Well the dirt guy in me is rather curious about that culvert. After the first fail, I researched the plastic pipe we had been using and for the size, it did not need any elongation during installation. But, based on what has happened again, perhaps it should. This involves bracing the pipe into an egg shape. Then fill around the pipe to just above it, tamping and compacting all the material. Remove the braces and finish filling over the top of the pipe. I was part of a crew that did this on a 4 foot diameter pipe, 40 feet down, 150 feet long.
Two other options exist though. Cement pipe or steel culvert. Either way, expensive....