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TBM progress continues on Seattle tunnelling project

Tunnelling could resume by Christmas on the Highway 99 tunnelling project in Seattle. 

According to an update by the Washington State Department of Transportation, Seattle Tunnel Partners continues to prepare the SR 99 tunneling machine for mining. Crews have spent several weeks conducting a series of preliminary tests – including rotation of the cutterhead in the open air at the bottom of the 120-foot-deep pit that was built to access and repair the machine. Over the weekend, STP began filling in the access pit with sand and soil – the final step before they resume mining.

These graphics illustrate this phase of STP’s repair effort.

During the backfilling process, STP will incrementally turn off the dewatering wells that crews have been using to control groundwater. The company will continue to monitor movement of the ground, structures, utilities and the viaduct. 

STP's most recent schedule shows the machine will be ready to resume mining in the pit on Dec. 23. 

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. 

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:

  • A 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.
  • A 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.
  • A new Alaskan Way surface street along the waterfront that connects SR 99 to downtown.
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