After 430 ft of tunneling Bertha was already in need of a stronger bite.
The tunnel-boring machine—the largest in North America at 57.5 ft in diameter and weighing 7,000 tons—digging a new State Route 99 tunnel under downtown Seattle has cleared zone one of 10 planned zones, making a stop for over a week for scheduled maintenance.
And while a thorough testing of Bertha’s systems, really the first in-depth look at how the machine has been operating since launching on July 30, showed that Bertha has fared well in the early stages of the project, the constant chewing through concrete, rebar, soil and timber has taken its toll on the cutting mechanisms.
Crews changed out 24 of Bertha’s giant teeth in preparation to start digging again this week. Contractor Seattle Tunnel Partners hadn’t originally planned on swapping out so many teeth, instead expecting to wait until its next stop just south of Yesler Way, but the sharper teeth were the order of business.
The TBM also received a new style of rectangular teeth better suited for the glacial soils she’ll soon encounter instead of the round teeth she’s been using to tunnel through the much harder grout-injected soils in the early part of the route.
The first zone contained fill soils dumped by the city’s early settlers, which were removed or strengthened ahead of tunneling. The next zone, though, includes Bertha traveling about 15 ft below the Alaskan Way Viaduct’s foundation, which required crews to build angled walls beneath the foundation to protect it during tunneling.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for TIME, Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.