Since this is an alternative foundation there is no specific reference, however a treated wood foundation over a gravel base is prescriptive, I'm just noting that as a technicality. There is some precedence in the codebook for a gravel footing. Superior Walls precast foundations also use a gravel base and do have code approval. I was kicking ideas around with an engineer last week who was talking about gravel under ICF forms as another possible affordable foundation.
Anyway from the permanent wood foundation section, R403.2;
...Gravel shall be washed and well graded. The maximum stone size shall not exceed 3/4". Gravel shall be free from organic, clayey or silty soils.
Sand shall be coarse, not smaller than 1/16" grains and shall be free from organic, clayey or silty soils.
Crushed stone shall have a maximum size of 1/2"
Devildog, this is a neat place. The edge of the Blue Ridge and the Valley and Ridge topographies. If you order from Cardinal Stone near the parkway it will be crushed stone composed of gneiss, metamorphosed granite, that came from precambrian times, is some of the oldest rock in the appalachians. It is the hubs of hell, the basement rock, upheaval mountains of irregular plutons of magma that have no real pattern. Mount Airy granite is one of the largest granite quarries in the world and supplies the salt and pepper granite you see in public buildings everywhere. The band runs the escarpment and includes Buffalo Mt, Mt Rogers, Grandfather Mt, Roan Mt, etc. Pilot Mt below the ridge is the remaining core of an eroded volcano.
Order from Wythe Stone or any of the quarries to the northwest and it is limestone and sands from 600 million years or so later. Limestones, dolomite, lead, salt, iron etc. Still darned old but it contained organisms in the sea on that side of the mountain that could form limestone. That topography has regularly undulating long lined up ridges and valleys. Holston, Clinch, Big Walker, East River Mts. The depth of the sea created different deposits as well, in oxygenated shallow water the soils left behind are now known as Rich Valley and Burke's Garden. Just over that ridge deep acidic low oxygen waters left behind Poor Valley,("The last train from Poor Valley taking brown eyed Becky, Richmond bound") Cross through the tunnel into WV and you step through a few hundred million more years, enter the Allegheney plateau. A raised level plain of what flowed off of the mountains and buried forests and fern beds which were in place by then, forming coal. The mountains there are the erosion of that plain, Flat Top and the mountains northwest. That erosion slope ends in Ohio.
BTW Wisconsin architect Frank Lloyd Wright was an early gravel foundation proponent. The red granite from around Merrill, WI is some of the prettiest stuff I've ever seen.
Well, that wandered, washed pea gravel is what I've used from Cardinal
