Same technique with building a retaining wall in a yard. Dig about 1 foot (30cm) along the wall, plastic sheet liner, fill with gravel. Then add a few inches of

Same technique with building a retaining wall in a yard. Dig about 1 foot (30cm) along the wall, plastic sheet liner, fill with gravel. Then add a few inches of soil on top for grass, etc. This prevents the wall from slowly leaning away from the side with earth on it, pushing it. Especially important in climates that freeze. The gravel provides extra space for ice to expand into.