Two Brightwater TBMs hole through
Two Brightwater TBMs hole through Jun 2010
Shani Wallis, TunnelTalk
Good news this week from the Brightwater project in King County north of Seattle. Two of the four TBMs that have worked on the 27km water conveyance tunnel broke into their reception shafts.
Ground freezing stabilization of the high water table around the 62.5m deep Bellingham shaft allowed excavation of the last 45m and breakthrough of the 4.7m diameter Lovat EPBM through the tunnel eye to end the 6.4km long BT4 drive from the Point Wells Portal.
More significantly, one of the two Herrenknecht Mixshields working on the Central Tunnel section completed its east drive from the North Kenmore working shaft into the North Creek pump station shaft. The slurry TBM holed into a partially backfilled shaft of CDF (controlled density fill) to about 6ft above the crown of the tunnel eye and a further 30ft of water.

Lovat EPBM on BT4 breaks in

From mid-February the machine completed the last 4,000ft of the 11,600ft (3.5km) long drive after being held up for more than a year to undergoing repair of extensive wear of the cutterhead rim and leading edge of the shield. Dewatering from the surface as well as from holes drilled through the tunnel lining around the BT2 drive TBM lowered the water table sufficiently to complete repairs from within a chamber excavated ahead of the cutterhead and under compressed air pressures of about 1.3 bar. During the NAT conference in Portland, Oregon this week, Shane Yanagisawa of Frontier-Kemper and Deputy Project Manager of the Central Contract's Vinci/Parsons/Frontier-Kemper JV explained to TunnelTalk that more than 300 man entries into the 1.3 bar pressure repair chamber under some 300ft of cover, were completed during the months of November, December and January to repair the TBM and have it ready to restart on 15 February 2010. "Since then we have completed about a third of the total drive length in about 3.5 months of TBM boring to arrive at the North Creek shaft earlier than the anticipated August breakthrough date. The pumping station contractor, Kiewit, was working in the pumping shaft and we had to wait some weeks before we could push through," said Yanagisawa.
Judy Cochran, Construction Manager for the Project, also explained how hole through of the BT2 Mixshield was important for the overall project program. The North Creek pumping station shaft is needed for operation of the new Brightwater treatment plant, which is on schedule for being completed and commissioned by the Fall of next year. The Kenny/Shea/Traylor JV excavated the pumping station shaft as part of its East Tunnel contract and it was handed over to Kiewit for installing the pumping equipment in January 2009. "Once commissioned, flow from existing sewerage networks in the area will be directed to the new pumping station and from there through the completed East Tunnel drive to the treatment plant. The Central Tunnel's eastbound tunnel drive had to be completed before completing and commissioning the pumping station shaft's electrical and mechanical installations."
The repaired Mixshield completed the 4,000ft drive with two planned stops for maintenance interventions and a series of interim tool inspection stops, explained Cochran. "Further dewatering wells were installed to allow for the two planned maintenance stops to reduce the intervention air pressures and fortunately there was no further need for cutterhead rim or shield edge repairs."

Both Mixshields suffered excessive wear to the cutterhead rim and shield edge

The other Mixshield heading west from North Kenmore to the Ballinger Way reception shaft lies stranded about half way through its BT3 20,000ft drive and under more than 300ft of cover and a high hydrostatic head of ground water pressure. A repeat of the BT2 repair process was not considered possible within a reasonable time and cost. There was also a lack of confidence that, after repair, the TBM would be able to carry on in the complex abrasive glacial conditions and complete the remaining half of the drive with success. The alternative recovery plan is to have the Jay Dee/Coluccio JV continue its West Tunnel EPBM through the Bellinger Way shaft to complete the last 10,000ft of the Central Tunnel drive and junction with the stranded slurry TBM.
At NAT, John Critchfield, a senior manager with the project's construction management team explained that, with the TBM now in the Ballinger Way shaft, the cutterhead will be removed to the surface for a rebuild and the machine and its seals and pressure controlling systems will undergo a thorough overhaul. Once reassembled the TBM will be launched into the alignment on the opposite side of the small diameter shaft and advanced until the rear of the shield is in the shaft and the tail seal can be replaced and other refurbishments undertaken.
The machine's airlocks have also passed pressure tests to 9 bar. On its drive into the Central Tunnel alignment, interventions for inspecting the cutterhead and changing worn tools will require compressed air pressures of up to 7 bar.
The West Tunnel TBM is expected to be relaunched by September and will be serviced from the Point Wells access portal for its extra 10,000ft. Exactly how the two machines will junction is yet to be defined. The expectation is for it to meet the stranded slurry machine by Fall 2011.
The treatment plant can operate with finish of the North Creek pumping station and the East Tunnel drive but delay to commission of the conveyance tunnel to the effluent outfall into Puget Sound due to the Central Tunnel problems is estimated at more than a year.
"Despite the challenges, the breakthrough of the two TBMs on Friday 18 June for the BT4 TBM and Tuesday June 22nd for the repaired slurry TBMs is a big accomplishment," said Cochran. "They mark another important milestone on our overall project program. I can believe that excessive wear was a topic of conversation at the NAT conference. The challenge is to the chemical additive suppliers and machine manufacturers to develop methods for reducing the abrasive affects of these aggressive glacial deposits and to prevent excessively damaging wear."
Brightwater plan and rescue strategy - TunnelTalk, May 2010
Brightwater mobilization - TunnelTalk, Sept 2007
Brightwater update - TunnelTalk, Aug 2008
Sinkhole bothers Brightwater - TunnelTalk, Mar 2009
Brightwater TBMs in trouble - TunnelTalk, Jun 2009
Dewatering assists Brightwater TBM repairs - TunnelTalk, Oct 2009
Brightwater action to limit costly delays - TunnelTalk, Feb 2010

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