Big Bertha

July 20, 2013

Seattle is replacing its aging, elevated Alaskan Way Viaduct with a deep bore tunnel.  It will be the largest diameter deep bore tunnel ever dug and will be dug by the largest tunnel boring machine (TBM) ever built.  The machine's name is Bertha, and today was the one chance for the public to see it up close and personal before it disappears underground for the next couple of years.

Here's some information from an article I read.  It weighs in at 7000 tons, the equivalent of more than 30 orange and black BNSF Dash 9 diesel locomotives. Its 24 huge electric motors generate the 25,000 horsepower that will slowly rotate the cutterhead (at about one revolution per minute) as it chews and grinds the ground – TBMs don’t actually drill into the ground. The TBM’s 56 thrust jacks that push the rotating cutterhead (the business end of the boring machine) against the ground ahead exert 44,000 tons of thrust, or 13 times the thrust of the engine and booster rocket that lifted the space shuttle into orbit. The machine is powered by what is essentially a 26 kilovolt extension cord. Seattle City Light built the dedicated feeder line just for this purpose.

Here's a photo of Bertha.  It's very hard to get a sense of scale, but you can get some idea by comparing the machine with the people (the tiny, little people) on the left.