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.
The Problem: Accessing the front of a boring machine that's already belowground is hard, particularly in deep tunnels, where the air pressure is dangerously high. 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. After installing the new bits or disks, the team returns to the surface. 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 that people can work just behind the drill face at 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. The better equipment, combined with the safety and freedom from working at sea-level pressure, could make repairs about four times as fast.
This is a great article, the only thing missing is explaining why the drill is operating under 5 bars of atmosphere. Are the chambers the drill is operating in under 200+ feet of water? When I read this I'm thinking it's drilling through earth. I'm assuming that there is some artificial pressure in play here to prevent against a Lake Peigneur-type incident? http://en.wikipedia.org/wiki/Lake_Peigneur
Would have loved to reach more about the implications of pressure while drilling along with the logistics of the drill itself.
Google Deep Undergorund Military Bases.
I would imagine it helps support the tunnel walls and/or is due to the air having to support the tunnel walls.
Sorry but this is Not the largest. A German company recently build a 63 foot tunnel boring machine for a tunnel project under St.petersburg. Also this tunnel is very short so not that impressive compared to any of the Swiss Mega tunnels.
I believe johnt007871 is partially right. When they back the cutter head up the soil is no longer supported. They need to support it while workers are performing maintenance. One way they do this is by applying a bentonite slurry to the face of soil. This creates a seal and they increase the air pressure in the plenum which is like blowing up a balloon. This prevents collapse of the 56 foot tall "soil wall".
Another reason that these machines operate with pressure in the cutter head (aside form saturated soil conditions) is to help steering the TBM. Try this: push a pencil across your table at the eraser end you will find that it will start to get off track and "jack knife". Pressurizing the front of the machine helps to steer it. Now, while pushing on the eraser end again while applying a little pressure at the graphite end steering is more manageable.
I find this article boring.
But I mean that in a good way, lol.
Science sees no further than what it can sense, i.e. facts.
Religion sees beyond the senses, i.e. faith.
Open your mind and see!
Actually GreenMatrix, I'm pretty sure it will be the biggest........ for about 5 minutes.
The Herrenknecht 19.25m Mix Shield TBM machine for the Orlovski Tunnel hasn't been built as of yet (though the contract for it has been confirmed.
It is not expected to be delivered until after the Hitachi Zosen 17.6m TBM is delivered to Seattle.
So the Seattle TBM will hold the world record .....but not for long.