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 planned to dig the SR 99 highway tunnel under downtown Seattle would be 58 feet in diameter. The following is a picture of a slightly smaller TBM than the one for SR 99:
Photo source: http://www.cascadiaprospectus.org/2008/12/gregoire_advisor_tunnel_probab.php
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 2,642 square feet.
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Last modified: August 29, 2011