Transforming Bertha's finish line
The large pit north of Harrison Street that was the tunneling machine Bertha’s finish line has seen a series of dramatic transformations in the past few months. What was once a 90-foot-deep excavation to the west of SR 99/Aurora Avenue North will soon be entirely covered by a concrete lid, capping the future roadways and tunnel maintenance and systems corridors built within.
In early January crews placed 10 girders across the pit. These girders, each 70 feet long and weighing 35 tons, will support the final section of the north portal operations building (the building with the four yellow ventilation stacks) that will be built atop the pit.
This week, crews placed rebar and poured concrete to form a lid across the girders, burying the pit entirely from view.
Crews first began excavating the 450-feet-long and 90-feet-deep pit in 2013, in preparation for the completion of tunnel mining. Bertha broke through the pit's southern wall on April 4, 2017. Crews then used that space as the staging area for dismantling the 8,000-ton machine, a tremendous effort which they completed in August.
Once the final piece of the machine was gone, the pit was ready for its second use: becoming part of the tunnel itself. Since August crews have been building walls and decks within the pit that will connect the bored tunnel’s roadways to the short section of cut-and-cover tunnel just to the north. Building these structures has slowly obscured the disassembly pit from view of our construction camera.
In the coming weeks and months crews will begin constructing the final section of the operations building. They will also restore the block of Sixth Avenue North that has been used as a construction staging area for the north portal work.