December 8, 2014
SEATTLE (AP) - There are more problems for the Highway 99 tunnel project beneath downtown Seattle, as engineers study whether settling soil means work should be altered on a pit being dug to reach a stalled digging machine known as Bertha.
Tests over the weekend showed the settling around the pit is uneven, the Department of Transportation said.
No differential settling, which is potentially risky for structures, was detected on the Alaskan Way Viaduct, which remains safe for travel, the department said Monday.
The settling near the pit is apparently the result of groundwater pumped out by Seattle Tunnel Partners as it digs a 120-foot access pit to reach and replace the damaged head of the boring machine.
Bertha overheated and stopped a year ago. It has drilled about 10 percent of the way into a planned 2-mile tunnel to replace the viaduct.
State engineers are analyzing the settling data to make sure the pit, viaduct and nearby buildings are secure. On Sunday, the state said pumping necessary to reduce water pressure on the pit would be stopped. However on Monday, Laura Newborn, spokeswoman for the viaduct replacement project, said the dewatering was continuing.
Seattle Tunnel Partners had told the state it planned to resume tunneling in April on the $2 billion project. It's already about a year behind schedule. The opening was previously scheduled for December 2015.
Bertha is stuck about 60 feet under a street not far from the Seattle waterfront. To remove the 57-foot diameter cutting head, the 80-foot-diameter access pit needs to be about 120 feet deep. It's currently about 70 feet deep.
Engineers last week said the viaduct had sunk about an inch in the past month. That's in addition to earlier sinking after the 2001 Nisqually earthquake.
The risk of the 61-year-old viaduct collapsing in an earthquake is one reason the state pushed for the tunnel replacement project to carry Highway 99 traffic through the city. The viaduct carries about 100,000 vehicles a day.
Work on the access pit was stopped for several weeks this fall after clam shells were uncovered. Experts determined they were not archaeologically significant.