Jim,
I did find a table online years ago. Basically it said to put 12 inches over the culvert. That was good for a semi trailer. I tried to get 12 inches of ABC over mine. I'm close but I doubt its that deep. The culvert held up just fine during the house construction. At least one logging truck, dozens and dozens of dump truck loads of ABC and 67 stone was brought in, 7 concrete trucks, as well as bulldozers, backhoes, and an execavator went over that culvert.
I back filled the culvert with ABC which is the name of the road base here in NC. This stuff packs down almost like concrete. That culvert has not moved. The culvert was not placed in a ditch, its in a low area that had to be crossed. So the ABC had to cover and hold in the sides of the culvert. From the road, the drive drops a bit, climbs the culvert, dips again and the climbs a hill. One day I'll have another 15 tons dropped on each side of the culvert to take out the dip.
Later,
Dan