I think I remember that about 20" of dirt will protect from most radiation. To make a reinforced bunker that shallow is not too difficult, providing it's not real big. The longer the walls the more support you would need. But if you go deep the weight adds up fast, and not just from above but from the sides as well. Most septic tanks are only made of 4" thick concrete with wire mesh inside it. But these are small and no more than 2 ft deep. Once you get larger than 8x8' you need thicker rebar reinforced walls and roof. The strongest structure you can bury is a verticle round cylinder with a domed roof.Of course, the first question is....bunker for what? Biologic, Nuclear, hidden or above-ground protection, all of this will have different considerations. Going underground, you have to be sure it will stand the weight of the earth on top of it. That's where many go wrong.
storm shelter? I remember a few tornadoes in Fla., but a cat 5 hurricane is a distinct possibility.I usually remember it as 2' of dirt (so 24") but doubt there'd be much difference there protection wise. If you have concrete to go through too though, then even better, as that's a good radiation barrier also.
I've always toyed with the idea of making our hay shed (which is concrete block up to about 5' on the walls) into a shelter, but we're far away from any nuke plant, and even the closest real nuke target would have to be hit with nearly 10 megatons, for even the outer effect of the blast to reach us. So the expense of doing so simply is too great for the minimal risk for this kind of event affecting us.
I was 25 miles inland as well, in deland. It was a little buffer from the storms. I built the house there without cutting hardly any trees, just kind of nestled in the woods. Anyways, during one of the storms when we had those all in one year, I went outside after it calmed down. I wasn't worried about the house as I had built it pretty tough, but around the house I had 6' deep of tree debris, all the way around the place. Amazingly, not a single tree landed on the house, but I had to climb over and under in all directions. I remember calling my insurance co. and the guy asked how was the house damaged. I looked it over and it was fine, so the guy tells me they only cover the structure, I was on my own on the thirty something trees piled everywhere! Took me weeks to get the yard cleaned up! I was lucky but did learn to respect Mother Nature. Since I grew up in fla. I kind of got used to the storms, but it did seem like we were having more in the last few years.No real need there (storm shelter). The house is also concrete block, and no crawlspace, is solidly on foundation. We have boards we fasten in place over the windows when battening down for a bad storm. Even the stable has storm doors we can close, and we can shutter the windows there. (for any horses in the secondary stable, we simply bring them into a stall in the main one, during a storm). Secondary stable is cheaper, so usually have a boarder or two there.
We're inland, so by the time a storm got here, it would have lost a lot of its teeth. My big worry is branches from large trees, but I try and keep those trimmed back from the house. Also, I'll pull one of the trucks into the stable alley, so it isn't damaged by debris.
Nothing is really going to protect you from a strong enough storm, hitting just the right way, but we've certainly got a lot to minimize the effect.