It sounds like a good concept, but, my plans are slightly different where my OTG (Off The Grid) home will consist of a couple self-sufficient independent buildings above ground connected via underground storage / living bunkers (think of a gopher home - several openings to the world above, but, tunnels under ground).
BunkerBob has an amazing OTG home and is in the process of building his underground portion for self-sufficiency ..
SixGunsRattlesnake, I'd be interested in hearing updates on your progress. I like the concept, would definitely want help packing the dirt in, would like to have an indoor greenhouse along one exterior wall instead of tires, for a narrow patio living space with entry doors at both sides.
What type of ceiling/roof materials are you planning on using? Maybe a rain catch area as well?