Ouch! This looks messy guy. Love youre enthusiasm but please be very carefull. I have done excavation for one of my very many contracting jobs and what you are doin is out of the ordinary and very unsafe.I Like the idea of what you are doin but you are gonna have ta dig the hole wider then that and build some sort of retaining wall for the walls not ta cave in from the weight of the car. Then you can cement over it. You can use green treated so the wood does not decay as fast but youre best bet would be ta make the retaining by a none corrosive alloy. And while pouring the cement dont forget ta mesh rebar into it for support. Another thing you can do for retaing support is mason it. Might prove more cost efective. Again just use lots of rebar and poor the walls thick. Being that its gonna be under dirt and the elements till you can close it off. Keep it up ta maintnince. Cracks in the mud and season changes will change the outcome of how you built it. The ground always shifts! You want sometin done do it yourself but be safe
