I'm no structural engineer but you'll need more sopport - 4x4s or lally caolumns to keep the roof from caving in. Dirt is heavier than people think and you'll die in there if you fuck it up. Also wrap the top and sides with thick plastic sheeting and make sure there's proper drainage around the sides so it doesn't flood the floor constantly (could also build a raised plywood floor on cinder blocks or the like).
Ventilation is important but especially if you want to cook or have a fire, I would put a smoke/carbon monoxide detector in it and checking for radon might prevent lung cancer in 20yrs.
https://www.youtube.com/watch?v=o5EVC3rgJ-o 