Seattle Borer Drills Past Viaduct
Bertha, the world’s largest tunnel-boring machine, made its way under Seattle’s Alaskan Way Viaduct faster than expected, limiting road closures last week and moving the tunneling project close to the 2,000-foot mark.
The Washington State Department of Transportation closed the Viaduct to traffic April 29 while the borer, 57 feet in diameter, tunneled underneath the structure. The Alaskan Way Viaduct carried State Route 99 through Seattle, and will eventually be replaced by the tunnel that Bertha is creating.
The closure, put into place to protect the already-fragile Viaduct from vibrations caused by the borer, was expected to last two weeks, but wrapped up early, on Sunday evening, May 8. Traffic was flowing again for rush hour last Monday morning.
The intention was to let the borer (and, more importantly, the crews operating it) rest a few days after it made a 385-foot advance under the Viaduct, but after it got that far, officials opted to keep it going, telling the Seattle Times only that they wanted “to reach a better location for the machine to stop.”
Reinforcing the Viaduct
The bore took place about 80 feet below the surface; part of the preparation for the dig involved reinforcing the Viaduct’s footings and wrapping the structure in a reinforcing carbon fiber, to reduce the effects of vibrations.
As Bertha makes its way north—about 6-and-a-half feet at a time—concrete ring segments are left behind it, creating the shell of the tunnel, which will eventually carry northbound and southbound traffic on separate decks.
Crews add concrete ring segments for Bertha to place in the tunnel.
Bertha’s work on the tunnel project was fraught with issues and delays early on. After making it about 1,000 feet in 2013, work was halted when the borer stopped; at first, officials thought it had been impeded by a steel pipe, but it turned out dirt had gotten into a bearing, overheating the machine and damaging its seal system.
It took two years to get the huge machine, built by Hitachi Zosen Corp., back in working condition. Drilling started back up in December 2015, and with the latest stretch of boring complete, Bertha has made it 1,945 feet. The total planned tunnel length is 9,270 feet.
The Alaskan Way Viaduct replacement project began after the structure sustained damage from a 6.8-magnitude earthquake in 2001. The Viaduct was built in the 1950s.
Right now, Seattle Tunnel Partners, the contractor overseeing the work, expects the tunnel to open by mid-2018. After the tunnel opens, the Viaduct will be demolished, and plans are in place to create a waterfront park and trail.
Washington State DOT recently posted a drone-camera video of the inside of the tunnel, showcasing the size and scope of the project. Watch below.
A drone’s-eye-view of the SR 99 tunnel under construction.