As a case study I go back about 3 years ago to Dublin, New Hampshire along route 101 near Keene. I was passing through the area I noticed this beautiful dry stack stone wall (no mortar). It was beautiful until I got to the last 30 feet which was now just a pile of rock.
When this wall was built route 101 was probably nothing more than a local road between Peterborough and Keene. Now it is a major trucking route between Manchester and points west. The vibration of the trucks passing through was probably never envisioned by the craftsman who put this wall up.
I came to an agreement with the owner and went to work. I added many more through stones to cross brace the wall to help stand up to the trucks. The results can be seen here.