Each of the three tunnel boring machines (TBMs) used to dig out the twin subway tubes under Capitol Hill and the Ship Canal that will convey Seattle's University Link light rail are 21 feet in diameter. Here is a photo of one of the machines:
Photo source: http://www.kplu.org/post/sound-transits-giant-tunnel-machine-nearly-finished
The single tunnel boring machine that has been built in Japan to dig the SR 99 highway tunnel under downtown Seattle is 57.35 feet in diameter. The following is a picture of the machine that has been named Bertha, taken in December 2012 as the machine is delivered to the tunnel construction company.
Photo source: http://www.youtube.com/watch?v=EMIorr7DJ7c by Tunnel Talk Videos
Using high school geometry, we can calculate that the area of the digging face of the light rail TBM is 346 square feet. The cutting head area on the larger machine for digging the highway tunnel under downtown Seattle is more than seven times bigger at 2,582 square feet.
Web cam photos of the TBM rescue mission underway in March 2015 on the big machine pictured above are posted here. See photo labeled "SR 99 access pit #2."
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Last modified: March 08, 2015