Plastic is better than metal for several reasons. First, it will never rust out on you. EVER. Second, get the type with the smooth inner wall. It stays cleaner and doesn't hold silt and debri. As for strenght, both rely on preasure from the fill material being equal on all sides. You need a certain amount of fill on top based on the size of the pipe you install to reach maximum strength. For an RV, you will need this. For a car or light tractor, the amount on top isn't as important.
Since you want to create a crossing over a ravine, go as big as you can. That ravine was created by allot of water. The size of it shoud tell you the amount of flow you have to deal with. 24 inches is a huge pipe and should be able to handle most situations, but there is a reason they make bigger pipes and/or install two side by side.
Putting grates in front will cause more problems than it would solve. Small leaves will wash right through it, but with a grate in front, they will create a dam. It's very rare to see one along highways or anyplace that they are put in profesionally, so I'm thinking you don't need one. I don't have any on mine and so far haven't had any issues.
Backfill is usually with clean soil, but sometimes sand is used instead. Rock isn't any good because of the voids and dificulty in getting it all compacted. At the entrance to the pipe you will need to put something down to stop erosion. I've used sacks of cement for this with good results. Of course, it's not as attractove as rock. If you use rock, be sure it's big enough not to wash away or let the water through it. Small rock is useless during a heavy storm.
And always plan for the very worse that can happen.
Eddie
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