We are getting ready to pour a new concrete slab for a daylight basement. The slab will be a 3 1/2 in thick reinforced slab over 4" of compacted sand but the whole system will be over between 2 1/2 to 3 feet of clean, native, non-expansive infill. This was not compacted in lifts although the exavation equipment was used to rough tamp it. A vibratory pad will be used to further compact it, however. No water is available to wet the soil down thouroughly which might have been useful. Does anyone see any potential pitfalls?

The slab should be a full four inches thick, and should be placed over a gravel bed four inches deep, with a properly installed and sealed vapor retarder between the concrete and the gravel.

Not enough is known about the subsoils to comment on possible pitfalls, but the composition of the soils upon which the fill was placed is important to know, and the fill ought to have been placed in lifts and compacted. Heavy equipment does not compact soil. Think about it...the tires or treads or rollers, or whatever it runs on, are designed to distribute its weight so that the unit loads are small, and so that it doesn't sink into loose soil. That won't achieve any compaction to speak of, at all.

I'd consider removing the fill and having it placed and compacted properly.

Several,
You need at least four inches of cement. Also you need to lay 3/4" crushed gravel down. Not sand. You also need to place a radon mitigation pipe under the new slab as required in most states these days.RAdon systems require communication under the slab that gravel provides with sand you would not be able to remove radon if it was present with conventinal methods. What about interior drainage system? With sand if you have any moisture issues the water cannot be deverted to the drainage pipes with sand. It flows through the gravel.Do not forget the plastic over the top of the gravel as a moisture barrier. What about foam insulation below the cement for warmer floor?