For a frost protected shallow foundation how do I compact gravel?

A homeowner planning a garage build in a cold climate raised a practical question about frost protected shallow foundations, a foundation type that skips deep footings by using rigid insulation and a bit of foundation depth to keep frost from reaching the soil below. The building code calls for a 12-inch thickened concrete edge around the perimeter, a 5-inch slab in the center, and 6 inches of coarse, free-draining gravel underneath both. The catch: that kind of gravel, often called #57 or #5 stone, has no fine particles to lock it into a dense mass, so it does not "compact" in the way soil does.

People answering the question explained that crushed, angular gravel (not smooth river rock) will interlock and settle into place even though it never becomes truly dense. Running a compactor over it still helps, because it presses the bottom layer into the ground below and settles the stone so it will not sink further once the slab is poured. For the sloped transition between the deeper edge and the shallower center slab, the gravel can generally hold a slope of about 1 foot of run for every 2 feet of rise, so a 7-inch height difference would need roughly 14 inches of horizontal space to taper.

There was some debate about why building guides call for a sloped gravel transition instead of a simple vertical step between the thickened edge and the slab. The best guess offered is that a sloped edge may reduce cracking as the concrete cures and shrinks, though no one had hard data to back that up. For most homeowners, the underlying message is that a properly built slab, poured and cured correctly with control joints, matters more than the exact shape of that transition.