What's this tunnel going through, U?
From what I've read, mostly mud and fill.
Depends what portion you are talking about. It's not one tunnel, is't several tunnels constructed using nearly every soft-ground tunneling technique available.
When the NYC tunnels were created, they were in virgin ground. Much of the boston tunnels are under existing highway,partially in fills, partially in clays that woudl not support you if you were to stand on them, and constructed all while maintaining surface traffic.
Yeah, there are some serious screwups, and political crap, but the fact it could even be completed is amazing if you took time to study the entire project.