A few years ago I installed a patio off of our house with retaining walls on two sides for privacy (see images below). However, in recent winters the corner where the walls meet have started to separate as shown in the pictures. So far, the walls do settle back into position in the spring time but I am worried about the long term consequences of this movement and the possibility that the gravel/sand foundation is being washed away. I am confident this has to do with poor drainage and possibly a poor foundation (gravel and sand, rather than cement) but I am hopeful that someone here could provide some advice on how to remedy this issue without starting over from scratch. I'm curious if installing a dry well or putting a waterproof membrane around the patio would remedy or mitigate this issue. I would really appreciate any advice I can get on this topic.