The geotechnical investigation found fill. Now what?
Normal slab fill evaluation flow chart would look like this approximately:
Is the fill organic - if yes remove and replace.
If fill clean, do stepped test pits under the grade beam locations, through the fill, with densities and proctors. Steps provide access and benches to do densities without tieing up the backhoe. Mind OH&S. Under grade beam locations will less sensitive to the new backfill of the test pits.
If fill is more than a year old, and density above 98%, good.
If density 98% to 95% expect some settlement, and allow for mud jacking in a few years, if settlements occur and is a problem.
If density <95%, rework or expect a lot of settlements, or structural pile supported slab. This can be done just in the office, where settlement is likely an issue.
It is the clients decision. The numbers may change with depth of fill and expected loads.