To thinkgreen this is what I was talking about... http://www.mortonarb.org/deeptreeroo...ms.html#advent
scroll down a bit and there will be some pictures.
To Kiril I agree wording is everything and I should have read through my first posting a little closer to avoid confusion.
Anyway I agree that it could kill the tree, but a tree with a canopy that size is being sustained by more than that small mass that was behind the wall stones. Pruning could be done to the canopy to lessen the stress of the root pruining and he could at least work through some of the smaller roots to see what lies beneath all of the soil. Leaving all of the larger roots for a second opinion.