Shani Wallis, TunnelTalk
- All TBM excavation on the University Link extension of the Sound Metro LRT metro in Seattle is complete and for the Traylor Frontier-Kemper (TFK) JV on contract U-220, its two Herrenknecht EPBMs completed the 2-mile (3km) drives three and four months ahead of schedule. The two 6.59m o.d. machines were launched in May and June 2011 on an expected 13-month timeline to complete the 2 mile drives through variable glacial deposits in which abrasive ware and high groundwater pressures were expected.
- When the machines holed through at the Capitol Hill Station box in March and April 2012, the first in the southbound tunnel was three months ahead of schedule and the northbound machine came in four months earlier than program.
- "The machines performed exceptionally well and we had great crews on the shifts," said Mike Krulc, the JV's Project Manager with Traylor Bros. "These were anticipated to be difficult drives with tough geological conditions under a high overburden and groundwater table that would require pressures of up to 6 bar for man-entry maintenance. As it happened we had no interventions under pressure. All were achieved in geological areas that could support man-entry under atmospheric pressure."
- The number of interventions was also less than anticipated, explained Krulc. "There are 16 cross passages to connect the parallel drives and the contract specified an intervention at each. This was reduced to a total six interventions for both machines with one full set of tool changes on both cutterheads at about mid-point, before passing under the highest overburden and highest anticipated hydrostatic pressures."
- The reduced number of interventions is attributed to various factors. First, the cutterheads of the TBMs were described as the most resilient in design to ware. The
- Seattle Sound Transit announces the underground University Link light rail extension will open to passenger service in March, six months ahead of schedule and more than $150 million under budget.
Inside the U-Link tunnel Photo: John Walser
- “University Link opens March 19, changing forever how we move around Seattle,” said Sound Transit Board Chair and King County Executive Dow Constantine. “With fast, frequent trains bypassing some of the region’s worst traffic, thousands of people will now be able to get to work, school and appointments on time, every time.”
- The 5km extension to the existing light rail line that opened in 2009, includes underground stations at Capitol Hill and at the University of Washington (UW) near Husky Stadium. The trip from UW to downtown Seattle will now take eight minutes, bypassing some of the most congested traffic in the region.
- Early opening of the new line is largely down to early completion of EPBM tunnelling by both the Traylor Bros/Frontier-Kemper JV, using two Herrenknecht machines for execution of Contract U220 between UW and Capitol Hill; and the Jay Dee/Coluccio/Michaels JV, using a Hitachi Zosen machine for execution of Contract U230 between Capitol Hill and the working shaft to the south west.
- Fewer-than-expected hyperbaric interventions, and a better-than-expected advance rate through the prevailing geology of variable glacial deposits, ensured that mechanised tunnelling was completed by August 2012.
- Through the efforts of US Senator Patty Murray, a senior member of the Senate Transportation, Housing and Urban Development Appropriations Subcommittee, and other congressional leaders, Sound Transit secured an $813 million federal grant towards the $1.8 billion project.
- “As the Puget Sound region continues to grow, we need to make transportation investments that make our communities more liveable, create jobs, improve access to education centers, and support our local small businesses,” Senator Murray said. “That's why I am so thrilled to see Sound Transit reach this milestone on this important project for commuters and communities, and it’s why I am going to keep fighting for local investments like these that help our economy grow.”
- TBM excavation by the Jay Dee/Coluccio/Michaels JV continues on the 7km Northgate Link that extends the University Link further north; while SEM construction is scheduled to begin later this month (February) by Atkinson Construction for the underground Downtown Bellevue section of the new East Link.
Sound Transit News Release
- machines were dressed with double disc cutters to attack hard over-consolidated clay and deal with anticipated boulders; a foam soil conditioning system minimized wear of machine body parts; a wear detection bar warned of any excessive ware; and lateral wear protectors around the rim of the cutterheads had additional tungsten carbide bits that had them emerge after 2 miles as if new.
Fig 1. Complex geology and steep gradients conquered on the U-220 contract from UW to Capitol Hill stations
- The machines were also fitted with a camera that could look into the excavation chamber and check for signs of excessive ware, bypassing the need for the frequent and regular man-entry inspections. The camera on each machine is fitted to an aperture in the bulkhead and provides a back view of the cutterhead when the muck in the chamber is drawn down. A second aperture provides light into the darkness. The camera looks through the lens aperture into various parts of the excavation chamber and can identify the amount of material or water passing through the cutterhead into the emptied excavation chamber.
Breakout for cross passage excavation
- According to Krulc, half of the earlier than program progress was due to advancing faster than expected through the geology and the other half was the saving of long pressurized man-entry interventions that had been built into the program. "The experience proved that EPB was definitely the way to go," he said. "There is more clay in the ground than would have been efficient for a slurry system. The biggest problem we had was preventing the material from clogging at the transfer from the screw conveyor."
- A continuous conveyor muck haulage system was specified by the contract to cope with steep gradients on the vertical alignment. These performed well once early problems of material running back down the uphill running conveyors and during heavy rains at the working shaft were sorted. In-depth testing of anti-clogging agents and soil conditioners, together with better understandings of the ground behavior, saw progress rates and performance improved significantly.
- Other positive outcomes for the drives included no stoppages for dealing with boulders that were anticipated by the contract's GBR (geotechnical baseline report) and assistance of specified belt weighers on the muck haulage conveyors. "We hit pockets of cobbles that did not really affect us," said Krulc, "and once everyone understood their limitations, the belt scales provided useful information. Everyone had to understand that the data they provided was good to use in relative relation to past data and that it was not necessarily numerically accurate."
TBM recovery and cross passage progress
From their launch in the University of Washington station box at the north end of the U-Link, the machines broke through into the Capitol Hill station box and had to wait for access to the U-230 contractor's job site to lift them and have them trucked to a storage yard about 60 miles away. With the running tunnels of the Seattle LRT Northgate extension to be constructed, the JV is eyeing up a roll-on for both the TBMs and the tunneling teams.
Bored tunnel now ready for finishing
- In the meantime, excavation of the 16 cross passages for the U-220 contract is progressing. NATM or SEM excavation of the 6-7m long passages started and advanced concurrent with progress of the southbound TBM. From a raised platform in the running tunnel, topheadings of each passage progressed while maintaining service train traffic to the TBM under the platform. When TunnelTalk contacted the job site in early August, fifteen of the passages were fully excavated, with the final lining of a PVC waterproofing membrane and cast in-situ concrete complete in two.
- Work on the passages was reported as progressing well except for one. "Cross passage 9 under the highest overburden and high groundwater pressure was expected to be problematic, but it was moved to better ground and was completed without issue," said Krulc. "It is cross passage 17, also under high overburden, that is experiencing more water ingress than anticipated. The topheading is complete and we are installing dewatering wells from the tunnel as well as from the surface to stabilize conditions and get it finished."
- With the TBM in the parallel northbound drive also finished, the tunnel is clear for laying the track bed and all is on target to close out a successful and profitable contract. In progressing the contract, the JV has encouraged a group of new union-labor recruits into the tunneling industry. Of these a notable set, for a total of some 6% of the full JV's work force, are women. Video interview of a lady loco operator confirms that, for her, the work, the workplace environment and her colleagues make for an enjoyable and rewarding job.
Tunneling on the U-220 contract for the Seattle LRT - TunnelCast, August 2012
Success in Seattle as TBM holes through - TunnelTalk, March 2012
Mobilization of Seattle's U-Link extension - TunnelTalk, July 2010
Owner's approach to Seattle's U-Link extension - TunnelTalk, July 2010
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