Retaining Wall Under Deck . Second, a retaining wall must have properly compacted backfill. Crucial for supporting the rest of the wall, a good base is made of compacted soil and at least a six inch layer of compacted sand and gravel. Large Outdoor Fireplace and Retaining Walls in Columbus Patio deck from www.pinterest.com Working one post at a time, use the laser level to. Paver patio under deck with retaining wall & steps. We decided this space was too short to be included in the under.