Steel pipe blamed in Bertha blockage but more uncertainties could be ahead
An abandoned well pipe is likely blocking the tunnel-boring machine beneath Seattle's waterfront. The Washington State Department of Transportation, the agency overseeing the State Route 99 tunnel project, had the pipe placed in the ground back in 2002.
A WSDOT contractor installed the 8-inch diameter, 119-foot long steel pipe to conduct a test on the Alaskan Way Viaduct, following the Nisqually earthquake. The location of the well site was included in planning documents for the tunnel. Chris Dixon, a project manager for Seattle Tunnel Partners, said the tunneling contractor had assumed the pipe was removed when the well was decommissioned. He and a WSDOT representative would not comment on who was responsible for the pipe's removal.
Seattle Tunnel Partners and WSDOT, Dixon said, are “looking at the documents” to see if there might be any other pipes in the machine's path.
“What we’re faced with right now is a cutter head that needs more cutting tools replaced before we can mine forward,” Dixon said. “We need to remove whatever pieces of pipe or steel are out in front of the machine at this time.”
The cutting tools are designed to be replaced over the course of the project, but they are only intended to cut through soft material and are showing signs of unusual wear and tear. Dixon could not say if the pipe had damaged any other part of the machine, commonly called "Bertha." All but 15 feet of the machine's 57-foot "cutter head" is currently under water. There is a considerable amount of groundwater pouring into the soils where the machine is digging.
Matt Preedy, WSDOT Alaskan Way Viaduct Replacement Program deputy administrator, said determining whether the pipe is definitely the root cause of the Bertha's problems will require further investigation.
The boring machine hit the pipe on Tuesday, Dec. 3 and on Thursday, Dec. 5 started to experience “increased resistance” moving forward, Dixon said. On Friday, Dec. 6 Bertha “pretty much ground to a halt.” The current stoppage began on Saturday, Dec. 7.
Crews have pulled a 55-foot length of the pipe out of the ground. About 60 feet of pipe apparently remain buried in front of the machine. A drilling contractor has bored 12 holes to determine what is blocking the machine, Dixon said. Six of those holes, he said, found evidence of something hard and metallic at the same depth across the machine’s face. On their website, WSDOT reported on Friday that 17 holes were drilled and four encountered obstructions.
WSDOT’s Preedy would not comment on the cost implications of the stoppage, daily operating expenses or how many unanticipated down-days were built into the plan for the tunnel.
The $3.1 billion project has a $40 million contingency fund. “I can say with confidence that some of those funds will get used,” Preedy said.
The 1.7-mile tunnel is about 10 percent complete. Bertha began digging in Sodo on July 30 last year and was slated to emerge in South Lake Union about 14 months later. The Tunnel will eventually replace the Alaskan Way Viaduct and is scheduled to be open for traffic in late 2015.
“Tunneling is hard,” Preedy said. “There’s always going to be some unknowns anytime you’re underground.”