AASHTO Journal, 7 November 2014
Bertha, the tunnel boring machine, being assembled in her launch pit in May 2013. WSDOT photo.
Washington State Department of Transportation said state archaeologists gave Seattle Tunnel Partners the go-ahead on Nov. 2 to resume digging a rescue shaft toward a broken tunnel boring machine, part of a $3.1 billion Alaskan Way Viaduct Replacement project.
Contractors halted work Oct. 23 after workers digging the vertical shaft toward the giant tunnel digging device found shell material and called in experts to determine if it was related to the history of native tribes in the area.
But after days of examining the material, and triggering an established process of notifying area tribal authorities, archaeologists believed the shells came from commercial shellfish activities by early Seattle settlers around the start of the 20th Century, WSDOT explained, and allowed the construction work to resume.
It was a weeklong delay that triggered some concerns for the big project, which aims to replace a 1950s-era highway that was damaged by a 2001 earthquake. The job planned to use the giant “Bertha” machine to bore a long, horizontal tunnel shaft, but its drill head area broke last December to bring the work to a halt while managers developed a plan to rescue and repair it.
That rescue plan was to dig the vertical shaft to reach the front of the tunnel boring machine and bring that section up for refitting.
In a Nov. 3 update on the developments, WSDOT said “no one wants to stop work. We understand and share the urgency of the public in wanting to resume tunneling as quickly and safely as possible. But we also have an obligation to follow state and federal laws, and exercise due diligence when dealing with potentially significant discoveries. That’s why we have a very prescriptive process in place for dealing with this type of incident.”
Although the shaft digging can go forward, a “full laboratory analysis” will finally determine what’s in the excavated material. “All parties agreed that if the shell deposits prove to be significant from a regulatory perspective, the investigation we conducted, and the report we’re in the process of completing, constitutes sufficient mitigation,” WSDOT said. “That is, our efforts to characterize the archaeology to date will provide a scientific and historical record of what was found that offsets the destruction of the deposits from excavation of the access pit.”