Retaining Wall Drainage: Gravel, Pipe and Fabric
Why water is the main enemy of retaining walls, and how drainage stone, a perforated pipe and fabric work together behind the wall.
Most retaining wall failures are really drainage failures. Soil behind a wall that’s soaked with water is heavier and pushes harder, and in cold climates the water freezes and expands. A wall that drains well carries far less load.
The three parts of wall drainage
1. A zone of clean gravel behind the wall. Typically about 12” wide, from the base to a few inches below the top. Use clean, washed stone such as ¾” crushed (#57): no fines, so water flows straight down through it. Fill the hollow cores of the blocks with the same stone.
2. A perforated pipe at the base. A 4” perforated pipe laid behind the first course, at the bottom of the gravel zone, collects water and carries it out. It needs to slope to an outlet: through the face of the wall, out one end, or to a lower part of the yard.
3. Fabric between the gravel and the soil. A non-woven filter fabric separates the drainage stone from the soil behind it. Without it, fine soil washes into the gravel over time and clogs it.
How much for a 20 ft wall, 2 ft tall
| Item | Quantity |
|---|---|
| Clean drainage gravel | 2.59 tons (1.85 cu yd) |
| 4” perforated pipe | 30 ft |
| Filter fabric | 90 sq ft |
Note that the drainage zone runs the full block height, including the buried course, so it’s taller than the part of the wall you see.
Where the water goes
The pipe is only useful if it has somewhere lower to empty. Options:
- Out the end of the wall to lower ground.
- Through the wall face via outlets every so often, if the ground in front is lower.
- To a dry well or another drain, where allowed.
Keep a slope of at least about ⅛” per foot along the pipe. Never point the outlet at a neighbor’s property or your own foundation.
Keep surface water out
Drainage behind the wall works best when less water reaches it to begin with:
- Cap the drainage gravel with a few inches of compacted soil, so rain runs off the surface rather than straight down behind the wall.
- Grade the ground above the wall so it doesn’t funnel water toward the wall. A shallow swale above a tall wall can catch runoff.
- Point downspouts away from the wall.
Signs of trouble
Bulging, leaning, blocks pushing out at the bottom, or soil washing out through the joints all point to too much water pressure. Small walls can sometimes be fixed by taking down the top courses and adding drainage. On larger walls, get a professional to look at it.
The Retaining Wall Planner includes drainage gravel, pipe and fabric for any wall size, along with the blocks.