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.
I'm installing this exact situation now. The conduits are being laid in a 6" deep trench (there's a buried structure below it) and concrete will be poured just so people won't have to drive over the round conduits.
Just started plan review for an upcoming major project at one of our water utility treatment plants. Two (2) pages of details on the duct banks, some with burial depths of 56" to top of concrete. Details, details, details, which I will 'review' in the next week or so. It is a substantially large project, 350+ page plan set. Funny how the duct bank jumped out as I moved the plan set to my drafting table Saturday afternoon.
I may have used the word "drive" a bit too loosely. The conduits need to be driven past to get to the last 4 parking spots at the far end of the lot. There's no thru traffic and anybody going over them should be traveling at a walking pace (unless they're very late for work).