I chopped down a Lleylandii a few years ago. It was perhaps 40 to
60 feet high and perched right behind a retaining wall. It
withstood many a gale and didn't damge the wall so I guess the
root system must have been fairly substantial.

This summer did I removed the part of the wall next to the stump
due to another problem. The roots are thick and strong four feet
down and go deeper. Perhaps if you trim the tree to keep it hedge
size it needs less support (mecahnical and nutrients) so the root
system isn't forced to develop beyond a certain point?

W.