hello, sorry to admit that I am mainly just a lurker on most of these sites, but I read a few of the early posts on this thread and just skipped to the end to add my experience.
I had the pleasure of helping construct almost exactly what you are talking about a few years ago. we did 2 40footers side by side, and had the same idea about "they must be strong, look how they stack them on ships". WRONG. dont get me wrong, this does work, but they must be reinforced before you bury them! We made a giant whole, and had to pull/push them down a ramp into the hole with the tractor because the excavator couldn't lift the 40 footers that well.
anyway, when we began to backfill, the sides of the containers began to bow inwards quite a bit, same with the top, and we had less than 1 yard of dirt on top. We ended up reinforcing them from the inside, using hydralic jacks, and postmals and stuff, we framed them with wooden posts, and after that they were fine. It took a long time though, and would have been much easier if it was done before hand.
Take my word for it. please.