I'm sure you did your homework, but around here the state and some counties will not allow plastic culverts of any size. Cities/townships are not so picky. The ones I've installed for filling in a ditches have cost about $5/foot. Plastic can be as much as double and the spillways/aprons and other fittings are 2-3x the cost of steel. Doesn't answer your question directly but I have seen as much just afew inches of crushed stone on top. And I have never seen one collapse reagrdless of size or the weight on it.

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