April 2016, Vol. 71 No. 4


Bertha Resumes Digging Seattle’s Route 99 Tunnel

In mid-January, the world’s largest tunnel-boring machine (TBM) sat under downtown Seattle with its large diameter cutterhead completely static due to a sinkhole – it seemed the TBM, dubbed Bertha, couldn’t catch a break on the State Route 99 tunnel project.

Bertha was designed to bore a 1.7-mile tunnel beneath downtown Seattle, aiming to supplant the Alaskan Way Viaduct, a seismically vulnerable, double-decker section of Route 99 along Seattle’s downtown waterfront. After nearly two static years spent replacing Bertha’s main bearing and other maintenance fixes, the machine began tunneling again in December 2015, only to be halted on Jan. 14 when a sink hole opened up directly behind the TBM – the most recent delay on the project, now more than 27 months behind schedule.

Seattle tunnel project

Seattle Tunnel Partners (STP), the contractors on the job owned by the Washington State Dept. of Transportation (WSDOT), chose to fill the sinkhole with 250 cubic yards of concrete overnight – a decision that caused the WSDOT to suspend all Bertha’s tunneling operations until STP completed an analysis and modified its tunneling operations, ensuring appropriate ground control.

The project resumed for a 160-feet demonstration period put in place by the WSDOT, Feb. 23 – the period was cleared by STP, March 7, and Bertha will continue digging another 120 feet north until it reaches a planned maintenance stop. The maintenance could take up to several weeks, but once completed, Bertha will begin its digging under the Alaskan Way Viaduct, closing the highway for approximately two weeks.

The latest construction schedule at the time of this publication cites spring 2018 as the estimated time when the Alaskan Way Viaduct replacement tunnel will open for traffic. www.wsdot.wa.gov/Projects/Viaduct.

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