Bunker, mad respect brother. its like you took the idea out of my head & used it at your place. only difference is have been thinkin aboot a garage bunker underneath a new 3 bay garage. same exact principle. i can appreciate the hard work you done being in the GC world. i just need to buck up & buy a place. i just cant seem to let go of the convenience of being able to pack up & hall ass if needed. Well done bro, well done.
That would be a great way to do it, just build a full basement to your garage. If you don't have a garage, you kill two birds with one stone. The additional cost of doing that shouldn't be too painful. I've parked a skidsteer on top of this, so I'm pretty sure a it can handle a car. After the slab hardens, the weight is distributed over the walls. With a larger area you might need ot look into iron support beams. You'd probably need 2 support beams for a 3 car garage, no more than 8' span unsupported.
Fun fun fun!