SR 99 TBM Advances Out of Access Pit

By Jim Rush January 8, 2016

Crews working for Seattle Tunnel Partners watch as the SR 99 tunneling machine’s cutterhead rotates during testing on Dec. 16, 2016. Crews began filling the pit that STP used to retrieve and repair the machine on Sunday, Dec. 13. (Photo from WsDOT).

Crews working for Seattle Tunnel Partners watch as the SR 99 tunneling machine’s cutterhead rotates during testing on Dec. 16, 2015. Crews began filling the pit that STP used to retrieve and repair the machine on Sunday, Dec. 13. (Photo from WsDOT).

Bertha, the SR 99 tunneling machine, is now tunneling in Seattle soil after breaking through the access pit wall late Wednesday, Jan. 6. Seattle Tunnel Partners (STP) has mined 73 ft and installed 12 concrete tunnel rings since the TBM first moved forward in the pit on Dec. 22. video of the TBM in action can be seen here.

Now approaching South Main Street, near Pier 48, the TBM is digging well below the area’s notorious fill soil. The top of the machine is approximately 80 ft below the surface in a mixture of glacially compacted material. Washington State DOT’s website has additional information about the conditions that crews will face over the remainder of the tunnel drive.

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. The revised schedule anticipates tunneling to be complete in January 2017.

Additionally, crews have begun disassembling the massive crane that was used to lift Bertha’s front end out of the access pit.