They worked over the holiday season to identify the obstruction that had halted Bertha, the world’s largest tunnelling machine.
Seattle Tunnel Partners (STP) - a joint venture of Dragados USA and Tutor Perini - is Washington State Department of Transportation’s design-build contractor for the project, which involves building a tunnel to replace the Alaskan Way Viaduct. The contractors spent much of December reducing water pressure in the ground around the machine to make it safe for crews to inspect its excavation chamber. STP also drilled 17 exploratory probes in front of the machine to look for objects that might be blocking the path.
The machine – which has a record-breaking diameter of 17.5m - had completed her first 1,000ft (305m) of tunnelling a day before coming to a halt.
By this month, STP had reduced water pressure enough to visually inspect a portion of the excavation chamber. A piece of an 200-mm diameter steel pipe could be seen protruding through an opening in the machine’s 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 what’s obstructing the machine,” said Matt Preedy, deputy administrator for the Alaskan Way Viaduct replacement programme. “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.
“It’s still too early to know how this issue will affect the project’s schedule and budget,” Preedy said. “Our focus right now is on resuming tunneling as quickly and safely as possible.”
As the process to find the blockage continues, workers are conducting maintenance, inspecting and replacing damaged cutter tools on the face of the machine.
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