The transformation of a tunnel construction site into an actual tunnel includes reshaping the launch pits at both ends of the tunnel bore. And no transformation has proven quite as dramatic as in Seattle where the two pits—one at the south end of the project and the other at the north end, near the Space Needle—will require full transformations to serve the needs of the double-decker tunnel roadway.

After the 1.7-mile tunnel mining concluded in April 2017, contractor Seattle Tunnel Partners began the transformation of Bertha’s launch pit into a section of the State Route 99 tunnel for owners Washington State Dept. of Transportation. During the mining, the original launch pit served as the key access point for delivering tunnel ring segments and supplies to the machine. The pit also housed a major piece of the conveyor belt that delivered muck from the machine to barges docked at Terminal 46 on Seattle’s waterfront.

With the mining concluded, the transformation could begin. In June, after removing the conveyor belt used on Bertha, one of the world’s largest-diameter tunneling machines, crews started converting the launch pit into a pivotal section of the project. Three months later, the transformation from launch pit to tunnel has almost wrapped up.

The first step involved building lower interior walls on top of the lower roadway. Then crews moved to build the upper roadway on top of the lower interior walls. The third step included building upper interior walls on the top of the upper roadway before the final step of building the tunnel lid on the top of the upper interior walls.

That final phase of the launch-pit transformation is underway now as crews must build the top of the tunnel, a 4-1/2-ft-thick slab of reinforced concrete that makes up the lid. By placing a lid on the pit it makes it possible to build streets atop the tunnel. In 2018, crews will rebuild part of Alaskan Way South directly atop what once was Bertha’s original southern launch pit. When the tunnel opens, three levels of roadway will move people north and south across what was once the launch pit.

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