jomama
I Think that the first part of your comment is to my point. You cannot support concrete with loose fill, and expect it not to break again. I suppose you are correct if the under-fill has settled sufficiently to carry the intended loads that are applied, but how would you know? I also guess you could mud jack again if it settles again. You can build a self supporting structural floor that will easily carry the weight of an automobile if it is designed correctly, but it would be easier to use the right kind of fill and compact it in lifts of a foot or less with a vibra-plate, but no builder does it, and no local building inspector that I know of demands it. They simply put in a caveat that cracks of up to 1/4 inch are not a reason for rejection. I can tell you right now that they are a reason for rejection, and it is entirely possible to build a slab that will not crack. ever walk around in a Home Depot or Lowes? How many cracked slabs do you find? Its all part of the "Build them fast, not to last" mentality of todays tract housing builders.