Typically in construction, when an area is excavated, the undisturbed level is where footings and other similar structural components are placed, to avoid settling. Other situations where it was over excavated, the back fill has to be suitable in gradation, moisture level, etc. and compacted in 12" lifts to a desired compaction level. Each lift can be tested easily for percentage of compaction with a (Troxler)nuclear density meter, in the old days before these they did proctor tests etc. Some materials like quality gravel, with composite sizes of aggregate that will compact to 95%-100%, which I believe is better than undisturbed, which if I recall correctly is around 85%. We used to build small earthen dams and the undercut was excavated out to a certain level and compacted 100% to make it impermeable. The material used had to be suitable with the right moisture content, or it would not compact.
The compaction you have depends on the material placed in the excavated area and how it was compacted. If it was a suitable material known to compact like mentioned above with a vibratory roller, plate compactor, split drum sheepsfoot vibratory roller like a walk behind in lifts or increments within the range of the particular machine, you would be fine to place a slab on it. Fill should be at least 85% like undisturbed, but the only way to know is testing each lift. Ideally you want suitable material as a subbase onto bearing undisturbed, like around here you can undercut to a relatively shallow level, fill with gravel, crushed stone road base like what NYS DOT calls Item #4 compact and place a slab.
If you have questionable material, or unknown fill from your site that you don't know how it compacts, has been filled in the cut and maybe tamped with a backhoe bucket, no vibratory roller or some other hap-hazard method to backfill, you may very well get settling. If you know the material does compact and can bear loading, but was not compacted using a vibratory roller or the like, yes, I'd wait til spring or excavate again, compact in lifts, up to your subbase, compact that, form, then place your slab. Its not going to hurt to let the weather settle it in, you could place some subbase material, grade compact, use as a temporary wear course this winter, then regrade, add more subbase etc. compact, then place the slab.
The fill material has to be suitable, with an optimum moisture content to compact properly, if not it may not, could be unstable, and do a myriad of things under different conditions. The underslab area should also be well drained, or constructed so it does not trap water.
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