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.