A dramatic scene unfolded Tuesday in a 90-foot-deep pit to the east of Seattle Center. A five-foot-thick concrete wall came tumbling down, revealing a slowly rotating cutterhead five stories tall. This was Bertha, the SR 99 tunneling machine, completing her 1.7-mile journey beneath Seattle.
|Bertha’s cutterhead made its appearance near the Seattle Center on Tuesday, April 4.|
Crews from the contractor Seattle Tunnel Partners began mining around 8 a.m. on Tuesday, breaking through the concrete wall around noon. We shared updates from the scene on Twitter, Facebook and Periscope, and hosted a livestream on the Alaskan Way Viaduct Replacement Program website and here on the blog. (The traffic overwhelmed our servers for a time; across all our channels, nearly a half million people tuned in to witness the event).
Bertha has been building the new SR 99 tunnel as she mines, erecting a 6.5-foot-wide tunnel ring and then pushing off that ring to mine forward (watch her build a ring). This new tunnel will carry SR 99 through Seattle, replacing the Alaskan Way Viaduct which is old and vulnerable to earthquakes. Once the tunnel opens, the viaduct will be torn down and a new Alaskan Way surface street built in its place, which will provide connections between SR 99 and downtown Seattle (this map shows how common trips will look when work is finished).
With tunneling complete, Bertha now sits at the north end of a 9,270-foot tunnel stretching from the stadiums to the Seattle Center area. In the coming weeks, STP will move the machine into its final position in the disassembly pit. Then the machine will be cut up and the pieces loaded onto trucks and hauled away. Some parts of the machine will be recycled while others may be reused.
The (literal) road yet to come
While Bertha tunneled forward, crews working behind her have been building the structure of the highway inside the tunnel. These walls and decks will form the tunnel's double-deck highway, plus the ventilation, maintenance and emergency exit corridors on the sides of the roadway. When complete, the tunnel will have two lanes in each direction, plus an 8-foot shoulder and a 2-foot shoulder on either side of the roadway.
|Crews have been construction the roadway of the new SR 99 tunnel behind the Bertha tunneling machine.|
Building the roadway is one of four big steps remaining before the tunnel opens to traffic, currently scheduled for early 2019.
- Disassembling and removing the tunneling machine
- Building the roadway inside the tunnel
- Installing the tunnel's mechanical, electrical and plumbing systems
- Testing and commissioning the tunnel