Let the mining begin!
That was the sentiment today at an event hosted by Sound Transit for media outlets around the region in downtown Bellevue, where Atkinson Construction is beginning the excavation of the tunnel that will eventually carry ST’s East Link light rail under downtown. The tunnel is being partly financed by the City of Bellevue, which contributed $100 million to make the sequentially excavated tunnel a reality.
Several members of the Bellevue City Council were on hand to praise the project’s progress, along with U.S. Representative Adam Smith and King County Councilmember Claudia Balducci, who also represented the Sound Transit Board. (Sound Transit CEO Peter Rogoff and Board Chair Dave Somers were not able to participate in the event due to other obligations.)
Unlike the tunnels that carry Central Link trains underneath Capitol Hill and points north, the downtown Bellevue transit tunnel will be somewhat short — less than half a mile long, in fact. Trains will enter the tunnel through a portal just south of Main Street and exit the tunnel as they pull into the Downtown Bellevue Station at NE 6th Street. From there, they cross I‑405 to reach the Overlake Hospital station.
Here’s a video showing this segment of the alignment:
“Because of the short tunnel length, and to minimize impact on neighboring homes and businesses, Sound Transit is constructing the tunnel using the Sequential Excavation Method (SEM) rather than using a tunnel boring machine or digging a large trench that is later covered,” Sound Transit explains.
“SEM removes soil in small sections or bites using an excavator and cutting equipment. As soon as soil is removed, pressurized concrete, called shotcrete, is sprayed on the tunnel’s sides, ceiling and floor.”
“Lattice girders provide additional structural support for the tunnel. Advantages of SEM include minimizing disruption to traffic, reduced noise and dust impacts to neighboring residents and businesses versus cut-and-cover tunnel construction, and elimination of utility service disruptions.”
Vital tunnel statistics:
- Tunnel is approximately 2,000 linear feet long (one-third of a mile)
- Height of the tunnel is 27 feet, 10 inches
- Overall width (north and south-bound sides) is 34 feet.
- Each side is 16 feet, 3 inches wide, with a center dividing wall.
- Tunnel is 30 to 60 feet below the surface.
SEM, also known as the New Austrian Tunnelling Method, was invented in Austria in the 1960s, and has become a rather popular method of digging tunnels. It has seven basic elements that make it both practical and cost-effective.
There are several basic steps involved in digging a tunnel using SEM. As explained to NPI by Chad Frederick of Sound Transit, they are:
- Excavate earth (not more than four feet at a time)
- Spray with two inches of shotcrete
- Put a lattice girder in — that’s a steel rib
- Come back and put on another eight inches of shotcrete (in order to fully encapsulate that lattice girder)
Construction teams will start by digging the top heading of the tunnel first, and then advance to the bench and invert sections. “We call it [the middle section] the bench because we set up on top of it,” Frederick says.
In this NPI photo, you can see the digging has begun at the south portal. The big block in the middle of the hole is the face wedge. It consists of dirt and concrete and it’s there to hold the face in. Structural integrity is important to a tunnel dig!
Because Sound Transit and the City of Bellevue opted against a “cut and cover” tunnel, it won’t be necessary to shut down and rip up 110th Avenue NE in order to construct this segment of East Link. To minimize noise pollution, Atkinson has set up large noise walls around the construction site.
During today’s media briefing, reporters were allowed to ask the contracting team questions about the project. I asked about the possibility of sinkholes.
Here’s a transcript of our exchange:
NPI’S ANDREW VILLENEUVE: What’s the risk of sinkholes, and what can be done to mitigate against that? I remember there was a sinkhole on the Beacon Hill project.
CHAD FREDERICK, SOUND TRANSIT: So there’s always the risk of minor sinkholes. We have settlement monitoring in place that will catch that well before it happens… or it should.
That settlement monitoring runs all along the alignment of the tunnel, out here in the 110th right-of-way.
ARCHIE COLE MORGAN, ATKINSON CONSTRUCTION: We also have people in the tunnel all the time who are watching that. We got experts out here that can see what’s going on. Unlike [with] a machine job, where you got a big machine and you can’t see what’s going on outside the machine, [with] this tunnel project, you’ll be able to see the ground — you’ll be able to see what’s going on. If you’ve got an issue going on now, you should be visually able to see it.
CHAD FREDERICK, SOUND TRANSIT: And of course, it’s continually monitored now.
BOB ADAMS, ATKINSON CONSTRUCTION: One of the other concerns about [tunneling here] — [is] not necessarily sinkholes, but utilities — there’s some very mission-critical utilities that are underneath 110th [Avenue]. And particularly at the north end of the tunnel, it gets fairly shallow, the cover, and so there’s a lot of concern about those utilities. So one of the early work items we did on this job was to dig all the soil out from around those utilities, and bury it in a low-strength concrete. CLSM was the term for it. But we wanted to make sure that area where we had the least cover was very secure and would not collapse as we mined underneath it.
All in all, it was an exciting afternoon in Bellevue. It’s nice to see East Link construction is on track and proceeding as planned. Atkinson Construction are tunnel-building veterans, having been involved in mining many of our area’s most famous and well-used highway and rail tunnels.
- Mike Lindblom for The Seattle Times: Bellevue transit tunnel underway, but no giant drill for this job
- Graham Johnson for KIRO 7: Digging begins on Bellevue light rail tunnel