Seattle pushes on with LRT extensions
Aug 2012
Shani Wallis, TunnelTalk
- Positive progress on the current U-Link extension, from downtown Seattle to the University of Washington, is being studied and brought forward into final designs by Sound Transit for the confirmed extension of the line to Northgate. Three EPBMs recently completed 3.1 miles (5km) of difficult U-Link twin tube tunneling well ahead of program and work by separate contractors to fit out two open-cut underground stations and install the track-work and services are preparing to gear up. A repeat of the process is planned for the 4.3 mile (near 7km) Northgate extension that includes 3.5 miles (5.6km) of twin tube tunneling under similar geological and logistical conditions.
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Fig 1. Sound Transit's LRT route north
- "We are very pleased with progress on the U-Link project and compliment our contractors who have done a great job not only with the tunneling and excavation to date, but also on the management of their work in the city, moving muck trucks and undertaking heavy civil work in sensitive urban areas and at the University campus," said Joe Gildner, University Link Executive Project Director for Sound Transit. "All three TBMs completed their work ahead of schedule, which is a major advantage to the contractors on their fixed price contracts, and we are about 55% through the construction program towards having the U-Link line open for service by Fall 2016. The critical path elements on the tunnel boring process are now passed and positive progress on this part of the overall project is allowing us to continue to track about 169 days of float in our master schedule. Critical path elements now move to managing the interface between the tunneling contractors and the follow-on station finish contractors and the contractor installing the systems elements taking the project through the commissioning phases to final operation."
- As an immediate follow on from the U-Link, the 4.3 mile (7km) long Northgate Link extension from the University of Washington Station to the Northgate shopping mall is a further investment by Sound Transit of some $2.1 billion. Voters approved funding for the Northgate Link Extension in 2008 as part of a $17 billion regional transit expansion package.
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Northgate extension program
"The Northgate Link has seven main contracts," explained Don Davis, Northgate Link Extension Executive Project Director for Sound Transit. "The largest of these is for all the excavation works. The intention is to award a single contract of an estimated $500-540 million for excavation of the 3.5 miles (5.6km) of twin tube running tunnels, 23 cross passages, two underground cut-and-cover stations, at Roosevelt and the University District Station at Brooklyn, and a North Portal structure for continuation of the line on an aerial guideway to and elevated station at a park-and-ride parking structure adjacent to the Northgate shopping complex." - Final design of the large N120/130 design-bid-build excavation contract is currently about 90% complete with the 100% design and bid documents signed off before the end of the year. A call for bids is then planned for the first quarter of 2013 with award of contract and a start of work by the fourth quarter of the year. Target contract completion is October 2020 for the line to be in service by 2021. Jacobs Associates is working with Sound Transit on the design of the project with Shannon & Wilson engaged for geotechnical work and input to the contract's GBR (geotechnical baseline report).
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Herrenknecht EPBMs finished months ahead of schedule
- The length of tunneling has been extended during the final design process. Initial plans for cut-and-cover running tunnels from Roosevelt Station to the North Portal has been converted to bored tunneling. "The alignment follows the I-5 highway corridor in that area and the open cut work was complicated by a steep slope to the I-5 highway above on the east side," explained Davis. "It was considered better to continue the alignment in bored tunnel, which added about 1,900ft or 600m to the TBM drives."
- The alignment of bored tunneling is just slightly longer than the work completed for the U-Link extension (23 cross passages compared to 21 on U-Link) and the geological conditions are expected to be similar. "The alignment passes through some steep gradients of up to 4% to follow the topography as it rises from the Montlake Cut at the University of Washington Station to the Northgate district," said Davis. "As an extension of the U-Link, the dimensions of the running tunnels are the same at 18ft 10in (5.7m) i.d. and closed face pressurized TBM excavation is specified through the silts, sands, gravels and clays of the glacial sedimentary deposits and the high watertable levels."
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Early breakthrough for Hitachi EPBM
- Given the logistics of the urban project, mucking of the tunnel drives is only permitted at the Roosevelt or North Portal construction sites. One possibility for bidding contractors is to work from Roosevelt station and use three TBMs, two working south to the U-Link station at University of Washington and another to work in the opposite direction, completing the two short drives to the North Portal.
- With TBMs and crews in place in Seattle, companies in the tunneling JVs that recently completed the U-Link drives - Traylor Frontier Kemper on U-220 and the Jay Dee/Coluccio/Michels on U-230 - have registered interest in bidding the Northgate extension work. Sound Transit also confirmed major interest from other US tunneling contractors and international firms that have interests in North America.
- "We expect good competition for the contract," said Davis. "As well as a poster display and papers presented at the recent NAT conference in Indianapolis, and we have conducted an outreach program into the contracting community. The U-Link experience has provided a great deal of information that we are building into our project and while the machine and method is not specified, we will be looking for modern state-of-the-art elements and features on the proposed machines that will prepare them for the wear and tear they will encounter on the job."
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Northgate challenges
As well as working in the heart of an urban and university campus environment, the Northgate contractor will face similar challenges to those mastered successfully on U-Link. These include tunneling under sensitive structures, under high overburden and shallow cover, on steep down and uphill gradients, and through complex geology with variable behaviors and conditions.
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Fig 2. East Link route
- "In bringing the U-link excavation work to an end, we must compliment our tunneling contractors [Traylor-Frontier Kemper on Contract U220 and Jay Dee, Coluccio, Michaels on Contract U230] again for they work," said Gildner. "The quality of work is excellent; their safety record has been very good; and their management of difficult logistics, including passing under the waterway (Montlake Cut) on U220 and under the I-5 highway on U230 with very shallow cover, has been outstanding, as has their management of heavy construction within urban communities."
- The U-Link and Northgate extensions in Seattle continue a long tradition the city has with tunneling and LRT work will continue. As well as the Northgate extension, Sound Transit is progressing the East Link extension through the existing Mount Baker Ridge highway tunnel, across Lake Washington on the I-90 highway floating bridge and on through a cut-and-cover tunnel in downtown Bellview to Overlake (Fig 2). Funding for the project is also in place and construction is programmed to begin in 2013, for an opening of service in 2020.
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Success in Seattle as TBM holes through - TunnelTalk, March 2012
Seattle LRT breakthroughs - TunnelTalk, November 2011
Owner's approach to Seattle's U-Link extension - TunnelTalk, July 2010
Mobilization of Seattle's U-Link extension - TunnelTalk, July 2010
Build-out plans for the Seattle LRT - TunnelTalk, July 2010
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