Not that I could cite.
When I was working on the MM99 toll booth on I-75 I did spend some time with the engineers and they say the spec's the duct banks should be 5-6 feet below the sub bed of the road. Florida DOT says there is quite a pounding going on under a road when you have big trucks and lots of traffic.
Most of the actual protection is the concrete itself and the pipe is just a place holder until the concrete cures.
