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Video | Seattle tunnel construction

The SR 99 tunneling machine, known as Bertha

Washington State Department of Transportation has released a new video that gives an insight into the latest tunnelling progress on the Alaskan Way Viaduct replacement project.

The footage shows how the curved concrete segments that form the outer wall of the SR 99 tunnel were constructed.

Earlier this week, the tunnelling boring machine (TBM) – named Bertha – had travelled a total of 1,072m and installed 533 concrete tunnel rings. The top of the machine is nearly 40m below street level.

Bertha’s ultimate destination is a receiving pit near Seattle Center, where the TBM will end its 2,825m-long journey beneath downtown.

Crews have now installed more than 5,000 curved concrete segments in the SR 99 tunnel – each one weighing an average of 16t. Bertha takes 10 of these segments and creates a five-storey-tall tunnel ring. It will take 1,426 of these rings – or 14,260 concrete segments – to create the nearly 3km-long tunnel.

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