Paula Wallis, TunnelTalk

Dewatering wells are operating full time to lower the watertable and reduce the groundwater pressure on Seattle’s $1.8 billion Brightwater conveyance tunnel and allow repairs on two of the project’s TBMs at atmospheric conditions.

Brightwater conveyance tunnel plan

The slurry machines have been idle for several months as engineers devise and execute a plan to repair their cutterheads. The 17.7ft (5.1m) diameter Herrenknecht TBMs experienced unusual wear of the rim on the backside of the cutterhead.

Work is further along on the BT2 machine said Brightwater Construction Manager, Judy Cochran. “Crews have drilled in and are operating six dewatering wells from the surface and they have also drilled in some drains from inside the tunnel. This has reduced the water pressure down from about 5 bar to around 1 bar.”

Dewatering wells are also planned to depressurize the ground for repairs on the BT3 machine “The contractor is currently installing two exploratory bore holes,” said Cochran. “They will finish the first one this week (October 16) and will begin on the second one next week."

The BT2 machine will need extensive repair work, that will entail significant welding which was a key factor in the decision to do the work at atmospheric pressure.

The BT2 machine stopped mining on May 26 at about 7,000ft into its 11,600ft eastbound drive to the North Creek shaft and pump station on the Vinci/Parsons RCI/Frontier-Kemper JV’s Central contract.

The BT3 machine for the same JV was about halfway through its 20,100ft (6.2km) drive in the opposite direction from the common working shaft towards the Ballinger Way reception shaft when similar wear to the cutterhead was detected, although not as severe.

Cochran says the cause of the damage is the subject of a lot of discussions, but would not elaborate. She also would not give an estimated time line for the repairs or for when the TBMs were expected to resume excavation.

The repairs are impacting construction of the pump station under a different contract. “The receiving shaft for the BT2 machine has a build-out in it that was included in the pump station contract,” said Cochran. “Originally our plan was to complete the build-out after the tunneling work was finished, but as we don’t know how long the repairs will take we have made some changes to enable the pump station contractor to complete that work without delay.”

The LOVAT EPBM (BT4) for the Jay Dee/Coluccio/Taisei JV on the West Tunnel has completed 15,332ft of its 11,600ft drive for a total of 74% complete. The machine experienced ground pressures from 0-55psi between August and October. Since the installation of a California Switch in June the JV has made several maintenance stops to change out rippers, scrappers and the nose cone said Cochran.

Meanwhile, as work continues to repair the TBMs on the Central contract, engineers are assessing ways to mitigate delays and keep the project as close to its 2011 completion schedule as possible.

References

Brightwater TBMs in trouble - TunnelTalk, Jun 2009
Sinkhole bothers Brightwater - TunnelTalk, Mar 2009
Brightwater update - TunnelTalk, Jan 2009
Fatal collapse on Cologne’s new metro line - TunnelTalk, Mar 2009
Bored tunnel for downtown Seattle - TunnelTalk, Jan 2009

King County’s Brightwater Project website