rpc311,
Goofy answer: Drop your potato onto the rockfill. Record penetration depth. (Sorry, I couldn't resist!)
I am assuming that your description of the rockfill is inidicative of the particle sizes rather than the layer thickness.
The penetration will depend on the energy and/or momentum of the spud as it reaches the rockfill surface, and the geometry of the spud tip.
However, if someone put a gun to my head and forced me to guess, I would say that the penetration would be less than six inches, unless the rockfill was unusually open (i.e., has a LOT of void space). I would also not be surprised if the penetration was two inches or less, again, depending on the compactness of the fill.
Jeff