Seattle Tunnel Partners (STP) resumed mining following a temporary stop to change tools on the cutterhead of Bertha, the SR 99 tunneling machine, Washington State Department of Transportation reported. As of Sept. 12, crews were tunneling north approximately 170 ft below First Avenue, just north of Union Street.
Crews replaced a total of 14 large cutting tools during a two-week maintenance period. Bertha has moved from clay to a mixture of sand and gravel that wears down cutting tools more quickly. A photo comparing a new cutting tool with a worn cutting tool is available in this post.
The tools STP changed during this maintenance stop are the first parts of the cutterhead that contact the ground, making them crucial to the machine’s ability to excavate soil. STP will continue to inspect and replace these tools as needed during the course of mining. They are also planning Bertha’s next hyperbaric maintenance stop, the location of which is still being determined.
The machine has tunneled more than 4,135 ft and is nearing the halfway point of its 9,270-ftnlong journey.