Atkinson completed the design-build I-405/NE 8th Street to SR 520 Braided Ramps-Interchange Improvements project in 2012. This project improved conditions at the highest-volume interchange on the I-405 corridor. The ramps linking I-405 to SR-520 were braided—one above the other—to separate traffic entering NB I-405 from traffic exiting NB I-405 to access EB SR-520, thereby eliminating dangerous merging conditions and resultant congestion and delays.
Work included seven new bridges, a new bypass lane on SR 520, PCC panel replacement, paving, sign bridges, more than 30 retaining walls, recycling pavement and concrete slurry, major electrical work, and creating a wetland and stream migration site in Sturtevant Creek to meet environmental requirements.
The new NE 12th Street bridge accommodates the wider freeway below and incorporates improved local pedestrian and bicycle access. WSDOT's “stacked” trench concept would have required significant drilling and excavation adjacent to the Overlake Hospital Medical Center (OHMC) property line causing vibration impacts to sensitive operating suites and reduced traffic circulation on the hospital campus.The design-build team proposed constructing the NE 10th Street ramp on oscillated drilled shafts using a side-by-side concept. This reduced the amount of drilling, minimized vibration and noise impacts, alleviated air quality concerns, and enhanced traffic circulation on the campus.
As part of the project, crews tore down the old NE 12th Street Bridge, a post-tensioned box girder bridge, during a scheduled 53-hour closure of I-405. The closure required a comprehensive public information program and significant coordination with OHMC, the City of Bellevue, other WSDOT projects, and nearby businesses to minimize impacts to major stakeholders and the public. Our public outreach effort and education campaign reduced the 450,000 daily I-405 vehicles by almost 75 percent, minimizing delays and congestion around the site.