Crews will stop pumping groundwater from the soil surrounding stranded tunnel-boring machine Bertha, after state surveyors found Sunday that the Alaskan Way Viaduct is sinking in an uneven manner that could lead to damage.
This discovery will probably cause additional delays in Seattle Tunnel Partners’ effort to repair the world’s largest tunnel drill, which has barely moved in the past year. Bertha is stuck 60 feet below ground near South Main Street, along the waterfront.
The Washington State Department of Transportation (WSDOT) announced the new viaduct findings in a news release Sunday night. The department didn’t say exactly when water pumping would cease, but that STP would do so in a “deliberate manner” that protects worker safety and avoids damaging structures, including a big ring-shaped vault where STP has excavated 70 feet deep so far.
Matt Preedy, deputy Highway 99 administrator, said Friday that the settlement of the viaduct was happening evenly, among several spans of the elevated deck, at about 1.2 inches in the past month.
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As long as settling happens evenly, there would be minimal twisting and stress on the 61-year-old concrete structure. During that WSDOT interview, Dave Sowers, a geotechnical engineer with the department, said settlement appeared to have stopped last week, and groundwater pumping could continue unless the sagging worsened.
WSDOT said Sunday night that its team didn’t find signs of actual damage to the viaduct, and that it remains safe and open for drivers.
WSDOT provided no new measurements in Sunday night’s message, which said the agency will be gathering more data on Monday and would have no further comment until Monday afternoon.
The $2 billion tunnel is already about one year behind the scheduled December 2015 opening.
Groundwater pumping is an essential part of STP’s excavation of a giant access pit, 120 feet deep, so that a crane can pull Bertha’s front cutter and drive assembly to the surface for repairs.
Groundwater must be removed to reduce pressures — similar to those against an undersea diver — that could blow clay soils up through the bottom of the pit.
Even before the viaduct settling came to light, WSDOT has been saying lately that STP’s March goal to restart the machine would likely slip.
Excavation has been slow, and the effort lost a week or two this fall when a layer of old shells had to be investigated, in case it contained artifacts from native tribes, which it didn’t.
Chris Dixon, director of STP, didn’t respond to phone calls Friday afternoon and Sunday night.
Ground settling happened in 1994 around a rescue pit about half this size in Sarnia, Ontario, Canada, during repairs to tunnel machine Excalibore, which eventually finished drilling its train tunnel to Port Huron, Mich. But that site wasn’t surrounded by historic buildings nor a busy highway.
Besides changes in groundwater-pumping methods, injecting concrete grout is another method that can be used to stabilize weak soil in tunneling projects.
The viaduct settlement this fall could mean that some portions of the structure have sunk more than 6 inches since the 2001 Nisqually earthquake — exceeding a safety limit that WSDOT publicized last decade.
Since 2009, some columns were reinforced by steel foundation rods, and braces above ground.