Next year, workers will start digging a 1.7-mile tunnel underneath downtown Seattle using the world's largest tunnel-boring machine. The 57.5-foot-diameter, $80-million drill, which is currently under construction for the State Route 99 project, has about 600 cutting tools-steel bits and spinning disks on the borer's face that break up dirt and rock. The tools may need to be inspected as often as every 400 feet, or about 20 times over the course of construction.
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