Investigating the Seattle mega-TBM stoppage
03 Jan 2014
TunnelTalk reporting
- It is an 8in diameter piece of steel pipe in the cutterhead that is reported as causing the Seattle TBM stoppage, which is now in its fifth week.
- After spending most of December reducing groundwater pressure around the machine, a WSDOT news release explained that a safe man-entry intervention on 2 January found the 8in (200mm) steel pipe protruding through an opening in the machine's cutterhead. A set of 17 exploratory probes drilled from the surface to look for objects that might be blocking the machine's path also detected metal in front of the machine.
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Casing caused trouble for the mega TBM
- Of greater implication, the news release by the project client, said that the existence and location of the steel casing was known of and documented in reference materials supplied for preparation of the project's design-build contract. It is identified in the documentation as one of many borehole casings installed in 2002 following the 2001 Nisqually earthquake to help geologists better understand how groundwater moves in the area. Although included in contract documents, it nevertheless provided reason enough to stop excavation by the 17.5m diameter machine and investigate why positive progress of 7m x 2m rings per shift in previous days was being slowed and baulked.
- In explaining the situation at a media event on Friday 3 January, Matt Preedy, Alaskan Way Viaduct Replacement Program Deputy Administrator, admitted that "this is just the first step in determining how this might have been obstructing the machine" and that "further investigation is needed to see if there are other factors that could have contributed to the blockage."
- In the weeks since the stoppage was announced, there have been many suggestions and ideas of what it is that might be causing the hold-up. 'Monday morning quarter backs' and 'armchair engineers and geologists' have been second-guessing the engineers who worked on the design and manufacture of the machine and who have been working on the project since its early planning stages and the start of the project by STP (Seattle Tunnel Partners, the JV of Dragados/Tutor Perini) as the design-build contractor for the WSDOT (Washington State Department of Transportation) contract.
- Suggestions of insufficient site investigation seem to have been debunked with the recent announcement that the borehole casing was known and included in the contract documents. Accusations of deficient design of the machine and its systems could also be levelling criticism from the point of view of hindsight. Other possible causes are offered by engineers who draw on extensive personal experience.
- The facts, as reported, were that, after good progress in the previous shifts, advance gradually slowed, torque increased and the decision was taken to stop and investigate. An immediate possible cause was an obstruction in front of the machine, either manmade or natural such as a foundation pile, a buried obstruction or a massive boulder. These were possible causes that harbored the least amount of anxiety for all involved. Other manageable causes stem from possible mistakes in operating the machine during its learning curve with the chamber becoming packed solid with material, or inefficient or ineffective application of the vital EPB conditioning system, the conditioning materials and the injection processes.
- Other possible explanations, that no one dared utter, centered on major mechanical failure of some part of the cutterhead drive unit or a situation of some part of the shield causing the cutterhead to jam and prevent its rotation - things that would cause major delay, expensive recovery, and the straining of contractual relationships.
- The agreed plan of action to install a set of dewatering wells to draw down the ground water and allow for extended man-entry interventions in lower compress air pressures had the contract or client presenting a unified front at the start of the stoppage when Matt Preedy of WSDOT and Chris Dixon, Project Director of STP presented the first media briefing.
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Managers explain TBM stoppage in Dec
- News of the hold-up also had many second guessing decisions made by STP in being selected to build the world record setting double-deck highway tunnel and in selecting a 17.5m diameter EPBM from Hitachi Zosen in Japan for the excavation. Some decisions that could be questioned include the decision to take off the disc cutters and replace them on the cutterhead with excavation bits, although the response is that the quality and performance of bits is now far superior to what it used to be, and more fundamentally the decision to use EPB in preference to slurry technology which was the strong recommendation by Herrenknecht in its response to bid for supply of the machine and as stated again at the WTC conference in Geneva in June 2013. In speculating about a more serious mechanical issue, many recall the mechanical problem found during fully assembled commissioning of the machine in December 2012 that delayed its shipment from Japan.
- Whatever else may yet be confirmed about the hold-up, STP tunnelling crews carry on with removing the irksome steel pipe and exploring for signs of any other potential trouble.
- In speaking about the possible consequences to the project, Preedy said that "it is still too early to know how this will effect the schedule and budget" but running now behind its planned progress program is a given and time that STP will be looking to make up on other parts of the project.
- This begins with STP workers preparing the launch pit with the systems needed to progress the planned concurrent program of works to build the highway structures inside the tunnel as excavation progresses.
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Dewatering to help deal with stuck Bertha - TunnelTalk, 20 December 2013
Technical details of Seattle mega-TBM - TunnelTalk, 20 December 2013
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