Drainage materials keep water pressure from building up within or behind the wall.
For all walls, a column of drainage aggregate is placed directly behind wall units that are above grade. The drainage aggregate should be clear, angular gravel that is free of fine soil. It should be placed to a minimum thickness of 12 inches.
For walls over 3 feet high, a perforated drainpipe at the base of the drainage aggregate is suggested to quickly remove large amounts of water. The drainpipe should be sloped for gravity flow. Collected water is sent out and away from the wall through solid outlet pipes placed at the wall ends or through the wall face every 40 to 50 lineal feet of wall.
If the groundwater level around the wall is high or the wall is along a shoreline, additional drainage materials may be needed behind and below the wall system. Also, filter fabric may be needed around the drainage aggregate to keep fine soil from migrating into and clogging the aggregate.