Tunnel Business Magazine

AUG 2018

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Page 21 of 43

Next Phase of Tunneling As part of its long-range vision, Sound Transit is in the planning phase for the West Seattle Link Ex- tension (revenue service 2030) and Ballard Link Extension (2035). The Ballard Link Extension includes a new downtown tunnel that would parallel the existing Downtown Se- attle Transit Tunnel. Final design for the Ballard Link Extension and second downtown Seattle Light Rail Tunnel is expected to begin in 2023 and be completed in 2026. Construction and testing for the Ballard Link Extension and downtown Seattle Light Rail Tunnel is expected to begin in 2027 and be completed in 2035. Sound Transit is soliciting public input and considering alternatives for these extensions. The board will decide the preferred alternative and other alternatives to advance to envi- ronmental review in April 2019. TUNNELINGONLINE.COM TBM: TUNNEL BUSINESS MAGAZINE // AUGUST 2018 chines, in part due to the availability of used machines in that size range. In addition to the mainline tunnels, cross passages were needed every 800 ft. The cross passages, which ranged from about 15 to 25 ft in length, were built using the sequential excava- tion metho vancing in 3-ft rounds and supported with shotcrete and lattice girders. "Even though the cross passages were short in length, excavating for them was very com- plex due to the ground condition," Capka said. Ground Freezing In conditions where groundwater was present, ground freezing was per- formed to allow cross passage excava- tion in a watertight zone. This marked the first time that Sound Transit had used ground freezing on its projects. In addition to cross passages, ground freezing was used to create a support canopy f eak-in into the University District Station site. Ground freezing was used on 11 of the 23 cross passages, 5 using vertical drilling from the surface and 6 using horizontal drilling from within the completed tunnel. Original plans had been to dewater the 6 locations of horizontal ground freezing, but actual ground conditions encoun- tered rendered the dewatering op- tion not feasible. "We found ground freezing to work well in every instance we used it," Capka said. "We had concerns over the freeze-thaw effects, including the potential for heave or damage nearby utilities or the tunnel itself, so we im- plemented additional monitoring. In our case, we did not experience any significant issues." While freezing from within the tunnel itself comes with logistical challenges and can impact other ac- tivities within the tunnel, it has its benefits too, according to Capka. "By drilling from within the tunnel, you minimize some of the challenges of working from the surface," he said. "There are fewer issues in working around existing utilities, and you are not impacting the neighborhood with noise or road closures. Sometimes there are issues getting temporary power to support the freeze plant. By drilling within the tunnel, you don't encounter any of that." Closed Book te Link Extension is approximately 65% complete and is on track to open in 2021. Tunneling was completed in 2016 and all construc- ontract has been completed, moving Sound Transit one step closer to providing better service to residents. "The highlight of the project has been the successful collaboration, once again, between Sound Transit, the final design team, the tunnel con- tractor and the construction man- agement folks, which has resulted in te Link Extension staying on target to open on schedule," Davidson said. "Following on from the success- ful U-Link project it's been great to see the continuing expansion of rail tran- sit options in the region." "Overall the tunneling went well," Capka said. "When we were develop- - len Jacobs Associates, we said we tar- geted a tunnel contract start in 2013 and finish in February 2018. With all the complex conditions we faced with tunneling and some of the unan- ticipated challenges that came up, we finished in February 2018. Keeping to that schedule is crucial in allow- ing our follow-on contractors to start their work and mak te Link Extension a reality." The next phase of Link light rail expansion is the Lynnwood Link Ex- tension that will extend the line from te Link terminus 8.5 miles northward to Lynnwood City Center. Construction is anticipated to begin in mid-2019 with revenue service tar- geted for 2024. C O V E R S T O RY 2 2 PROJECT TEAM Owner: Sound Transit Contractor: JCM Northlink LLC (Jay Dee/Michels/Coluccio) Design Team: McMillen Jacobs Associates Construction Management: NorthStar (CH2M/Jacobs) Major Subcontractors: DBM, Case Foundations, Bencor, Elcon, Sundancer, Hayward Baker, Soldata. Suppliers: TBM manufacturers: Hitachi Zosen and Robbins. Segment manufacturer: CSI/Hanson.

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