Seattle Tunnel Partners passed the 5,000-foot mark of the SR 99 tunnel drive late last week. Crews have mined 5,085 feet of the 9,270-foot-long tunnel and built 774 concrete rings. Bertha, the SR 99 tunneling machine, is now located below First Avenue, just north of Pine Street.
STP is currently performing routine hyperbaric maintenance on the machine, including the inspection and replacement of cutterhead tools (this post explains more about the different methods for changing tools). Previous hyperbaric stops have lasted as long as seven weeks. However, STP has indicated they may choose to do shorter and more frequent hyperbaric stops moving forward.
Now more than 200 feet below the surface, crews are approaching the deepest point in the tunnel drive. A map and description of the tunnel route – as well as twice weekly progress updates – are available on our Follow Bertha page. You can also follow Bertha on Twitter @BerthaDigsSR99.