Today, Sound Transit invited representatives from the mass media, its contractors, and its interagency partners at the state, local, and federal level to downtown Bellevue to commemorate a literal breakthrough: the completion of the mining of the tunnel that will carry trains underneath the city’s main business district.
Completed five months ahead of schedule, the new tunnel is approximately one third of a mile long (1,985 linear feet, to be specific). It is twenty-seven feet and ten inches tall, and thirty-four feet wide. Excavation began a year and a half ago, around the time NPI’s Permanent Defense celebrated its fifteenth anniversary.
“This tunnel is the culmination of over a decade of vision, hard work and tenacity by local and regional leaders, planners, workers and many, many members of the public who all insisted on a great transit connection through downtown Bellevue.”
“East Link — and this tunnel — will connect our communities, support our economy and provide a fast and reliable way to get out of traffic for people all over the region,” said Sound Transit Boardmember Claudia Balducci, who joined CEO Peter Rogoff and Bellevue officials for a celebratory speaking program at the site.
After the conclusion of the speaking program, the media was invited to join Sound Transit staff members for a tunnel walk-through, beginning from the south portal and ending at the newly-created north portal (where crews are still working).
It took about ten minutes of walking at an unhurried pace to traverse the length of the tunnel. Once we reached the other end, Atkinson Construction workers demonstrated the equipment they’ve been using to dig the tunnel.
Unlike the tunnels dug by WSDOT’s Bertha, and by Sound Transit’s smaller TBMs (the Emerald Mole, Brenda, and Pamela), this tunnel was dug using the sequential excavation method, also known as the New Austrian tunnelling method.
Sequential excavation involves several steps that are being constantly repeated: soil removal, the spraying of pressurized concrete known as shotcrete for stabilization, and the installation of lattice girders for additional support.
Unlike with a tunnel boring machine, there isn’t a giant cutterhead and a trailing apparatus that has the length of a football field. When I visited the construction site last year, we were able to walk right up to the spot where the digging was going on and watch soil being loaded into dump trucks for removal.
According to Sound Transit and Atkinson Construction, 72,000 cubic yards of soil had to be removed to dig the tunnel, 9,000 cubic yards of shotcrete were applied to stabilize it. And 479 lattice girders were put in for additional stabilization.
The tunnel is twelve feet from the surface at its shallowest point, and thirty feet from the surface at its deepest. It crosses under Main Street and then follows 110th Avenue NE, crossing NE 2nd Street and NE 4th Street. It then ends at the site of the future Bellevue Downtown Station, which abuts City Hall.
Here’s an excellent map of the alignment, courtesy of Sound Transit:
The City of Bellevue provided $100 million worth of in-kind contributions to help Sound Transit deliver the tunnel. These included property and utility relocations. The contributions were formalized in a 2015 Memorandum of Understanding approved by the Sound Transit Board and the Bellevue City Council.
The choice of a sequentially excavated tunnel allowed Sound Transit to avoid tearing up 110th Avenue NE for an at grade alignment or a cut and cover tunnel.
The end result is a light rail alignment that goes where the people are (instead of an alignment crammed into the I‑405 right of way) that did not involve shutting down a major artery in downtown Bellevue to construct.
The entire East Link project includes ten stations linked by fourteen miles of track. Construction is scheduled to be complete in time for a 2023 grand opening. When operational, it will be possible to go between the Microsoft campus and downtown Bellevue in just ten minutes, south Bellevue and Sea-Tac Airport in just fifty minutes, or Mercer Island and the University of Washington in just twenty minutes.
NPI congratulates Sound Transit on today’s big milestone. It is wonderful to see progress being made on making East Link a reality all around the Eastside.