Seattle Tunnel Partners resumed mining today following a temporary stop to change tools on the cutterhead of Bertha, the SR 99 tunneling machine. As of lunchtime, crews were tunneling north approximately 170 feet below First Avenue, just north of Union Street.Crews replaced a total of 14 large cutting tools during the 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 feet and is nearing the halfway point of its 9,270-foot-long journey. The entire tunnel route, including descriptions of each of the 10 zones through which Bertha is mining, can be found on our Follow Bertha page. Tunneling statistics are updated on that page on Mondays and Thursdays. You can also track Bertha’s progress on Twitter @BerthaDigsSR99.Recent tunneling updates— more —
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.Crews are replacing worn cutting tools (left) with new ones (right).Recent tunneling updates— more —
After tunneling more than 1,000 feet since their last maintenance stop, Seattle Tunnel Partners has paused to inspect and replace some of the larger cutterhead tools on the front end of Bertha, the SR 99 tunneling machine. STP chose to check the tools – and change them as needed – because Bertha has moved from clay into a mixture of sand and gravel that will more quickly wear them down. Replacing the tools now will preserve the machine and ensure it continues to function well as it mines toward STP’s next planned maintenance stop.This work does not require crews to work in hyperbaric conditions as they did during the previous two maintenance stops. Still, their task is challenging. Crews must climb into tight quarters inside the spokes of the cutterhead and remove tools weighing up to 600 pounds with hoists and chains. Each tool can take 20 or more hours to remove, inspect and reinstall.Crews have inspected four tools since they stopped mining late last week, and they could inspect up to eight more in the coming days. STP will resume mining when tool replacement is complete, no sooner than next week.The cutterhead is approximately 170 feet beneath First Avenue, near Union Street. The entire tunnel route, including descriptions of each of the 10 zones through which Bertha is mining, can be found on our Follow Bertha page. Tunneling statistics are updated on that page on Mondays and Thursdays. You can also track Bertha’s progress on Twitter @BerthaDigsSR99.
Recent tunneling updates— more —
The public may not be able join Bertha as she tunnels beneath downtown, but that doesn’t mean you can’t track her journey at street level. We recently launched a bicycle tour that leads attendees along Bertha’s tunnel route.
The approximately two-hour tours depart from Milepost 31, our information center in Pioneer Square. Attendees pedal with a guide from the south portal to the north portal, learning about the historical, archaeological and engineering aspects of the … more