ROAD TUNNELS: WSDOT starts on access pit to repair S.R. 99 TBM

With environmental review complete, STP begins assembling pit equipment

May 09, 2014

Environmental review of Seattle Tunnel Partners’ (STP) plan for accessing and repairing the S.R. 99 tunnel boring machine (TBM) is now complete. The on-schedule completion of the review clears the way for STP to build the 120-ft-deep pit that will allow crews to access the machine, which is located about 60 ft below the surface in a fenced-off construction zone between South Jackson and South Main streets.

The ground above the machine has been closed for construction since 2012 by the Washington State Department of Transportation (WSDOT), but further environmental review was required for this additional work.

Pit construction will be hard to see in person because crews recently built a wall to shield neighbors from construction noise. The best spot to view STP’s work is online at WSDOT's newly installed time-lapse camera (

The camera went live on Saturday, May 3. On May 7, following completion of the environmental review, STP began assembling equipment where the pit will soon take shape. This work is consistent with the schedule STP released last month. Crews are relocating utilities and will spend the next week injecting grout into the ground above the machine. Later this month, they will begin installing the pit’s underground walls.

STP’s schedule shows that excavation of the pit will last from late July through September. When the pit is complete, the machine will tunnel into it. Crews will then partially disassemble the machine and make repairs. These conceptual drawings illustrate the basic idea (…).