Seattle TBM starts tunnelling again
Bertha, the SR 99 tunnelling machine, is now tunnelling in Seattle soil after breaking through the access pit wall on January 6th. After being out of commission for about two years, Seattle Tunnel Partners (STP) has mined 73 feet and installed 12 concrete tunnel rings since Bertha first moved forward in the pit on Dec. 22.
Now approaching South Main Street, near Pier 48, Bertha is digging well below the area’s notorious fill soil. The top of the machine is approximately 80 feet below the surface in a mixture of glacially compacted material.
Currently, STP has two tunneling crews consisting of approximately 25 members each. Each crew is working six 12-hour shifts per week. Mining progress during each shift may vary significantly from day to day. Some days crews may mine a long distance, while other days might be devoted exclusively to planned maintenance.
According to STP’s most recent schedule, Bertha is expected to reach a planned underground maintenance stop just south of Yesler Way in late January. Once there, crews will spend approximately three weeks performing final maintenance before the machine tunnels beneath the Alaskan Way Viaduct.
WSDOT will fully close SR 99 through downtown for approximately two weeks while the machine passes beneath the viaduct. STP’s latest projections show that the closure will occur in March, but the actual closure date will depend on Bertha’s progress and the state cannot verify the contractor's schedule.
One last thing of note: Crews have begun disassembling the massive red crane that was used to lift Bertha’s front end out of the access pit. We'll post photos of this work in the coming days.
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.
Last 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.