiered permanent retaining wall to support the maintenance facility that will be built into the slope beneath the east approach bridge. Other design items included developing a temporary dewatering plan to facilitate construction of two cofferdams, bridge foundation design, pile-capacity and driveability analysis, and earthwork design.”

“During this phase of the project, we are contributing a variety of construction-related geotechnical engineering services to the project team,” Phelps said, “including field design modifications to the retaining walls, foundation excavation shoring and construction dewatering. We are currently observing, monitoring, testing and documenting construction and excavation on land and over water.”

Phelps explained that the east approach site is a very complex and crowded one, made more challenging by its very steep slope, artesian groundwater conditions, location on the Lake Washington shoreline and the need to maintain traffic on the existing bridge. To facilitate construction access to the site, GeoEngineers designed temporary modular block walls and soldier-pile walls that are being constructed in conjunction with the permanent walls and bridge foundations now being installed.

At the Kenmore site, GeoEngineers has been working with the KGM design-build team to complete wharf upgrades and prepare the casting facility site necessary to fabricate gravity anchors (constructed on barges), fluke anchors and pre-cast roadway deck sections. The anchors, which will secure the new floating bridge section from shifting during wind and wave action, will be floated into place on Lake Washington later this year.

One of the 35 x 26 x 18-foot, 100-ton fluke anchors built at the Kenmore Industrial Yard.

Those who have crossed the bridge recently may have noticed excavation and pile-driving equipment, drill rigs, cranes and barges afloat on the lake. KGM is using these to construct the project, including the on-shore and in-water cofferdams, the temporary sheet-pile retaining wall enclosures that will facilitate excavation of the 140-foot-long by 40-foot-wide by 12-foot-thick spread footing foundations that will support the new transition span bridge piers.

According to the Washington Department of Transportation, the SR 520 floating bridge replacement project is slated for completion in late 2014. Demolition of the existing bridge is planned in early 2015.

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