Tunnel Business Magazine

AUG 2018

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Seattle Voters Approve ST3 . 8, 2016, voters approved Sound Transit 3 (ST3), a ballot measure that provides $54 billion in funding to expand and improve transportation in the Seattle area. ST3 will add 62 miles of light rail, completing a 116-mile re- gional system. "Our region has embraced a genera- tional opportunity to move forward with a transit network to connect millions of people across three coun- ties," said Sound Transit Board Chair and King County Executive Dow Con- stantine. "After decades of waiting, we are ready to start building a light rail system that will grow our economy, improve our quality of life, and ensure access to jobs, education and all the Central Puget Sound has to offer." By 2021, Sound Transit will com- plete light rail to Roosev - gate Link and in 2023 trains will reach cer Island, Bellevue, Overlake, Shor e Terrace and Lynnwood. From there, Sound Tran- sit will keep building until the agency has completed a 116-mile regional sys- up will be getting light rail to Federal Way, downtown Redmond, Tacoma, West Seattle, Ballard, Everett, South Kirkland and Issaquah. TUNNELINGONLINE.COM TBM: TUNNEL BUSINESS MAGAZINE // AUGUST 2018 been previously used on a project in Singapore. Tunneling was completed Sept. 1, 2016, with the second hole- through at the University of Washing- ton Station. In addition to the main running ontract included con- struction of 23 cross passages between tunnels that will serve as emergency evacuation routes and will contain controls for electrical and mechanical systems. Remaining work under sepa- rate contracts includes installation of track beds, rails and communications systems, and fit out of the stations. Urban Tunneling About 3,800 ft of the tunnel align- ment traverses under the Univer- sity of Washington, which created a unique set of challenges for tunnel construction. The school is renowned as a leading research institution, and had strict limitations for noise and vibration – during construction and operation – to ensure that its facili- ties were not impacted. To further reduce impacts on the university, an agreement was reached to limit the construction duration for tunneling under campus. "We had 304 days to build the tunnels fr crossed under campus," said Rick Cap- ka, Deputy Project Director – - gate Link. "That entailed completing two tunneling passes as well as ex- tracting, refurbishing and relaunching y that we were able to meet that milestone with room to spare." Tunneling under campus also re- quired innovation from the design side. "The tunnels under the Univer- sity of Washington campus provided a series of design and construction chal- lenges," said Gregg Davidson of - len Jacobs Associates, Sound Transit's design consultant. "These ranged from analysis, design and testing of a float- ing track slab in the tunnel under the noise and vibration sensitive campus buildings, to schedule constraints on how long the could actually work within the campus boundaries." Basically, the floating slab comprises precast concrete structures that sit on natural rubber pads, resulting in a sort of shock absorber. Sound Transit had tested a 400-ft prototype floating slab in the University Link tunnels. To the north, the alignment crosses a densely populated and growing residential area, so the decision was made to keep the tracks underground. "Tunneling was the best option in that area due to minimizing construction impacts on residents as well as mini- mizing property acquisition in such a densely packed area," said Kimberly Reason, Senior Public Information Of- ficer for Sound Transit. Like many projects in the Seattle area, the ground conditions posed challenges. Typical reaches f - gate Link include glacial soils compris- ing sands, silts, gravels, clays and scat- tered boulders under the water table. In addition, crews had to navigate a section of hard glacial till. To tackle the ground conditions, earth pressure balanc e selected. Un- like the U-Link project, Sound Transit allowed the use of refurbished ma- C O V E R S T O RY 2 0

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