I'm interested in building some tunnels as well, combined with a root cellar and "hidey hole" for tornadoes. Other questions you might consider is the total depth underground you want, how permanent you want it be, your area's seismic activity, and what kind of subsoil (or huge boulders) you have.
Around here, if a boulder can't be moved by a large back hoe, it is jackhammered into smaller pieces. If it's too massive for jackhammering, then it's drilled and blasted with dynamite. From my test holes, I've determined that my rocks can be handled by a back hoe, so I'm considering using one to dig a deep trench for the tunnel. Then I'll form it up for pouring concrete. I'm still debating on whether to use an arched ceiling, or a flat, poured concrete ceiling. On top will be about 18 inches of subsoil and whatever topsoil I have left. Then I'll attempt to grow grass.
A neighbor of mine built a seriously large bunker under his new house. The walls and ceiling are two-feet-thick poured concrete. For my tunnel, eight inches should suffice. I do not plan to use any steel rebar because it rusts and actually breaks up concrete over time. My concrete will be a stiff mixture containing rough aggregate.
If you need to burrow though a mountain (or hill), then my plan won't work for you. I've heard of tunnelers using "Gunnite" concrete to line it, but it requires a special machine for spraying it on. Tell me more about your terrain and the other considerations I mentioned at the beginning.