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SCOTT SIMON, HOST:

Contractors working for the State of Washington are planning a high-stakes operation to try to rescue Bertha. That's the nickname for the world's largest tunneling machine which was digging a highway under downtown Seattle. But Bertha has been stuck in the mud and rocks since December.

NPR's Martin Kaste reports from Seattle.

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MARTIN KASTE, BYLINE: Bertha is right here, on Seattle's waterfront halfway between South Main and South Jackson Streets and about 60 feet straight down. At first, they thought the machine was being stymied by a big glacial rock. Then the attention focused on the chewed-up remains of a metal pipe. But now, now it seems Bertha's ailment is actually mechanical.

JOSEPH WARTMAN: The problem with Bertha is located right behind the cutter head, which is the very front of the machine.

KASTE: This is Joseph Wartman, an associate professor of geotechnical engineering. At the University of Washington, he points out the key parts of a scale model tunneling machine. The cutter head is the cheese-grater-looking thing at the front. On Bertha, that cutter head is five stories tall, a world record. The problem seem to be in the bearing that rotates that cheese-grater. Apparently some sand got in there.

WARTMAN: Usually there are seals placed to try to keep sand out of the bearings because of course the sand, as you might imagine, is going to wear the bearings down. But it has somehow gotten into the bearings and likely through some kind of failure in the seal system itself.

KASTE: That's bad. The prospect of metal grinding on metal always makes engineers wince. How that happened is still a matter of inquiry and probably litigation down the road. Some fingers are pointing at Hitachi Zosen, the Japanese company that built Bertha. It reportedly found a similar problem during testing, and the fix may not have been good enough. That company is now devising the rescue plan, which is no simple task.

WARTMAN: This isn't like a problem where you pull your car onto the side of the road, get out, open the hood take a look at the engine. The front of the tunnel-boring machine is pressed up against soil, and the tunnel boring machine is not only underground, it's underwater.

KASTE: Still, it could be worse. The good news is the breakdown happened here, on the waterfront. Chris Dixon is project manager for the main contractor on the tunnel.

CHRIS DIXON: As unfortunate as this event has been, we're very fortunate that it occurred where it did, and we're able to access it from the surface.

KASTE: There's a vacant lot here, enough room to dig a 120-foot-deep pit needed to expose Bertha and lift out the parts that need replacing. If the machine had broken down further along, under, say, the skyscrapers of downtown Seattle, digging down would not be an option. They wouldn't be able to back the machine up, either. A person close to the project says they probably could still fix it, but he says that would be sort of be like a dentist doing a root canal without direct access to the patient's mouth. Martin Kaste, NPR News, Seattle.

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SIMON: And you're listening to NPR News.

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