Both Vancouver and Seattle have wanted to put huge tunnels under the city in muck. Both are now experiencing settlement problems which is a bad thing in earthquake country.
But Seattle is much more. Just barely into the tunnel, this monster, the largest ever, sprung it's bearing. Since the entire machine is just the bearing, this cannot happen, or should not. I saw one of this things being made once, just a small one. Unbelievable machining on that bearing, which is virtually whole cost of the thing.
Now, it is very common that when one thing works, somebody says "Let's make a bigger one.". We know what happened at Darlington -- sooner or later you run into new physics. With a tunnel boring machine I think you run into several exponentials:
1. Flaws in the steel. The probability goes up as an exponential.
2. Hydraulics. This is my main guess. The whole thing is a hydraulic motor, and you have to get the oil to every nook and cranny for lubrication. All sorts of room for non-linearities.
The now have to sink a shaft 120 feet into muck. This is as bad as sinking a shaft for the Bruce Black Hole. The pressure goes up linearly, but that squares for the force. Then they have to lift the sucker. Good luck with that.
Reference Seattle
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