<font color=blue>I have three concrete culverts</font color=blue>
When I said culvert pipes have little crush resistance of their own, I was referring to metal and plastic pipes. Concrete pipes are different. They have a
lot of crush resistance.
I'll go along with the "just ain't necessary", though,
if certain other conditions are right. I think the soil is the most critical factor. If it's largely organic and compresses when you drive over it, the pipe will wind up taking the full load. Your clay and gravel sounds like pretty sturdy stuff once it's compacted.
The 12" rule of thumb shows up in virtually all of the culvert documentation I ever found (again, referring to plastic and metal pipes). The more specific rule is a percentage of the diameter of the pipe itself, but of course I can't remember what that number is. Come to think of it, for the smaller pipe that Cisco is talking about, that would probably work out to less than the 12" rule of thumb. /w3tcompact/icons/crazy.gif
The anecdotal "proof" that backfill matters could be evidenced by the septic lines that a contractor ran under one of the roads on my tractor property. It was the plastic stuff, buried about a foot, but just casually backfilled with the same stuff that came out of the ditch. Within 2 years we determined that the pipe was partially crushed from the little traffic that passed over it.
What really ticks me off is that the plastic septic tank they installed was backfilled the same way, and
it is now partially crushed, too. /w3tcompact/icons/mad.gif
Our soil is a mixture of organic and rocks. /w3tcompact/icons/wink.gif