After 15 months of relatively easy digging, Sound Transit celebrated the completion of East Link tunnel excavation in Downtown Bellevue. The tunnel will carry East Link trains from East Main Station (at 112th Avenue and Main Street) to Bellevue Downtown Station (at Bellevue Transit Center and the city hall), traveling for 1,984 feet under 110th Avenue, at a depth of about 12 to 30 feet below street level.
Unlike the neat and tidy bores left by the tunnel boring machines on University Link and Northgate Link, the Bellevue tunnel was dug using the sequential excavation method (SEM; also called the New Austrian tunnelling method), which involves removing soil with heavy machinery and spraying pressurized concrete to support the void. Additional waterproofing and steel lattice girders (479 in total) were then added to support the new tunnel, which moved at a rate of a few feet per day.
The tunnel measures just under 28 feet in height and is 34 feet wide. Atkinson, the project’s lead contractor, excavated about 72,000 cubic yards of material and applied about 9,000 cubic yards of pressurized concrete (shotcrete). SEM was chosen in part due to its ability to reduce noise and general disruption to nearby residents, compared to cut-and-cover construction and tunnel-boring machines.
While the tunnel now has daylight pouring out of both portals, there’s still major work to be done. The tunnel will be divided by a center wall, with each half carrying a set of track. The outer walls will be reinforced and fitted with all the bells and whistles needed to run light rail trains. Construction within the tunnel is expected to wrap up in mid-2020, a full three years before East Link trains are scheduled to carry passengers.
Bellevue’s new tunnel will be the final tunneling project in the ST2 package, coming years before the West Seattle and Ballard extensions are planned to kick off. It’s a fitting role for one of the most controversial sections of the light rail system, which nearly sunk East Link well after it was approved by voters. The original package did not include funding for a downtown tunnel, but earlier environmental reviews had studied various alignments that stretched as far west as Bellevue Way. Bellevue’s city council endorsed a $600 million tunnel under 106th Avenue in February 2009, but Sound Transit responded a few months later by choosing a surface alignment in its preferred alternative.
The city council later accepted a compromise tunnel under 110th Avenue, with a single underground station serving the transit center. As the recession deepened, however, the plan was scaled back to a station-less tunnel that would cost only $285 million. While the city council’s members duked it out over the alignment around Mercer Slough in south Bellevue, the tunnel was approved by both Sound Transit and the city in 2011. Bellevue agreed to contribute $100 million to the cost of construction, property acquisition, and utility relocation related to the tunnel.