Sand is not stable fill -- one leak, and your fill is gone.

If you use concrete fragments, you have to try to use something on top that will not migrate into the spaces between the fragments of concrete. Of course, you can grind the concrete up intoin to gravel sized pieces.

Dunno what short of fill you have available in your area. Around here, the tendency would be to use chert backfill, and a 'whaley-packer' to compact it in 4 - 6" layers, that is, if the contractor were doing it 'right'.