Excavation crews for Seattle Tunnel Partners (STP) have reached the entire 120-foot depth needed to repair stalled tunnel-boring machine Bertha, state lawmakers were told Friday afternoon.
The next step is to build a concrete floor, including a “cradle” to support the cylindrical machine.
Then Bertha will attempt to grind forward, through the south side of the concrete pit wall, into open air. If that fails, workers will break a hole from the inside of the vault toward Bertha, allowing the giant machine’s thrusters to push it forward with minimal resistance — a more time-consuming operation.
A giant red crane, for which assembly is nearly finished along the waterfront, will straddle the vault and hoist the 4 million-pound front end to the surface for repairs by Bertha’s manufacturer, Hitachi-Zosen of Japan.
The Washington State Department of Transportation (WSDOT) posted an update Friday, along with a video of the rescue-pit excavation work, which began in May.
Bertha has moved only a few feet since Dec. 6, 2013, when the front end overheated, and the cutter spun without breaking dirt. The main bearing is being replaced, stronger seals are to be installed and the entire front end strengthened with steel plates.
The original late-2015 completion date for the Highway 99 tunnel, replacing the old Alaskan Way Viaduct, is now forecast by STP in late 2017.
Groundwater pumping, to reduce pressure around Bertha, will continue until repairs are done. Water migration is suspected of causing Pioneer Square soils to sink about an inch last fall, though WSDOT says the ground has remained stable since Thanksgiving.