So, we'll all be mole people? Given the amount of land I already talked about, trying to put that all underground makes even less sense.

A small enclosed area propped up by the buildings themselves, would potentially make sense, you'd still be able to get sunlight for energy and growing food, it's just not that practical to do that for like a few dozen square miles. But it's a hell of a lot more practical than essentially doing exactly that but also digging out a few dozen square miles of underground territory...

As for blocking blasts - why. Radiation could be as simple as water, tbh. It's what we use in nuclear reactors NOW. Iirc a 30 foot deep pool with nuclear shit at the bottom, you'd be safe from the radiation on the surface.