How to backfill trench under slab in Los Angeles
You need to use a tamper and tamp each 6&quot;-12&quot; of fill as you go:

It is also important that you have a plan to tie the new concrete to the existing slab, to help prevent independent movement. For mine we drilled horizontally into the edges of the existing slab and used epoxy to set bent rebar rods into it, then tied those to the rebar grid for the new concrete.