“I’m not sure about that I’ve never really thought about it, but most Road boring is shallow work usually no more than 30 feet deep.”
While conceding no technical knowledge about this; I’m guessing that in order to clear all the already present underground infrastructure along his proposed urban east coast route he will have to tunnel much deeper than thirty feet.
I was doing this in and around Dallas Texas so I’m sure you’re correct about the depth along the east coast they can go as deep as they need to go , but I was familiar with work in Texas mostly shallow work