I picked-up a couple magazines this past weekend. One is entitled, "Home Power" and the other is "Backhome" magazine. Both are geared towards sustainable living. An article in "Home Power" disussed a completely offgrid, cordwood home. The link is below and very interesting. http://www.cordwoodmasonry.com/
I've looked into cordwood before. I like that you can determine the thickness of your walls with the wood cuts. So you could have 36" walls!
8 feet. I'm sure that would also depend on what kind of wood. Eg, Cork may need more and Petrified less http://survivalmonkey.com/50bmg_penetration_table.htm