Sept. 8 project update: Cutterhead maintenance continues

Posted on Sep 8 2016 4:25 PM
Seattle Tunnel Partners is continuing to change large cutting tools on the cutterhead of Bertha, the SR 99 tunneling machine. Crews have inspected 12 tools – replacing 10 of them – during the current maintenance stop. They could change up to four additional tools before they resume mining. 
This is a normal part of tunnel mining. Cutting tools wear down over time and must be replaced regularly. Bertha recently transitioned from soft clay to a more abrasive mixture of sand and gravel, resulting in greater tool wear. 
The tools STP is focused on 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. Replacing them now will preserve the cutterhead and keep the machine in good working order as it continues mining north beneath downtown.
The current maintenance does not require crews to work in hyperbaric conditions, as they did during the previous two maintenance stops. That’s because the cutting tools along the spokes of the cutterhead can be accessed from within the machine, at regular air pressure. Crews performing the work must climb into tight quarters inside the spokes and use hoists and chains to remove the tools, which weigh up to 600 pounds. 
STP will resume mining when the tool replacement is complete, no sooner than next week. We’ll continue to provide updates as this work progresses.


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