I'm curious if you plan to approach this design problem with FEA. Long ago, I did a study on deep burial polyethylene pipe, from a pipe resin longevity standpoint. Look up the 1988 Ontario Ministry of Transportation study...I think it is still online courtesy of the concrete pipe lobby group. To summarize my thinking, deep burial polyethylene pipe should be designed so that, to the greatest extent possible, all loads are compressive. Where loads unavoidably become tensile, perhaps at a rib, the stresses should be checked against the hydrostatic design basis of the polymer, a measure of its long term creep-cracking resistance of pressure pipe. If the polymer does not have such a rating, it may still work, but any tensile stresses become more worrisome. To be precise, it is dilational stress that causes voids to form and merge as cracks, so use of Von Mises stress, as is common in FEA is not going to tell you what will happen. Von Mises stress is zero for a pure dilational stress state of any magnitude.
I'm happy to discuss what little I know.
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