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Normally what takes place is that a soil test is done beforehand and the design follows from that. In this case, it looks like a typical footing detail was copied and pasted onto the drawing.
Building sites normally get excavated down to the point where all of the organic material on the top is removed and you're down to soils with a suitable bearing capacity. It may require additional material be brought in, placed and compacted, to get the elevation back up to the correct level. Putting in that much clean rock, topped with sand, will be difficult to compact and may result in settlement over time. Think of the sand working it's way down through the voids in the rock, over time...
If the organic material had been the typical 6" or so that is usually found in most locations, I'd have suggested digging a foundation trench 12" wide and leaving the middle undisturbed. Quicker, faster and less expensive in the long run.
Anyway, that option is now gone, so good luck with the project.
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