TLDR: Site is almost impossible to reach with heavy machinery, considering use of a two-way suspended slab in lieu of a traditional slab on compacted gravel because I can hire a concrete pump but I can't pump gravel.

Context:

I'm currently planning out the foundation for a 20X30 machine shop in Connecticut. Initially, the plan was to put 6" frostwalls reaching down 42" below grade on footings, with a conventional insulated 5" slab on compacted gravel as the finished floor.

However, a few things have gone poorly. The soil at the site has almost no bearing capacity; I sink in when I'm in my boots in some places. I am able to drive a pickup or small equipment on it, but that's unfortunately about it. I'm not concerned about this as far as the concrete goes because I was planning on hiring a pump anyway and it's only ~150 yards from the road, but I do not like my odds of getting a dump truck back there. At the same time, we've dug down and discovered that the soil is still pretty mushy where we were planning on pouring the footings, so we are going to have to go down another 30", and we will have to replace most of the dirt that was to support the final slab with more compacted gravel.

The combination of these two problems gave me a thought. I am more than happy to pay more in concrete to avoid schlepping many tons of gravel up the hill a thousand pounds at a time; what if we were to fully excavate under the future slab, pour two colums along the centerline of the building at 10 foot spacing, thicken up the frostwalls to 13" with a corresponding increase in the thickness of the footing, and pour a 7" suspended slab supported by the frostwalls and columns? That way, with just a thin 2" slab along the bottom we'd have a convenient crawlspace we could use for louder equipment, an easy place to run utilities, and I wouldn't have to find a way to move all the gravel I'd need otherwise.

Obviously, I'll still need a lot of gravel for the footings as well as for drainage. I've tested it, and it seems like I can fit 1000 lbs in a yard trailer or about the same in my truck bed before I start running into issues. That said, if this saves me extra weeks of loading gravel by hand a half-ton at a time, I think it's worth it.

My problem with this plan is that I'm certain that there's something I'm not understanding. I'd appreciate any pros who could tell me why I'm an idiot and this plan makes absolutely zero sense. Obviously, I'm not going to put my stamp on this and will certainly hire an actual civil engineer to look over this once I finish drafting this up, but I want to know if it's even worth going that far.