I agree that a waterproof, concrete, underground shelter is the gold standard for an SHTF dwelling. Really, we're talking about a bunker here and that's exactly what you want.
However, got a question for you on your approach. I can see the value of the pre-cast concrete culverts, but I see a few issues that I wonder how you solved.
1.) A pre-cast culvert is basically a concrete box with two open ends. My question - what did you do about the open ends? Minimally, I'd like to buy a concrete box with at most one open end, that way I could buy two and I could join them at the open ends and end up with a larger closed six sided box. Is this what you did or did you cast the "ends" in place?
2.) To get more square feet I could see the need to join two or more pre-cast culverts. I know they have tongue and grove joints at the ends, but did you do this and if so did you have any problems getting the joints to be waterproof? If so, what did you do?
3.) Did you coat the outside of the culverts with any sort of waterproofing to ensure a dry underground space? I'm thinking I might want to spray on the green elastomeric stuff I see builders spraying on in-ground basements. What do you think of that?
Appreciate any insight you might provide.