Seattle Tunnel Partners has begun injecting grout into the soil near tunnel- boring machine Bertha to prepare the ground for when the giant drill begins working again.
Seattle Tunnel Partners has begun injecting grout into the soil near tunnel-boring machine Bertha to prepare the ground for when the giant drill begins working again.
It’s unclear yet whether STP can meet its goal of Nov. 23 to restart the Highway 99 tunnel dig to South Lake Union.
The team has not yet begun open-air testing of Bertha’s rotary cutter and soil mixing arms, scheduled for this week and next.
Workers are continuing to reattach wires and hoses inside the steel cylinder, said a Washington State Department of Transportation update Wednesday afternoon.
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Construction teams have been at work day and night to finish reassembly of the $80 million machine, built by Hitachi Zosen in Japan.
The tunnel machine — 57 feet, 4 inches in diameter — overheated Dec. 6, 2013, as its seals failed around the main bearing. Several parts were replaced or reinforced this year.
The most recently published STP schedule, issued in August, aims to open the four-lane tube to traffic in March 2018.
Grout is an essential feature in Seattle tunnel projects, including the smaller Sound Transit light-rail tubes, to control wet soil and aid accurate drilling.
But in this case, the purpose is more specialized.
The tunnel machine will be churning northbound, when it leaves the vertical, ring-shaped repair vault. Geometrically, that means the west and east edges of the disc-shaped cutter would reach dirt while the top and bottom areas are still grinding within the curved wall of concrete pilings.
Therefore, STP will grout alongside the ring, so the entire cutterhead scrapes a consistently hard surface, said Laura Newborn, spokeswoman for the WSDOT.
Before drilling begins, contractors will also pour sand and soil into the vault surrounding Bertha.
At that point, engineers can gradually turn off their temporary dewatering wells, allowing groundwater to return at up to four times atmospheric pressure — while striving to avoid sudden ground heaves or shifts that could threaten structures.
“As we backfill the shaft, we’re going to be incrementally turning off those wells, and we’ll be monitoring that, to make sure everything’s stable,” said Chris Dixon, project manager for STP, in a recent video.
Not quite a year ago, contractors building the repair vault were pumping huge volumes of groundwater away from Bertha, a change suspected of causing the Alaskan Way Viaduct and a city water main to sag 1¼ inches. Seattle Public Utilities has dug open Western Avenue to replace the water line.