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SR 99 tunnel
A view south of the SR 99 tunnel from the back end of Bertha in May. Here the tunnel takes a slight westward turn as it heads under downtown Seattle near Yesler Way. (Via WSDOT Flickr)

Since getting beyond the delicate work of crossing beneath the Alaskan Way Viaduct in May, Bertha the SR 99 tunneling machine has moved into a stretch that takes it below downtown Seattle for the first time.

The Washington State Department of Transportation’s most recent progress update says that Bertha’s boring work is now 25 percent complete. New images on WSDOT’s Flickr page capture the latest views from inside the tunnel as well as the work being done at the northern entrance.

SR 99 tunnel
Before and after images at the north end of the tunnel capture the rise of a nearby apartment building along with work on the roadway. (Via WSDOT Flickr)

The machine is in the third zone of 10 total on the tunnel route. It has traveled 2,400 of the 9,270 feet it must go to reach the finish.

In May, Bertha tunneled 722 total feet and forced the 10-day closure of the elevated highway it’s working to replace as it passed just 15 feet below the aged roadway’s foundations. The viaduct and ground around it held up and Bertha moved on.

SR 99 tunnel zones
The SR 99 tunnel machine is in zone 3. (Via WSDOT)

The machine has now built 361 of the 1,426 rings that comprise the tunnel wall. A YouTube video last month offered a glimpse of what that process looks like as wall segments were trucked in and moved into place by the massive machine.

According to WSDOT’s website, Bertha’s entrance into downtown means it is now tunneling through clay, which is dense and uniform, and other glacially compacted materials. The machine has traveled 86 feet so far in June on a route that will pass beneath Western Avenue near Marion Street toward First Avenue.

The top of the machine is approximately 105 feet below the surface as it approaches Marion Street.

WSDOT’s photographs also capture the progress being made on the operations building, on-ramp and off-ramp at the northern end of the project.

SR 99 tunnel
The SR 99 tunnel’s north portal operations building. WSDOT says buildings at both portals will be the tunnel’s nerve centers where staff will manage lighting, ventilation, drainage and fire-suppression systems. (Via WSDOT Flickr)
SR 99 tunnel
It almost looks ready for cars: Drivers leaving the north end of the tunnel will take this off-ramp to get to the South Lake Union and Queen Anne neighborhoods. (Via WSDOT Flickr)
SR 99 tunnel
At the tunnel’s north portal, landscaping is installed along the future Sixth Avenue North. Drivers will access the southbound on-ramp from Sixth Avenue North. (Via WSDOT Flickr)
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