I-405/SR 520 Bellevue Braids
This project improved conditions at the highest-volume interchange on the I-405 corridor. Atkinson's design braided the ramps linking I-405 to SR-520—one above the other—to separate traffic entering NB I-405 from downtown Bellevue from traffic exiting NB I-405 to access EB SR-520. This design eliminated dangerous merging conditions and resultant congestion and delays. Work also included seven new bridges, a new bypass lane on SR-520, PCCP panel replacement, paving, sign bridges, more than 30 retaining walls, recycling of pavement and concrete slurry, major electrical work, and creating a wetland and stream mitigation site to meet environmental requirements.
The RFP’s 'stacked' trench conceptual design would have required secant pile walls that involved significant drilling and excavation adjacent to the Overlake Hospital Medical Center (OHMC)—causing vibration impacts to sensitive operating suites and reducing traffic circulation on the hospital campus. Jacobs and Atkinson designed and built the NE 10th Street ramp on oscillated drilled shafts and cantilevered the bridge over the hospital access road—this reduced the amount of drilling, minimized vibration and noise impacts, alleviated air quality concerns, and enhanced traffic circulation on the campus.
During the project, crews tore down the old NE 12th Street Bridge—a post-tensioned box girder bridge—during a 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 the public and major stakeholders. Atkinson's public outreach and education campaign reduced the 450,000 daily vehicles on I-405 by almost 75 percent—minimizing delays and congestion around the site. Atkinson constructed a new NE 12th Street bridge that accommodates the wider freeway below and includes improved local pedestrian and bicycle access.