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Bertha breaks 4K feet as Seattle tunnel approaches halfway point

The tunnel below Seattle as it curves under First Avenue. (WSDOT)

After tunneling 4,000 feet, Bertha is now about 165 feet under First Avenue at Union Street in downtown Seattle.

When the tunnel is completed, it will be 9,270 feet long. Currently, the tunnel is just shy of the halfway point at about 43 percent.

How tall Bertha’s tunnel is after the rings are installed

So far, crews have installed 607 concrete rings to form the tunnel walls. At this stage, crews are working amid a mixture of clay, sand, and gravel.

Above Bertha, at First and Union, are four buildings including an office building built in 1910 at the northwest corner; a portion of the Seattle Art Museum built in 2005 on the southeast corner; the Four Seasons Hotel built in 2006 at the southwest corner;  and a 76-unit condominium built in 1985.

This also means the machine is on the border between zones 5 and 6 — there are 10 zones that make up the entire project. The machine was able to move through zone 5 in about a month’s time.

The plan is for Bertha to keep boring under First Avenue for about 3.5 blocks to Stewart Street where zone 6 ends. The machine will then start to turn right and west. After that turn, the project will continue on to the Seattle Center area where the tunnel will end.

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