Photo courtesy of seattle.gov. A close-up look at the cutterhead of the world’s largest-diameter tunneling machine.
SEATTLE (AP) — Experts say they’ve at least partially solved the mystery of what’s been blocking a massive tunnel-boring machine beneath downtown Seattle for the past month: a steel pipe.
The $80 million, five-story-tall machine, dubbed "Bertha," is digging a new path for State Route 99, one of the region’s primary north-south arterials. But it became stuck Dec. 6, 60 feet below Seattle streets. In the ensuing weeks crews have probed, sifted and speculated in an effort to figure out what was causing the delay.
Much of the city got in on the act. Some fingered a giant boulder — a "glacial erratic," in the parlance of geologists quoted in the press. Students in a fifth-grade class wrote essays with their best guesses, including the remnants of a train left under the ruins when the city rebuilt after the Great Seattle Fire of 1889.
Ivar’s restaurant, a local institution known for its not-quite-credible publicity stunts, promoted the idea that it was a 70-foot "clamosaurus" that escaped the restaurant’s founder in 1937.
The real answer is less exotic, and only a little less baffling: an 8-inch-diameter steel pipe that the state Department of Transportation already knew about.
The department installed the pipe in 2002, shortly after a 6.8 magnitude earthquake in 2001, as a well casing to better monitor groundwater flows in the area. In a news release Friday, officials said the location of the pipe was included in materials provided to the contractor on the $1.4 billion tunnel project, Seattle Tunnel Partners. But the contractor reported that it didn’t know the pipe was there.
The boring machine, billed as the largest in the world, can grind through rock, dirt and timber — but not thick metal.
Crews worked extensively over the holidays to try to identify the mystery blockage. They examined Bertha’s excavation chamber, just behind its cutter-head; drilled wells to relieve water pressure around the machine; and earlier this week drilled 17 exploratory holes near the front of the machine to find the obstruction.
They encountered obstructions in four of the holes, the DOT said.
Finally, on Thursday, enough soil had been removed and the water pressure had dropped enough that crews could inspect the top 15 feet of the excavation chamber, the department said. That’s when they saw the pipe protruding through one of the many openings in the cutter-head.
Experts believe some of the obstructions found in the exploratory holes are pieces of the pipe, but the DOT also says it’s not sure the pipe is the only thing blocking Bertha. There could be other obstructions that aren’t yet visible, or changing soil conditions might have caused excessive wear of the machine’s cutting tools.
Seattle Tunnel Partners is considering several options for removing the pipe and identifying other potential obstructions. It’s too soon to know how much the delay will cost, or how it will affect the long-term schedule of the 1.7-mile tunnel project, officials said.