serious error (re. radiation shielding) in 1975 Summer Study?

When I read of bases on the Moon, there is usually talk of burying the living quarters under five metres of lunar regolith. I've wondered why they think they need two and a half times as much shielding as an O'Neill habitat would need, and figured that either they were being more conservative than the Stanford study or that loose regolith isn't as effective a shield as reclaimed industrial slag, so you need more of it. If the shield has to be five metres thick instead of two, that's not a big deal, but it is something we need to know.

Now in this thread I saw a figure of fifty metres of water, apparently enough to shield against the worst of the radiation. This much water would mass (Wiki, math math...) 45,850 kg/m2. The material for a passive shield, mentioned in this thread, is fused silica glass. So, how much fused silica do you need, how thick, to get 45,850 kg/m2?

The answer I'm getting is a shield 22.727272... metres thick, which you might as well round up to 23. Now, this isn't undoable, but it's a lot different than the six and a half feet we've been thinking! Then again, i could be making errors all over the place.