Paula Wallis, Reporter

Two of three TBMs on Seattle’s $1.8 billion Brightwater conveyance tunnel are at a standstill for major repair work that could take months to complete.

Brightwater Construction Manager, Judy Cochran confirmed that the cutterhead on the refurbished Herrenknecht machine (BT2) from the West CSO project in Portland is experiencing unusual wear and will need extensive repairs. "The contractor has identified quite a bit of wear at the outer rim of the BT2 machine’s cutterhead,” said Cochran.

Brightwater conveyance tunnel plan

“The wear is not on the ground surface side, but on the backside of the cutterhead where it meets up with the shield.”

The excessive wear was identified about a month ago, shortly after the TBM resumed its eastbound drive in March to the North Creek shaft and pump station. This followed a lengthy shut down in December for extensive wear repairs to the cutterhead that took three months to complete.

Cochran said the BT2 machine stopped mining on this occasion on May 26 after the contractor, Vinci/Passons RCI/Frontier-Kemper JV, tried moving forward to find a spot where they could do repairs at lower pressures. The machine was driving at 5.3 bar and above, and is about 7,000ft into its 11,600ft drive.

“The JV is working on a plan to relieve water pressure and preliminary, that plan is to drill in some dewatering wells from the surface and hopefully do the repairs at atmospheric pressure or at the very least at low pressures,” said Cochran. “I assume it will be a welding repair. They will have to rebuild the edge of the cutterhead rim and reinstall hard facing.”

In the meantime the contractor inspected the second Herrenknecht machine (BT3) and discovered similar damage to the cutterhead, although not as extensive. That machine was purchased new for its 20,100ft (6.2km) drive in the opposite direction from the common working shaft towards the Ballinger Way reception shaft and is about halfway though at roughly 10,000ft. The machine also has a broken mixer bar in the cutterhead that needs repair.

“They are on stop too,” said Cochran. “Unfortunately they have increasing pressures ahead of them, increasing depth, and they are close to the edge of this Lake Forest Park aquifer that we’ve all been very concerned about. They’re also in potentially squeezing ground. So they would like to accomplish their repairs sooner rather than later.”

March, 2009 sinkhole

BT3 machine had been mining in fits and starts for the last two weeks looking for a better place to stop for the repairs. Its alignment is running below homes and the team wanted to be at least between homes to conduct the repair work. This is the same machine that was responsible for a large sinkhole that opened up in March at the bottom of a residential driveway. The sinkhole was blamed on over-excavation by the TBM.

The cutterhead damage is a new concern. Determining the cause of the unusual wear, how to repair the damage, and how to ensure the TBMs will be able to finish their drives will be the focus of a Friday, June 12 meeting with the Contactor, Client and representatives from Herrenknecht.

“We all believe the TBMs can finish the job,” said Cochran. “Both of them have lots of challenges ahead including crossing an Interstate highway and improving their chances is on every bodies minds." Cochran wouldn’t specify how long the repairs would take, but agreed it would take several months.

The LOVAT EPBM (BT4) for the Jay Dee/Coluccio/Taisei JV on the West Tunnel is a little more than halfway through its 11,600ft drive and is expecting to see higher pressures. The first half of the drive was mostly in 1 to 2 bar, and they are expecting to be under 5 bar shortly. The contractor had planned a major stop to put in a California switch, do some continued testing on the machine to demonstrate that the team could do maintenance at the higher pressure if they needed to, do a major change out on cutting tools, and also complete some work on the guillotine doors. Greg Hauser, Project Manager for the contractor said the TBM stopped mining May 20 for roughly three weeks to complete the necessary work. The machine resumed operations June 8 and completed 400ft its first week back up.

TunnelTalk caught up with Cochran as she was coming from the project’s regular quarterly DBR meeting. She characterized the discussions as good over all in as far as understanding the technical issues in front of the contracting teams. “They have been more than willing to share with us their findings and have ask us to make sure we have people with expertise available, even at this meeting on Friday, because if there’s something we see that they didn’t happen to catch, questions or issues that we have, they’d like to hear about it from us. So I think communications on building the job and doing the best we can to get it completed, which is in every bodies interest, is pretty good,” she said.

However the client is expecting to receive a claim shortly from the Vinci/Passons RCI/Frontier-Kemper JV, which will present a case that the overall ground conditions differ materially from what was presented in the contract documents.

Meanwhile the repair work will likely push the project off its 2011 completion schedule. By how much remains to be seen.

References

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