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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.
Engineers on the Seattle project have modified the design of the drill, manufactured by Hitachi Zosen, so that workers can replace the teeth from inside the safety of the machine itself. The new borer is large enough that people can work just behind the drill face at aboveground atmospheric pressure. The better equipment, combined with the safety and freedom from working at sea-level pressure, could make repairs about four times as fast.
Read more at PopSci.com.