JP, Eddie is right. A retaining wall is a structure. It must sit on a foundation that has some strength. Wet soils, with weight on top, will shift. Also, if a retaining wall is tied well back into the shore with some type of anchor, it stands a better chance to stay put laterally. I've seen lots of retaining walls. Each one that didn't have a good footing AND was anchored into solid soils behind it, shifted. Every one.

Teach a man to grow fish...
He can teach to catch fish...