quote from that page:
Begin reinforcing your shelter from the inside. Do this by stacking dirt around the walls or anything else you can find. If you are in a trench then create a roof, only if the materials are nearby; you don't want to expose yourself when it isn't necessary. Canvas from a parachute or tent will help stop fallout debris from piling on to you, though it will not stop the Gamma rays. It is impossible, at a very fundamental physical level, to completely shield from all radiation. It can only be reduced to a tolerable level. Use the following to help you determine the amount of material you'll need to reduce radiation penetration by one half:[7]
Steel: 21 cm (0.7 feet)
Rock: 70-100 cm (2-3 ft)
Concrete: 66 cm (2.2 ft)
Earth/Wood: 2.6 m (8.8 ft)
Soil: 1 m (3.3 ft)
Ice: 2 m (6.6 ft)
Snow: 6 m (20-22 ft)
/end quote
Don't really know the source of the information, but it appears to be in error. .7 feet of steel for a half value layer??? 8.4" of steel..
while each source of radiation has it's own specific HVL I can't think of one that would require that amount steel. I'd have no problem standing on the otherside of a 8.4" thick steel wall with 200 ci of Ir192 in contact with the opposite side. In contact it would be in excess of 11K Rem/hr. However; there is a reason IR192 cannot shoot a weld 8.4" T no matter how strong it is. If your standing somewhere that receives a gamma burst that requires that much material, your probably screwed anyway.
I think the idea of surviving a global nuclear war is as insane as the idea that such a war could be won. The earth may or may not still be here, but I don't know that even if you did survive it, that it would be worth doing so.
The only scenerio's that could be survived would be limited theatre, or EMP, but in all cases, the biggest threats would be the EMP and or fallout.