Hi all,
We are excavating for a 16×26 frost-protected monolithic slab on grade.  The site was a former slightly downhill sloping area that was built up with rock and concrete excavated from stone ledge that was broken to accept another slab (a few years ago) and pieces of concrete removed from around the old house that was attached to that slab.  I used a 500 pound rock breaker to explore how stable the fill was, for example, in an area that is to be thicker to receive a center post to hold the ridge beam mid-span.  The hammer pounded for a few seconds then went right through.  I suspect there are lots of voids in this site.

We plan to bring in crushed stone to compact over the area and excavate down 24″ (or until we hit bedrock) on the perimeter.

I am concerned about the thickened part, the rest of the slab, which is planned to be 5″ thick, and the downslope edge.  This edge is almost 100% fill, so 24″ might not get us to ground.

But forgetting about any particular edge, is there best practice for working with a site with filled rock with voids?  If we secure the edges and build them back up, are we OK with a 6″ gravel base under the slab, and 5″ of concrete despite that there are voids, likely undiscovered?

Or, sadly, would the whole site need to be excavated and refilled, filling, compacting, and eventually adding gravel?

Thanks, in advance, for any advice you might have.