Changing The Teeth On The World’s Largest Tunnel-Boring Machine

By Tim Newcomb

Drill Dentists Kevin Hand

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, 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 throughout construction.

The Problem: Accessing the front of a boring machine already belowground is hard, particularly in deep tunnels with dangerously high air pressure. Repairing cutting tools, therefore, is typically a task for workers who must spend time in hyperbaric chambers each time they visit the machine to acclimate to the pressure. (Two hundred feet belowground in an enclosed tunnel can get as high as 5 bars—the equivalent of being 165 feet deep in open water.) Crews retract the front of the machine to create a space ahead where a group of about five workers operates while wearing special helmets for breathing in those conditions. They use pneumatic wrenches and hammers to loosen the teeth, and pneumatic pulleys, hoists and chains to tug them out. The team returns to the surface after installing the new bits or disks. Replacing a single tool could take up to four hours.

The Solution: 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 for people to work just behind the drill face aboveground atmospheric pressure. An automated system retracts the cutting tools into the chamber, where a crew can make repairs. The chamber is also roomy enough to accommodate hydraulic pulleys and other hydraulic machines, which are more powerful than their pneumatic counterparts. With the safety and freedom from working at sea-level pressure, the better equipment could make repairs about four times as fast.