That really looks like the general burial requirements and I am not sure you even need enough dirt to grow grass over a concrete duct bank if there is 2" of concrete to the first raceway.
It does seem that when you go too deep you may be making ampacity adjustments but that is starting to get up in that engineering supervision stuff.
I really only dealt with this once, the ducts were over 6 feet down and they had engineers on site. He showed me stamped plans and I said OK.
