Dewatering to help deal with stuck Bertha
20 Dec 2013
Shani Wallis, Peter Kenyon and in Seattle, video reporter Sterling Noren reporting for TunnelTalk
- Six of an increased number of ten dewatering wells had been installed by last Friday (20 December) in Seattle to aid investigation of what has caused a forced stop of Bertha, the world's largest ever 17.5m diameter EPB TBM excavating the Alaskan Way highway viaduct replacement tunnel.
- Information in a press release from WSDOT, the Washington State Department of Transportation client for the project, said that more than 300,000 gallons of water had been extracted by operating wells to that point and that more would be extracted before the groundwater table around the machine was lowered sufficiently to allow for safe and extended man entry into the cutterhead chamber to deal with whatever is holding up progress.
- The 17.5m diameter machine currently stands about 60ft (18m) - still with only just one TBM diameter of cover - below the junction of South Jackson and South Main streets, to the west seafront side of the viaduct (Fig 1). On Wednesday of last week (18 December), crews were able to turn the machine's screw conveyor and remove some material from the excavation chamber to allow a quick view through one of the man locks at the top of the machine, states the WSDOT release. It continued that "nothing extraordinary" was observed in the chamber before it began to fill quickly with water and the lock door had to be closed.
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(Images and video by WSDOT)'>
Fig 1. The TBM stands about 18m below the surface and to the west of the highway viaduct which it will pass under to arrive at the next and last safe haven - see the route to be travelled
(Images and video by WSDOT) -
Fig 2. Construction of the interior highway will progress as a concurrent operation with TBM excavation
- Since the brief visual inspection into the chamber yielded no obvious clues as to the cause of the holdup, the WSDOT statement explains that lowering of the ground water around the machine will continue through the Christmas holidays to reduce the necessary compressed air pressures and allow for full man-entry interventions and inspections out in front of the cutterhead in the early new year. During the holidays, crews will also undertake maintenance of the machine and the launch pit will be prepared for the planned concurrent program of works to build the highway structures in the tunnel as excavation progresses (Fig 2).
- The machine and came to a halt in early December when design-build tunnel contractor STP (Seattle Tunnel Partners - a JV of Dragados and Tutor Perini) experienced "unanticipated and increasing resistance" to TBM progress. As explained by Chris Dixon, Project Manager for STP, and Matt Preedy, Deputy Project Administrator for WSDOT, in the video reports below, STP took the proactive decision to stop the machine and plan for an unscheduled intervention to investigate the possible causes.
- With more than a bar of pressure difference across the vertical height of the 17.5m diameter cutterhead, the high groundwater table, in combination with loose soils of the area close to the sea front and still beneath a shallow cover of just one TBM diameter, has prevented safe man intervention under pressures that would have been needed without lowering the ground water table.
- Before the current holdup, the TBM was making impressive progress with seven 2m wide rings in a 12 hour night shift recorded as the best advance to date. To early December, the machine had advanced 300m (1,000ft) into its 2.7km (1.7 mile) drive to excavate the double deck highway tunnel and had completed some 243m (800ft) in the 10-week period from restart of mining after mucking away by barge from the waterfront resumed on 23 September 2013.
- Since 17.5m diameter TBM Bertha launched on her world record drive beneath the streets of Seattle on July 20 (2013), the project has been hit by a set of advance delays.
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Chris Dixon, Project Manager, STP |
Matt Preedy, Deputy Administrator, WSDOT |
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TBM Bertha is at the 1,000ft mark
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Muck is now being barged away after labor dispute was resolved
- On August 20, a labor dispute broke out between STP and the Longshoremen's Union over whether its members or those belonging to STP's construction union should load muck onto barges at the wharf. No progress was made for 34 days and at one stage STP made contingency plans to use trucks to move muck away from the job site. Eventually the intervention of Governor Jay Inslee helped the disputing parties to reach an agreement.
- Then on November 14, following a further two-week shutdown for machine adjustments and the fitting of new cutting tools, progress was halted over night to infill a 7ft deep sinkhole that formed west of the Alaskan Way Viaduct at South King Street. The possibility of the small sinkhold was predicatable under shallow cover of less than half a TBM diameter at the start-up, after leaving the treated ground of a second safe haven, and after advancing from beneath the concrete slab that had afforded buoyancy protection in the loose waterbearing soils and the minimal overburden at the early stages of the tunnel alignment.
- TBM crews reported reaching the 1,000ft mark on December 5, 450ft short of reaching zone 2 of the drive at a depth of 60ft.
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Video: Technical tour of the record setting TBM - TunnelCast, December 2013
Dawn restart for trouble hit drive - TunnelTalk, September 2013
Seattle TBM hit by labour dispute - TunnelTalk, September 2013
Inauguration of world record EPBM - TunnelCast, July 2013
Seattle prepares for mega-TBM assembly - TunnelTalk, May 2013
Mega-EPBM for Seattle tested in Japan - TunnelCast, January 2013
Keeping Seattle safe on Alaskan Way TBM drive, - TunnelTalk, February 2012
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