Progress Made In Seattle’s Tunnel Blockage

February 2014, Vol. 69 No. 2
The SR 99 tunneling machine’s cutterhead was held aloft by a large gantry crane before joining other pieces of the machine in th

The Transportation Department says help is on the way for the massive tunneling machine Bertha that became stuck on Dec. 3 while boring a tunnel under downtown Seattle, WA.

Bertha became stranded on Dec. 6 after running into the eight-inch diameter, 199-foot-long well casing that had been left in the ground in 2002 to measure groundwater in the area.

The SR 99 tunneling machine, manufactured by Japanese firm Hitachi Zosen Corp, is stopped about 60 feet underground and one-tenth of the way toward completing a 1.7-mile tunnel. It will carry Highway 99 traffic and allow the removal of the Alaskan Way Viaduct.

To determine what was obstructing the machine’s progress, the department said contractor Seattle Tunnel Partners (STP) started drilling 17 five-foot wide shafts in early January in front of the machine. They encountered obstructions in four of the holes. The water pressure was low enough and enough soil was removed from the excavation chamber to inspect the top 15 feet of the chamber. Subsequently, the inspection showed an eight-inch diameter steel pipe protruding through one of the many openings in the cutterhead. The location of this pipe was included in reference materials in the contract.

STP is considering several options to remove the steel pipe and identify other potential obstructions.

The $2 billion tunnel budget includes a $40 million risk allowance for repairs and inspections near the front of the rotary cutting face — plus a $105 million general contingency fund to deal with crises. Matt Preedy, deputy Highway 99 administrator for the state Department of Transportation, said some of that money will be consumed.
The costs will be determined later through negotiations between the state and Seattle Tunnel Partners (STP), maybe even a legal dispute.

Other work on the Highway 99 project has included extensive construction on the north and south ends of the tunnel; preparatory work to dismantle the push frame and temporary tunnel rings in the launch pit; and preparatory work to reconfigure the launch pit and build the roadway within the tunnel.