I would suggest backfilling with fill sand, then a 8" final layer of aggregate (we used 1" minus crushed concrete). Pit run Sandy loam is what we use(compacts nice). Pack it in lifts no deeper than a foot thick. The flood it idea works great! Put the aggregate down, pack it, wet it some, pack it some more. Let it set, pack it again! Grade it out as smooth as possible to maintain a consistent thickness of poured concrete. Standard is 4" thick here, we go 5" to be safe. Most all cheapskate contractors here pour on fill dirt the whole thickness... This is cheaper, but yields a weak stability to pour on. I don't like pouring on only fill sand. Sorry. I've worked Excavating my whole life, so I am speaking from experience. My 2cents...