Seattle Tunnel Partners (STP), Washington State Department of Transportation’s (WSDOT) design-build contractor for the SR-99 (Alaskan Way Viaduct Replacement) Tunnel Project, has taken the first step toward identifying and removing a potential TBM obstruction that halted the progress of the 57.5ft (17.52m) Hitachi Zosen EPB machine ‘Bertha’ last week.
STP is installing up to eight wells approximately 120ft (35m) deep on the construction site near the machine, which is located about 60ft (18m) below the surface, between South Jackson and South Main streets, to the west of the Alaskan Way Viaduct. The dewatering wells will lower the groundwater in order to create a safe environment for workers to enter the excavation chamber and begin to assess the blockage.
“We’re confident STP has the right people and the right approach in place to safely determine what’s causing the obstruction,” said Matt Preedy, Alaskan Way Viaduct Replacement Program Deputy Administrator. “Once they know what the obstruction is, our contractor will be able to select the best path forward.”
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On Saturday, December 7, STP proactively stopped tunneling after unanticipated and increasing resistance was experienced, possibly due to an obstruction, which significantly decreased the advancement rate of the machine and the volume of tunnel muck on the machine’s conveyor belt. Since then, STP has been evaluating various strategies to determine what is slowing the machine’s progress. The shallow depth of the mammoth TBM, the high groundwater table, and the challenging nature of the historic fill soils at this location have been key considerations.
Well installation started last Thursday (December 12) and will continue this week. STP hopes to begin inspecting the machine by the end of this week.
To date, crews have installed more than 1,000ft (300m) of tunnel. Back at the beginning of the year, testing at Hitachi Zosen’s manufacturing plant in Sakai revealed a problem with $80 million machine’s main drive unit, requiring crews to make appropriate repairs to the machine before shipping from Osaka Bay, on March 19, 2013. Bertha passed all further tests following the repairs, and was due to officially become the property of STP once she had tunneled approximately 1,000ft (300m) without any issues.
Further delays to the schedule occurred shortly after the TBM’s launch, when a labor union dispute led to a blockade that effectively shut down the site for a month.
Work currently continues inside the tunnel structure as well as in the north and south portal areas.
For more information about the SR 99 Tunnel Project, part of the Alaskan Way Viaduct Replacement Program, visit: www.alaskanwayviaduct.org.