How much would it cost?

How much would I cost to shore up a 500 foot tunnel that is 3 feet tall and 3 feet wide? Also, what material would hold up the weight of the dirt above best? Wood? Concrete blocks? Cinder blocks? I haven't started the tunnel yet, I'm in the planning stage. But I did get a permit approved for a bomb shelter (because that's exactly why I'm making this tunnel...). But seriously, what materials should I use and how much will they cost?