[ajb]

Well at least one of these was originally a quarry, so yes. But it would raise the cost a lot, if the excavate wasn't valuable as in this case.

[FFP999]

It'll definitely raise the cost, I acknowledge that what I'm talking about isn't going to be a small project. I just wouldn't necessarily assume that it would raise the cost so much as to make this infeasible. To be clear, it's also not like I know anything about civil or power engineering.