The Downtown Bellevue Tunnel
Lead Design Firm/Architect/Engineer: Hatch Mott McDonald, Jacobs Engineering, HNTB Joint Venture (H-J-H)
Subcontractors: Rebar International Inc.: F.D. Thomas; Amaya Electric; Conco; Condon Johnson; Gall Zeidler; Geo-Instruments; Grady Excavating; KLB Construction; O'Neill Service Group
Linking downtown Seattle to Bellevue with a 14-mile light rail extension required crossing a floating bridge over Lake Washington, passing through multiple cities and constructing a tunnel through downtown Bellevue. With a mixture of cut-and-cover construction at both the north and south portal and a nearly 2,000-ft-long sequential excavation method tunnel through the heart of downtown Bellevue, Atkinson delivered the Downtown Bellevue Tunnel on time and under the $121-million budget.
The first project in the larger $3.7-billion East Link light rail extension for Sound Transit, the sequential excavation method was chosen to ease the impact on the city during construction and speed the project’s timeline, but the tunnel required significant design changes to account for soft ground next to Bellevue’s high-rise buildings. The changes came with delays, but the team made up for the two-month delay, all while mitigating excavation issues around aging storm lines and granting early access to the tunnel’s utility contractor to allow the critical path activity for the overall project to begin earlier. The project embraced new uses of macro-synthetic fiber initial shotcrete lining, spray-applied waterproofing and shotcrete for the final tunnel lining, resulting in time and cost savings.
Atkinson crews removed 72,000 cu yd of soil in small sections and then supported excavated areas with spray-applied concrete.
The finished tunnel is 27 ft, 10 in. high and is 34 ft wide, split into two sides with a concrete dividing wall.
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