Hyberbaric inspections begin in Seattle 22 Jan 2014
WSDOT News Release
- Thirty five hours of man-entry time in Bertha's 5ft wide excavation chamber have revealed a large boulder or piece of concrete in a cutterhead opening, as well as previously reported metal well casing.
- Inspections of the 17.6m diameter Hitachi Zosen-manufactured machine under hyperbaric conditions at 1.4 bar pressure began on January 17, said Washington State Department of Transportation (WSDOT) officials in a statement released Tuesday (January 21).
Exploratory drilling around the cutterhead
- WSDOT confirmed that it had spent much of December, since tunneling was stopped on December 8, preparing for this week's hyperbaric intervention by drilling 10 wells - four on either side and two immediately ahead of the face - to lower the water pressure in and around the machine (Fig 1). But it is still too early to say what ultimately caused the machine to meet resistance, said officials.
- WSDOT said: "Since hyperbaric inspections began on January 17, crews from Seattle Tunnel Partners (STP) have spent more than 35 hours inside the excavation chamber gathering information about what might have caused increasing resistance at the front end of the machine before tunneling was stopped on December 6.
- "While the inspections are under way, it is too early to speculate on what led to the tunneling stoppage. Over the weekend, crews inspected a portion of the cutterhead as well as the cutting tools they were able to access. They also cleaned the spokes and removed a bent piece of metal well casing and plastic PVC pipe. Crews also identified a large boulder or piece of concrete material in a cutterhead opening.
- "The information from the inspection along with other data will be reviewed by a tunneling operation task force, which has been convened by WSDOT. These inspections are methodical. Workers are going spoke by spoke to clean off the tunnel muck, inspect parts and make necessary repairs. Air is being pumped into the chamber to stabilize the ground in front of the machine so crews can safely work in areas that would otherwise be filled with soil and water. After pressure in the chamber was lost, crews spent Sunday re-establishing the required air pressure in the chamber so inspection work could continue."
Managers explain TBM stoppage in Dec
- STP has already completed a number probe drills and three larger-sized exploratory shafts in front of the cutterhead as part of its efforts to ascertain the nature of the problem or blockage that was causing resistance to the machine. But visual inspections carried out by a team member who was lowered 60ft into the shaft were inconclusive, forcing STP to change tack and opt for inspection of the excavation chamber behind the cutterhead under hyperbaric conditions. The shafts all were filled in by January 13 said WSDOT, although a drilling a man-sized fourth shaft remains a possibility.
- The next step will be to rotate the cutterhead through 180 degrees to enable inspection of the lower section that is currently submerged.
Investigating the Seattle mega-TBM stoppage - TunnelTalk, January 2014
Dewatering to help deal with stuck Bertha - TunnelTalk, December 2013
Technical details of Seattle mega-TBM - TunnelTalk, December 2013
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