Tunnel-boring machine Bertha dug forward three feet Thursday and Friday, to get ready for fall repairs, the state Department of Transportation said Monday.
“We were just pleased we were able to move three feet, after it sat so long,” said Chris Dixon, executive director of contractor Seattle Tunnel Partners, in a phone interview Monday.
The giant drill’s cutter teeth now rest against the south edge of a concrete repair pit, Todd Trepanier, state Highway 99 tunnel administrator, told the Seattle City Council. Dirt will be removed from the 120-foot-deep, ring-shaped pit this month. Bertha will chew through a 20-foot thick wall into open air. Then a giant crane will lift the whole front end to street level, and take it apart. The main bearing and its rubber seals will be replaced, and steel reinforcing plates will be added. Gaps in the rotary cutting head will be enlarged, to increase the flow of excavated dirt toward the conveyor system.
The tests were meant to check Bertha’s performance and temperatures, to choose “operating parameters” for when the machine drills several more feet into the repair ring, Dixon said. Also, he said the 3-foot advance positioned the machine better for workers to add chemical grout at the back. The grout will form a barrier to block groundwater from flushing into the repair pit.
Meanwhile, construction continues on the north portal of the Highway 99 toll tunnel where it will emerge next to Seattle Center and the Gates Foundation. Concrete girders will form a lid for the tunnel’s cut-and-cover portion, where four lanes of traffic will ascend toward Aurora Avenue North.
STP continues to collect monthly payments under its $1.44 billion contract, with some funds subtracted until Bertha makes progress. STP expects to resume tunneling in March 2015. The state has posted more images through its time-lapse cameras, and on Flickr.