I asked a somewhat similar question about tunnels, and it seems that bombing them won’t cause a cave-in. So far, I’ve had made a tunnel that goes from one point to another. In-game time it take around four and the half hours to go from one place to another while dragging a wheelbarrow. So far, no cave ins have happened, so it’s probably safe.
Aside from that, jackhammer seems to be the fastest way to get it done. That four and a half hour long journey is pretty much a narrow tunnel. Used about two plastic jerrycans filled with gasoline to dig through that far.
