Bertha's dirty cutterhead has been rinsed to a shiny silver, as the machine advances Thursday into the repair vault.

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Tunnel machine Bertha is creeping forward again, after tons of debris were cleaned from the floor of its deep repair vault.

The move of approximately 30 feet will take many hours, even though the cutterhead has reached open air. This is because crews in the back are installing the tunnel’s next five concrete rings, each 6 feet, 6 inches wide. Bertha propels itself forward by pushing off each new ring, using 56 hydraulic rams.  Crews will occasionally stop to remove debris, a state update Thursday says.  Bertha broke through a 20-foot-thick concrete wall to reach the pit last week, operating at low speed to prevent overheating.

The state pit-cam shows the cutterhead’s green paint has been scraped away by soil abrasion, and Seattle Tunnel Partners (STP) rinsed off the dirt, leaving a silvery shine.  A curved concrete saddle has been built in the middle of the vault, so the machine’s front end can rest snugly. It could take weeks to remove the outer skin; to disconnect braces, motors and hoses; and to install the motors and gears atop the red Mammoet gantry crane.  At that point, the 4 million-pound front end will be lifted to the surface, next to the old Alaskan Way Viaduct at Pioneer Square.  Parts will be separated on the surface, and the entire bearing assembly will be replaced.

STP hopes to resume mining toward South Lake Union by late summer, and open the four-lane tube to traffic by the end of 2017.