Seattle's SR 99 tunnel blockage under inspection

8 January 2014


State Route 99 tunnel crews are busy trying to identify an obstruction that halted the TBM in early December 2013, Washington State Department of Transport (WSDOT) announced earlier this month. The TBM being used for the USD 2bn tunnelling scheme in Seattle has been at a standstill after experiencing unexpected resistance.

Seattle Tunnel Partners (STP), WSDOT's design-build contractor for the project, spent much of December reducing water pressure in the ground around the machine to make it safe for crews to inspect its excavation chamber. They also drilled 17 exploratory probes in front of the machine to look for objects that might be blocking the machine¹s path.

In early January contractor crews successfully reduced water pressure enough to visually inspect a portion of the excavation chamber. A piece of an 8in-diameter steel pipe could be seen protruding through an opening in the machine¹s cutterhead. Probes from the surface also detected metal in front of the machine.

The steel pipe is a well casing installed in 2002 following the 2001 Nisqually earthquake to help geologists better understand how groundwater moves in the area. The location of this pipe was included in reference materials in the contract.

"This is just the first step in determining what¹s obstructing the machine," said Matt Preedy, Alaskan Way Viaduct Replacement Program deputy administrator. "We need to investigate further to see if there are other factors that could have contributed to the blockage."

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

"It's still too early to know how this issue will affect the project¹s schedule and budget," Preedy said. "Our focus right now is on resuming tunnelling as quickly and safely as possible."

As the process to find the blockage continues, workers are conducting critical maintenance, inspecting and replacing damaged cutter tools on the face of the machine.

The TBM, Bertha, is being used by Seattle Tunnel Partners - a joint venture of Dragados USA and Tutor Perini ­ to build a 3.2km tunnel to replace the SR 99 Alaskan Way Viaduct in Seattle. In December 2013 Bertha tweeted that she is working fine but has encountered an obstruction.