Comment by idoh
9 years ago
If you start the tunnel down far enough, you could be below anything that could be hit. Would that make it less expensive?
9 years ago
If you start the tunnel down far enough, you could be below anything that could be hit. Would that make it less expensive?
You make it sound like all that's down there is uniform layers of perfectly intact rock just waiting to be bored.
No, it's a hellacious mess of rocks of different types, of muck of all kinds, of brittle, water-filled pockets of who knows what, and every inch you dig you find out there's another problem up ahead.
In the case of Seattle they planned, they surveyed, they did test drilling, and they plotted a course that should have avoided everything, yet they still managed to slam into a steel pole that shouldn't have been there. It set their project back months, the machine was trashed and had to be dug up and fixed.
Plus, these tunnels are only part of the package. You need those surface access lifts, and those may well be the most logistically complicated of the whole system. To make this accessible you'll need hundreds of them, potentially thousands, and each one is a mega-project unto itself if you've sunk the tunnel down deep enough to avoid all those hazards you're now digging straight through.
The deeper you go, the more those lift stations cost. The shallower you go the more you'll come into conflict with infrastructure. There's no easy win here.
Thanks for the info. I asked because I didn't know, didn't mean to say that it was a piece of cake or anything like that.