I had made a 24' x 16' slab out of doubled 2 x 6's and filled it with modified stone with the intent of having a shed laid on it. The shed company found that they could not get to that location on the property. It has now been two years and the stone seems fairly solid.
I'm now considering building the walls directly on the perimeter and raking a top layer of smaller round gravel over the modified to make for a smoother surface. Although I'm unsure as to how stable the sides of the slab will remain under the weight of the shed walls and roof and how the modified will work as a floor.
Appreciate any input,

You need to check your building codes, Personally if it was a shed I was building I would want a better foundation for it, if I wanted it long term.