Did you put your plastic and insulation right over the 304?When I built my 32' x 48' building I had to put in nearly 6ft of 304's or crushers run as some might call it limestone in one corner. It's a mixture of fines, #57's and #4's that really compacts in well. I didn't concrete it for quite a few years and did compact it while putting it in. When I concreted it I put in foam insulation and plastic under the concrete and so far no cracks and no settling. Don't let them put in regular dirt unless you can have it compacted before putting up your building so they can use a sheeps foot or vibratory compactor. Some around here have also used fly ash from the local power plants which compacts in well. Not sure on what others might have to say about any of this reacting with concrete but I've seen a lot of both used.
Good luck
Topstrap
I would always recommend to customers that they grade the land properly and then build if at all possible. In your case can you correct for the drop off, and then build?
Anything that does not contain organic material. The key is proper compaction.
Crushed concrete is fine, but larger sizes (> 1") are hard to compact with a plate compactor. If you are going to use crushed concrete, stick 3/4" down.
"Crusher run" is good because it contains different sizes material which helps it compact well.
Do NOT use "river rock", "pea gravel" or any aggregate with smooth round surfaces. It will not lock together when compacted.
You need to do your fill in "lifts" of about 4-6". After you have placed 4-6" of material, compact it. Take your time. Keep going over it until the surface is hard.
I would use sand for the final lift. Easier to spread and level.
Did you put your plastic and insulation right over the 304?