Worked this week on a repair to a circular patio retaining wall that someone had built less than 9 years ago. The wall looked like this when I went to see the job on Monday. It was leaning badly.
And it looked like this when I arrived Tuesday to start taking the wall down. (It had started taking itself down.)
There are many reasons this wall failed.The blocks had no strength laid along the wall this way.Why use blocks at all? The convex curve gave the wall no structural advantage at all. (Better to make retaining walls straight or concave if you can) The thin, one-stone-thickness of the wall in front of the blocks offered no resistance (mass or friction) to the surcharge force of the ground pushing out from the patio. Movement will always happen either from hydrostatic or frost pressure building up against the wall.
But I think the main reason the wall failed was the gravel. It had been poured in behind the stones and in front of the blocks instead of fitting in proper sized/shaped hearting stones.
'Gravel travels !' It is NOT structural. It just keeps slipping down and leaves gaps. It also pushes the stones outward where it piles up lower down at the base the wall. The clients told me the squirrels kept digging out the gravel from inside the wall too.
Never never use gravel in a dry stone wall.