South portal of the downtown Bellevue light rail tunnel
South portal of the downtown Bellevue light rail tunnel

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:

  1. Exca­vate earth (not more than four feet at a time)
  2. Spray with two inch­es of shotcrete
  3. Put a lat­tice gird­er in — that’s a steel rib
  4. 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.

FURTHER COVERAGE:

About the author

Andrew Villeneuve is the founder and executive director of the Northwest Progressive Institute, as well as the founder of NPI's sibling, the Northwest Progressive Foundation. He has worked to advance progressive causes for over two decades as a strategist, speaker, author, and organizer. Andrew is also a cybersecurity expert, a veteran facilitator, a delegate to the Washington State Democratic Central Committee, and a member of the Climate Reality Leadership Corps.

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