The 57-foot-wide Highway 99 tunnel drill has passed below the Battery Street Tunnel, heading toward a spring finish near Seattle Center.
Tunnel machine Bertha recently chewed its way beyond the Battery Street Tunnel and into the homestretch of its 1.7-mile trip north.
The giant drill passed 90 feet below the 62-year-old thoroughfare, without causing any damage to buildings and streets above. The dig that started in mid-2013 is now 83 percent complete, with less than 1,600 feet to go, the project team says.
Soil settlement has been “negligible” and there was no need to inject any grout to reinforce buildings or streets, said spokeswoman Laura Newborn of the Washington State Department of Transportation.
Bertha will soon go beneath the Seattle Center Monorail, whose shallow column foundations don’t require reinforcement.
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The machine is expected to reach daylight in late May, said Newborn. A concrete-walled retrieval pit has been built between Aurora Avenue North and the Bill & Melinda Gates Foundation headquarters.
Following more than two years of delays and repairs, the excavation has been virtually trouble-free since April 29, when a strengthened Bertha traveled under the old Alaskan Way Viaduct. The viaduct will be demolished after the tunnel opens.
The four-lane, tolled Highway 99 tunnel is expected to carry traffic bypassing downtown Seattle by early 2019. Design and construction costs for the deep bore tunnel are about $2 billion, plus roughly $500 million in overruns being disputed among contractor Seattle Tunnel Partners, insurance companies and WSDOT.