Big East Link milestone: Sound Transit’s light rail bridge over I‑405 is structurally complete

Per­cep­tive dri­vers stuck in grid­lock on I‑405 in Belle­vue have prob­a­bly noticed a new bridge being built above the inter­state over the past year or so. What they may not know is that in just a few short years, trains will be zoom­ing across that bridge as they car­ry pas­sen­gers to and from down­town Bellevue.

Con­struc­tion of East Link, Sound Tran­sit’s big light rail exten­sion that will con­nect Mer­cer Island, Belle­vue, and Over­lake with Down­town Seat­tle in 2023 and Red­mond in 2024, is more than 55% complete.

This includes sig­nif­i­cant por­tions through Down­town Bellevue.

East Link will have three sta­tions near the Belle­vue down­town core:

• East Main: south of Main Street on 112th Ave SE;
• Belle­vue Down­town: just east of the Belle­vue Tran­sit Cen­ter and south of the Mey­den­bauer Cen­ter; and
• Wilbur­ton: east of I‑405, at the inter­sec­tion of NE 8th St and 118th Ave NE.

To get light rail mov­ing through Belle­vue’s dense cen­tral busi­ness dis­trict with min­i­mum sur­face-lev­el dis­rup­tion, Sound Tran­sit con­trac­tors have dug a tun­nel under­neath the city, con­nect­ing East Main and Belle­vue Down­town stations.

This is a sequen­tial­ly exca­vat­ed tun­nel, where oppo­site-direc­tion tracks run right next to each oth­er — unlike the light rail tun­nels in Seat­tle, where two sep­a­rate tun­nels car­ry trains in between sta­tions on the under­ground alignments.

The big advan­tage of the approach uti­lized in Belle­vue is a reduc­tion in cost and risk: why dig two tun­nels when you could dig just one? Accord­ing to Sound Tran­sit engi­neers, many lessons were learned from pre­vi­ous tun­nel projects, includ­ing the infa­mous prob­lems with Bertha, which dug the State Route 99 tunnel.

Improve­ments to tech­nol­o­gy mean that larg­er tun­nels car­ry­ing two tracks can be dug reli­ably. In fact, the Belle­vue tun­nel was suc­cess­ful­ly exca­vat­ed five months ahead of sched­ule — fan­tas­tic news for the on-time com­ple­tion of East Link and a great fac­toid to share with friends and fam­i­ly who might not sup­port NO on I‑976 con­cerned about delays on big tran­sit projects!

The Belle­vue Down­town Link sta­tion is locat­ed right next to the exist­ing tran­sit cen­ter, con­nect­ing well to exist­ing and future tran­sit ser­vice. This includes the RapidRide B Line to Red­mond, the I‑405 Bus Rapid Tran­sit cor­ri­dor (future), as well as exist­ing King Coun­ty Metro and Sound Tran­sit ser­vice. While tour­ing the work site, NPI staff saw bus­es mak­ing fre­quent trips from the tran­sit cen­ter — includ­ing this dou­ble-tall Route 535 bus head­ed to Lynnwood!

East of Belle­vue Down­town sta­tion, East Link ris­es above I‑405 on a spe­cial­ly-con­struct­ed bridge six­ty feet above main­line express­way traf­fic. Con­struc­tion start­ed with met­al false­work (akin to an exoskele­ton) going up above the inter­state in ear­ly 2018 (link, page 19). Now, the bridge is com­plete, stretch­ing from the tun­nel por­tal to Wilbur­ton sta­tion before descend­ing to the ground.

The bridge is eight hun­dred and ten feet long, with a a three hun­dred and fifty foot span across I‑405. It was cast-in-place, mean­ing that con­crete was poured and solid­i­fied on the con­struc­tion site, unlike the bridge span­ning I‑90 in south Belle­vue, which is a bal­anced can­tilever bridge.

The con­crete bridge has been post-ten­sioned, mak­ing it able to with­stand a “2,500 year event” — mean­ing that even in the case of rare nat­ur­al dis­as­ters, light rail infra­struc­ture will remain safe and usable.

The main span of the bridge is hol­low, with four­teen-inch thick walls and ten-to-six­teen-inch thick hor­i­zon­tal slabs. Due to the cast-in-place nature of the struc­ture, it was hand-built entire­ly by work­ers on-site, and all mate­ri­als were U.S.-sourced.

Met­al sup­port struc­tures were used to sup­port the entire bridge as the deck and pil­lars were being filled out. Now that the bridge has been struc­tural­ly com­plet­ed, it is time for the exter­nal met­al that cur­rent­ly stretch­es between the con­crete bridge and active lanes of inter­state traf­fic to be removed.

Mov­ing to this phase of con­struc­tion means that I‑405 through Belle­vue will have to be closed dur­ing parts of the next two week­ends.

South­bound ramps will be closed the night of Fri­day, August 9th through Sat­ur­day, August 10. South­bound main­line traf­fic will then be closed from Sun­day, August 11th at mid­night until just before the morn­ing com­mute on Mon­day, August 12th. North­bound ramps will be closed the night of Fri­day, August 16th through Sat­ur­day, August 17th. Like­wise, north­bound main­line lanes will be closed ear­ly Sun­day morn­ing until the morn­ing com­mute on August 19th.

“Dur­ing the week­end clo­sures, crews will use an elec­tric winch to remove and low­er the heavy false­work to the road sur­face, where it will be com­plete­ly dis­as­sem­bled and removed,” Sound Tran­sit explained in an advi­so­ry about the work.

“With­out the week­end direc­tion­al clo­sures, this com­plex process would have tak­en more than twen­ty weeks of week­night work to com­plete,” the agency added.

Vis­it Belle­vue’s city gov­ern­ment web­site for detour infor­ma­tion and Sound Tran­sit’s web­site for more infor­ma­tion on the clo­sures.

Despite the clo­sures, light rail will be com­ing to the East­side in four short years. Once here, it will be here to stay, with light rail run­ning every six min­utes con­nect­ing Red­mond and Belle­vue to Seat­tle, North­gate, and Seat­ac for decades, even cen­turies, to come. It will rev­o­lu­tion­ize trav­el on the Eastside.