I-405/NE 8th St to SR 520 - Braided Ramps


Washington State Department of Transportation

Completion Date



Jacobs Engineering Group


Bellevue, Washington

Delivery Method




Contract Value

I-405 / NE 8th St to SR 520 - Braided Ramps

The project improved the highest-volume interchange along the I-405 corridor. Atkinson's design implemented a braided ramp configuration connecting I-405 to SR-520, with one ramp stacked above the other. This strategic layout separated incoming traffic to northbound I-405 from downtown Bellevue and outgoing traffic from northbound I-405 heading to eastbound SR 520. By eliminating hazardous merging conditions, the project successfully alleviated congestion and reduced delays. Additionally, the project encompassed the construction of seven new bridges, the addition of a new bypass lane on SR 520, PCCP panel replacement, paving, sign bridges, over 30 retaining walls, recycling of pavement and concrete slurry, extensive electrical work, and the establishment of a wetland and stream mitigation site to meet environmental requirements.

The original 'stacked' trench conceptual design outlined in the RFP would have necessitated secant pile walls involving substantial drilling and excavation near the Overlake Hospital Medical Center (OHMC). This approach posed potential challenges such as vibration impacts on sensitive operating suites and disruptions to traffic circulation within the hospital campus. An alternative solution was used by constructing the NE 10th Street ramp on oscillated drilled shafts and cantilevering the bridge over the hospital access road. This innovative approach reduced drilling requirements, minimized vibration and noise disturbances, addressed air quality concerns, and enhanced traffic flow within the campus.

As part of the project, the old NE 12th Street Bridge, a post-tensioned box girder bridge, was dismantled in a 53-hour closure of I-405. To manage this closure effectively, comprehensive public information efforts were undertaken, requiring close coordination with OHMC, the City of Bellevue, other WSDOT projects, and nearby businesses. These collaborative initiatives minimized impacts on the public and major stakeholders. Our public outreach and education campaign resulted in a nearly 75% reduction in the daily volume of vehicles on I-405, effectively mitigating delays and congestion in the vicinity. The project also involved the construction of a new NE 12th Street bridge, designed to accommodate the expanded freeway below and enhance local pedestrian and bicycle access.