Seattle's tunnel boring machine has drilled through the 20-foot pit wall so it can be repaired

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SEATTLE — Bertha, Seattle's tunnel machine, achieved what officials called "a significant milestone" on Thursday when it broke through a 20-foot-thick concrete access pit wall into a shaft so that it can be pulled out and repaired.

Hoses strapped to fences shot streams of water into the pit and plumes of dust wafted into the air as the cutterhead clawed through the concrete and emerged from underground into the 120-foot-deep rescue pit, where workers will remove the front of the machine, pull it to the surface and repair its broken parts.

"It's a very significant milestone but there's still a lot of work ahead," said Matt Preedy, Transportation Department deputy administrator of the Alaskan Way Viaduct Replacement Project, which stalled in December 2013 when Bertha overheated and quit after traveling about 1,000 feet on its 2-mile trip under the city.

The tunnel was the project of choice to replace the viaduct, which was damaged in a 2001 earthquake. Crews spent much of 2014 digging the pit to reach the front of the machine.

Chris Dixon, Seattle Tunnel Partners' manager, said they had developed a nine-step plan for moving the machine forward once the pit was completed.

As it slowly advanced in 6.5-foot increments, it built the rings that form the inside of the tunnel. During the first four steps the machine mined and built rings 151 and 153. By Thursday night, they were mining for ring 153, he said. After that, the front of the Tunnel Boring Machine will be all the way through the concrete.

"And then we're going to do five more shoves at 6 and a half feet, but we won't be excavating, we'll just be pushing the TBM forward," he said.

The crews carefully monitored the machine's temperature through the process and pumped a lot of grease into the seals to ensure that it did not overheat and shut down again, he said.

Dixon could not estimate how long the next phase would take, nor could he say when he thinks Bertha will be ready to start digging again.

But he did say he believed the tunnel-drilling portion would be completed by the end of 2016 and the entire $3 billion project would be ready for traffic by 2017 - two years behind schedule.

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