Seattle SR-99 Tunnel Fly-Through Video Released
The Washington State Department of Transportation has released a new fly-through video of the tunnel bored by its "Bertha" borer, threading through the tunnel’s north end then out the south end, located near the Seattle stadiums.
The video gives a bird's-eye view of 2 miles of current construction work, which will eventually replace the Alaskan Way Viaduct.
Bertha, the world’s largest tunneling machine, was built by Japanese manufacturer Hitachi Zosen, and was operated by Seattle Tunnel Partners, the design-build contractor overseeing the Alaskan Way Viaduct Replacement Program. The machine broke through at the tunnel’s north end on April 4.
The project will replace the Alaskan Way Viaduct—a stretch of SR 99 in the city that is subject to risk from seismic activity—with a new route that includes a nearly 2-mile-long tunnel. The project began after the 1950s-era viaduct structure sustained damage from a 6.8-magnitude earthquake in 2001.
How Bertha Did It
Bertha, with a diameter of 57 feet, was designed specifically for the ground conditions under Seattle, according to WSDOT. As the borer moved, it pulled pieces of precast concrete into place behind it, creating the tunnel structure. Crews have been working in the areas further back, creating the double-deck roadway inside it.
Some maintenance work involving the removal of cutting tools from the head of the borer had to be done in hyperbaric conditions, requiring crews to seal up the ground in front of Bertha with bentonite clay, then overpressurize the work area with compressed air.
Now that its mission is complete, the Seattle Times reported, Bertha will be almost entirely scrapped and melted down for reuse. The only parts eligible for recycling as-is, the paper says, are “generic parts such as hoses, belts, wires.”