The Transportation Department said crews are drilling "de-watering" wells Thursday to relieve pressure on a machine.
Spokeswoman KaDeena Yerkan said the wells should make it easier to identify what's blocking the machine known as Bertha. Then contractors will determine how to proceed.
The machine was halted Saturday by the mysterious obstruction. It may be a huge boulder.
The boring machine is 60 feet under the streets and about one-tenth of the way into the planned 1.7-mile tunnel that will carry Highway 99 traffic and replace the Alaskan Way Viaduct.
Chris Dixon said specially trained workers could be sent to the site next week, The Seattle Times reported. They would peek outside the tunnel machine's cutter head to see what's in the way.
The machine can retreat about 18 inches and compressed air would be forced into the small space in front of the cutter head to allow workers to get a look at the problem.
Since the top of the tunneling machine is about 60 feet below ground, professional divers used to working below the surface would be needed.
The leading theory is that the machine called Bertha hit a boulder last Friday and that the soil around it is too soft to hold it firmly and allow the cutter head to crack it apart. The machine was shut down Saturday about 1,000 feet into the 1.7 mile project.
The quickest way to remove whatever it is would seem to involve divers breaking it up with power drills and hammers, along with Bertha's drills, Dixon said.
Alternatively, contractors could drill down from above and break up the object or lift it out. That probably would require building a protective wall or pit to hold back sand and groundwater.
"It would take several weeks to build that," Dixon told a news conference.
The 58-foot diameter tunnel is scheduled to be completed by the end of 2015, creating a four-lane route for Highway 99 traffic between South Lake Union and the area south of downtown.
The $1.4 billion tunnel contract is part of the $3.1 billion project to replace the Alaskan Way Viaduct along the downtown Seattle waterfront.
Information from: The Seattle Times, http://www.seattletimes.com
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