20 feet of concrete.
And yes, I'm assuming that if they want to make a hardened bunker, they'll harden it with concrete instead of fluffing some vermiculite around it. But heck, I'll give you 150 feet. Using that on a bunker thousands of feet deep is still dubious and still won't contain a nuclear yield of ~1Mt.
Guided Bomb Unit-28 (GBU-28) specs from the FAS.