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Thu January 23, 2014
In Seattle, No Simple Answers For A Stalled Tunneling Machine
Originally published on Fri January 24, 2014 9:38 am
Ever wonder what happened with Bertha — the world's biggest tunneling machine, stuck under Seattle? We last checked in on the story right before Christmas, when engineers were preparing to send down inspection teams to identify the blockage. People (OK, the media) were having a grand time, floating ridiculous guesses about what the mysterious object might be. An old ship? Dinosaur bones? Bigfoot?
One month later, there's still no clear answer. Certainly nothing headline-grabbing.
At first, the blame was directed at a steel pipe: On Dec. 3, Bertha's cutting face hit the 8-inch diameter, 119-foot-long casing for a research well drilled a dozen years ago by teams working for the state department of transportation. The casing was never removed, and/or someone forgot that it was directly in Bertha's path. The blame game on that is just getting started.
You might wonder why a pipe would matter, since Bertha is designed to chew through boulders. But Bertha's five-story-tall rotating cutting face is meant to fracture rock, not ductile materials like steel. That pipe bent when Bertha hit it — imagine running over a jump rope with your lawn mower.
But now it seems the pipe may not have been the only cause of Bertha's troubles.
Even though the pipe fragments have now been removed, the giant tunneling machine remains stationary. For the past week, specially trained crews have been performing "hyperbaric inspections" in the claustrophobic space around the cutting head, and they've found more potential snags, including a rock that was almost exactly 3 1/2 feet wide. The rock was slightly too big to be "digested" by the conveyor system that moves rubble away from the cutting face, but too light to be fractured into smaller pieces by the cutting face. It's not something that could stop Bertha by itself, but it's a symptom of a bigger problem.
It's like trying to crack a nut with a hammer without being able to hold the nut down. Bertha has a harder time fracturing rocks when those rocks are light — or loosely compacted. At the moment, the machine is only about 50 feet down, in the loosely compacted fill of Seattle's waterfront. Once the machine gets deeper under downtown, engineers assume the more tightly packed soils will hold rocks in place firmly enough to let Bertha chew through.
So, no dinosaur bones. And to be clear, Bertha isn't really stuck. The department of transportation describes the situation as one of "increasing resistance" at the front end, and operators want a full picture of the situation before they put the $80 million machine back in gear.
Or, as the project manager put it to an impatient legislative committee (one more metaphor!): "If it was your car, and all your warning lights were on and you still needed to make a trip, would it be wise to make that trip?"