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[Gene GeRue <genegerue AT ruralize.com>]

I sashayed around Rob's question about slip-forming stone walls  
because I wanted to think about it. He asked, "What is a good  
thickness/ratio of concrete to stones . . . ." and I did not address  
that directly.

So out on the mower, where I do some of my best thinking, I thought  
about it. The answer is use enough concrete to fill all spaces between  
adjacent stones. Strive to avoid voids. With small stones--say, less  
than softball size--I would use small aggregate; rather than concrete  
gravel in the concrete mix I would use pea gravel. My normal mix is  
one part cement, two parts sand and three parts aggregate. As for wall  
thickness, it depends on tall it is, how much lateral pressure there  
is likely to ever be, what one likes esthetically, and whether one  
wishes to sit upon it or set objects atop it.

Leave weep holes at the bottom, where water can run out and relieve  
hydrostatic pressure behind the wall.

I found an example of what I consider poor workmanship on slip-formed  
stone walls at Taliesin West, Frank Lloyd Wright's winter home and  
school in Scottsdale. As always, he was short of funds and that, or  
use of architectural students as masons may be why there are great  
voids next to stones where any mason would have put concrete or  
mortar. The stones are large, with the largest flat face showing, not  
the strongest design. But those walls have now stood for over sixty  
years. Of course Scottsdale does not experience freezing conditions  
except in the shopping malls.