Big Bertha

"Bertha" is the world's largest tunnel grinding machine, built in Japan over the past two years and shipped in pieces to Seattle where it was reassembled. Bertha is 326 feet long and weighs 7,000 tons. Seattle's controversial big dig was officially underway yesterday, as the drill started slowly moving under the Seattle waterfront, boring out a tunnel 57 feet in diameter for a stacked, four-lane highway 2 miles long. It will be the widest single bore tunnel ever built. The 3.1 billion (and counting) project will eventually replace the Alaska Way viaduct.

Sometime in October, the machine will go under the old Alaskan Way Viaduct, considered the most sensitive part of the path, running only 30 feet below viaduct pilings and past historic Pioneer Square buildings. Monitoring devices will check for ground movements of a fraction of an inch, throughout downtown.

The machine’s mouth, called the cutter head, slowly chips away at the earth, advancing about 35 feet per day. It operates like a worm, “swallowing” the dirt and passing it back to a conveyer belt that leads out of the tunnel and onto a waiting barge in Elliott Bay.  

As the “worm” wiggles its way through the earth, 206 feet below downtown at the deepest, it sloughs off concrete rings that line the tunnel. The tunnel will ultimately consist of 1,427 such rings made from more than 244,000 tons of concrete.  Not everyone appreciates the tunnel, especially Seattle environmentalists who oppose spending $3.1 billion to serve automobiles.