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.

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.

The machines operated 24hr/day, 5 days/week with a full maintenance shift per week on each. The best advance rate for a 24 hour period was 175ft or 53m, with averages of 70ft/day set against design averages of about 50ft/day. As the TBMs progressed, they built the 5ft (1.5m) wide rings of bolted and gasketed precast concrete segments with five segments and a key in each. Excavation of the cross passages and....

....waterproofing set before casting concrete

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.

Gallery

References

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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