BridgeMan, I was planning to put in a gravel base under the slab for moisture reasons, but I never thought of the sulphate issue. Thanks for the tip.
Nealtw, we have very thick clay in this area of NC so I will definitely be able to cut my footing "forms" directly into the soil. That's what I did with my column footings and it worked great. Getting back to how the vapor barrier traverses the retaining wall, are you saying that I could just run it under the footing? That would seem to make a lot of sense in that it would not require a break between the footing and the wall, but my only concern is that I would probably need to be more careful digging my forms since any low spots would couse stretching of the plastic - although probably not enough to break a hole in it unless I get very sloppy.
Here is something else. I'd like to have footing drainage on both sides of the footing. Is it better to have separate pipes, running to daylight, or have a single drain pipe on either the inside or outside combined with a series of smaller pipes formed into the footing that lets water flow through the footing? I'm thinking two are better since I can install cleanouts would make maintenance of the drain system easier over the years. What do you guys think?