If you're using schedule 80 (grey) PVC pipe, it's got a pretty thick wall. P. nigra rhizomes are less than 1", and I'd guess that they would just go around the pipes, maybe deflecting them slightly.

Highly unlikely. A rhizome encountering a PVC pipe would be deflected AROUND the pipe by its cylindrical shape. You may encounter anecdotes about rhizomes piercing barrier plastic, but consider that those are much wider expanses of material and are often installed in a nearly vertical position. Like a well-cornered snake, that rhizome has to do SOMETHING even though it wasn't out looking for trouble when it encountered the inferior or deteriorated barrier material.