Canadian Underground Infrastructure Logo

Bertha cutterhead spins for first time in 9 months

Commissioning of the SR 99 tunneling machine continues as Seattle Tunnel Partners (STP) prepares to resume mining. STP began testing machine components earlier this month as crews continued welding pieces and reconnecting hoses and cables. Last week, STP began rotating the machine’s cutterhead a few degrees at a time, stopping periodically to take measurements before completing a full rotation.

Tuesday, Nov. 24 marked the first sustained rotation of the cutterhead since the machine broke through the southern wall of the access pit on Feb. 19. STP indicated they will rotate the cutterhead in both directions for about one hour at a time.

During the next phase of commissioning, STP will backfill the access pit with sand and gravel to prepare the machine for mining. STP's most recent schedule shows that the machine will be ready to resume mining on Dec. 23. The state cannot verify the contractor's schedule.

In summer 2013, Bertha, the world’s largest tunneling machine, began digging the SR 99 tunnel beneath downtown Seattle. In December 2013, Seattle Tunnel Partners, the contracting team hired to design and build the tunnel, stopped excavation approximately 1,000 feet into the dig after measuring increased temperatures in the tunneling machine. While investigating the cause of the high temperatures, STP discovered damage to the machine’s seal system and contamination within the main bearing. STP and manufacturer Hitachi Zosen have been repairing and enhancing the machine.

In August, Seattle Tunnel Partners and crane crews from Mammoet successfully lowered all of the SR 99 tunneling machine's pieces to the bottom of the access pit.

The TBM is an integral part of the Alaskan Way Viaduct Replacement Program. The Alaskan Way Viaduct, an elevated section of State Route 99 in Seattle, was built in the 1950s, and decades of daily wear and tear have taken their toll on the structure. Because of the viaduct’s age and vulnerability to earthquakes, replacing it is critical to public safety.

The Alaskan Way Viaduct Replacement Program includes projects led by the Washington State Department of Transportation, King County, the City of Seattle and the Port of Seattle. The Federal Highway Administration is a partner in this effort.

Major elements of the program include:

  • two-mile-long tunnel beneath downtown Seattle.
  • A mile-long stretch of new highway that connects to the south entrance of the tunnel, near Seattle’s stadiums.
  • new overpass at the south end of downtown that allows traffic to bypass train blockages near Seattle’s busiest port terminal.
  • Demolition of the viaduct’s downtown waterfront section.
  • new Alaskan Way surface street along the waterfront that connects SR 99 to downtown.