Analysis of information sources in references of the Wikipedia article "Fallout shelter" in English language version.
The 3-foot thickness of earth shown (or a 2-foot thickness of concrete) will provide an effective barrier, attenuating (absorbing) about 99.9%, of all gamma rays from fallout." "A right-angle turn, either from a vertical or horizontal entry, causes a reduction of about 90%." "...a large piece of 4-mil-thick polyethylene was placed over the mound. This waterproof material served as a "buried roof" after it was covered with more earth.