If you are going to bury these things, you need a couple pieces if large I beam welded to the bottom to keep them from floating. We made some pit blinds one time and they all floated out in a couple days. They had to rebury them with about 2-3 sticks of I beam sticking out 5' on each side to keep them down.
Adding some braces wouldnt be a bad idea either, from what Jack is saying. I believe he is right. While they are made to stack, thier strength is in the corners for vertical support. If you placed signifcant load on the sides or roofs, I can see them giving easily enough to collapse them since they are not built for that kind of pressure.
Personnally, I wouldnt want it buried and I would make some modifications by welding my brackets or shelves in. Yes, you burn paint off, but you can repaint. We had to fix quite a few that had holes in the roof. We would put plate over the hole and weld it. Then I think we used some kind of tar to seal it off even more.
Word of warning, if you get one and it has black stuff down the side, it's likely creosote. Grinding that off with a wire brush aint pleasant after about 20 minutes. My face burned for a while after I got it off of me. I didnt know what it was and it coated my face. Didnt burn me too bad, but my face was red for the rest of the day.