The world's largest tunnel machine, Bertha, is stuck 60 feet beneath Downtown Seattle. The tunnel contractor has drilled down, pumped out and pressurized a gap in front of the machine and is sending divers down 24/7 to try to figure out what brought the machine to a standstill nearly seven weeks ago.
According to the Washington State Department of Transportation (WSDOT), diver teams are working in shifts examining the cutterhead from an approximately 14-inch vertical crawlspace created between a claylike Bentonite barrier sprayed into the ground ahead of Bertha and the top half of the towering, five-story tall behemoth.
Depending on what they find, here are five potential scenarios for Bertha's future, ranging from inconvenient to disaster:
Scenario One: Pipe Gone Wild.
Divers may find more remnants of the 8-inch diameter steel pipe tangled within the cutterhead of the machine. If so, they'll need to clear it out and determine how badly the cutterhead may have been damaged by the encounter. That will take time, but the bigger question will be how much metal junk remains in the 9,000 foot path ahead of the machine. If there is more, then finding it, removing it — and being certain about it — will take time and resources.
Scenario Two: Big Rock Indigestion
Divers may find large rocks or boulders jammed into the machine. In that case, the fix may be to change some of the soil cutting teeth back to disc-style rock cutters like those used when the machine launched from its concrete starting pit. Disc cutters can break rocks, provided they can get a purchase on and exert enough pressure to do it. If not, then some other plan will be needed to remove large rocks. Bertha does not have a secondary rock crusher mechanism as do some other TBMs.
The tunnel route is a diverse patchwork of ground conditions including "non-cohesive" sandy, gravelly, clayish earth, all below the water table. Similar to the metal pipe, it is hard to know beforehand how many rocky boulders lurk in the mix along Bertha's path.
Scenario Three: A Bearing Surprise
At the core of any TBM is the vital main bearing. There is a small chance, observers worry, of problems related to those that Bertha's builder, Hitachi Zosen, had and fixed during testing of the machine at the factory. Bertha is a giant, and the pressures at the bottom of the machine vary significantly from those at the top of the machine, but TBMs are routinely built to cope with that fact.
It would take something truly extraordinary to prang the main bearing, but if it were damaged, it would be a major showstopper. UK Tunnel expert Alastair Biggart: "The heart of… any TBM, is the main bearing. If this should be damaged during tunnelling it is a major problem for the project. It is possible to change a main bearing within the tunnel, but extremely time consuming and expensive."
Scenario Four: The Biggest Problem
If the divers take a good hard look and don't find any obvious problems, that could mean the problem is Bertha's design itself. This would be the biggest problem of all. TBM selection is often the most crucial decision of a tunnel job. The situation in Seattle may turn out to settle a contentious technical question: Which of two competing types of machines was best for this job?
Tunneling here is known to be tricky. "We can never take for granted the tendency of the soils in Seattle to misbehave if given the opportunity," consulting project managers from the firm Hatch Mott MacDonald wrote in a report to WSDOT.
To this day, there is industry disagreement about what machine design is best for Seattle's conditions. In its "design-build" process, WSDOT specified a category of machine (closed face pressurized), but left the design choice up to the main contractor. Two different types of machines were contenders for the work: An Earth Pressure Balance machine (which Bertha is) or a Slurry/Slurry Mixshield machine. There are important technical differences between the two and each has vocal proponents.
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