Yeah man. Probably going to do the original driveway in millings also. Right now it's half n half. The wife isn't too keen on the non uniform colorA lot cheaper than crusher run or rock waste, which is what I used to cross one drainage ditch on my property. I have clay soil so one got put in the second ditch with just the clay which was very dry and crumbled nicely when I set the culvert. The biggest objective is to make sure you pack as you fill, unless you use sand/crusher/millings which will self-pack. The dirt is what's supporting anything that goes over the culvert. The culvert just makes a hole for water to get through. The double wall is to keep the inside of the culvert smooth so that silt/sand don't settle in the corrugations and give roots a place to get started which will lead to the culvert getting plugged up.
I'm about to test my theories about the strength of the culvert in a couple weeks. I put the culvert in shortly after I bought the LX, and its been holding up beautifully since. Having 4 trees taken down behind my shop, and the tree service is bringing in a monster to take them down in chunks. Sorta like the giant tree cutters, but just a straight boom on a machine similar to an industrial crane. I'm hoping to get a few saw logs out of all of it.

Guess I should call the county to see if I can just do it, or if I need some silly permit.