[DougWebb]

Boston's Big Dig project was an incredibly expensive tunnel project. I'm not sure how much of the cost overrun was due to the terrain they were digging through. I think it'll be a long time before another major tunnel project will be approved though.

NYC always has several big tunnel projects going on. The problem here is the very hard granite and the depth of existing infrastructure. New tunnels have to be very deep. At the moment I believe there is a large east-side subway project and a water main project. There was supposed to be a new west side tunnel for Amtrak and NJ Transit trains coming into Penn Station from NJ, but unfortunately NJ's ahole governor killed the project.

I'm not familiar with tunnelling projects in Philly and DC, or the areas between them. I suspect the terrain is a lot more varied than below the Channel though. It'd be challenging to tunnel the whole way.

[icegreentea]

Just to build on this a bit, the Channel Tunnel's current location was chosen because this way nearly its entire length runs through continuous chalk, which is incredibly easy (especially compared to granite, or wet silt) to tunnel through. It's soft enough to cut through without too much problem, but it's still rigid and dry enough that you don't have to worry too much about collapses.