Crushed Stone for Retaining Wall Backfill
Crushed stone for retaining wall backfill is what keeps a segmental block wall standing for decades. Water trapped behind a wall adds pressure and can push it over or bulge it. A zone of clean, angular crushed stone directly behind the blocks, often with a perforated drain pipe at the bottom, lets water drain away. Many wall manufacturers call for at least about 12 inches of drainage aggregate behind the wall, sometimes more for taller walls.
Volume and tonnage calculator →Which stone to use
| Stone | Use in wall construction |
|---|---|
| Clean 3/4-inch crushed stone, often #57 | Drainage zone behind the wall and block cores |
| Dense-graded base (crusher run, CA6, 21A) | Leveling pad under the first course |
| #8 stone | Sometimes used in block cores or as finer drainage stone |
| Native soil | Reinforced or retained soil beyond the drainage zone, if suitable |
Avoid rounded pea gravel, which does not lock together, and avoid sand or clay in the drainage zone.
How much stone you need
- Wall length in feet.
- Drainage zone width, often about 1 foot.
- Height of the drainage zone, usually the full wall height.
- Multiply length x width x height for cubic feet, divide by 27 for cubic yards.
- Convert to tons at about 1.3 to 1.4 tons per yard for clean stone.
- Add block core fill, listed on the block maker’s spec sheet.
Example: a 40-foot wall, 3 feet tall, with a 1-foot drainage zone: 120 cubic feet / 27 = 4.4 yards, or about 5.8 to 6.2 tons, plus core fill.
Leveling pad
The first course sits on a compacted leveling pad, commonly about 6 inches of dense-graded base, wider than the block. Bury the first course partly below grade, often about an inch per foot of wall height or as the maker specifies.
Drain pipe
A perforated drain pipe at the base of the drainage zone carries water to daylight or a storm system. It is commonly wrapped in fabric or placed in a fabric-lined trench. Outlets every so often prevent water from backing up on long walls.
Separation fabric
Placing geotextile fabric between the drainage stone and native soil keeps fines from migrating into the stone and clogging it. See separation fabric under a driveway base.
Taller walls
Walls over about 3 to 4 feet, walls with slopes or loads above them, and walls near driveways often need geogrid reinforcement and engineering. Many building codes require a permit and engineered design over a set height.
Installation tips
- Fill the drainage zone as each course is laid.
- Compact the soil behind the drainage zone in lifts, but avoid heavy compactors right next to the blocks.
- Cap the drainage zone with a few inches of soil so surface water does not pour in.
Bottom line
Use clean, angular crushed stone, often #57, for about a foot-wide drainage zone behind a retaining wall, with a perforated drain at the base and fabric against native soil. Figure tons from length x width x height / 27 x about 1.35. Build taller walls to engineered plans. See #8 stone for smaller clean stone options.
Questions people ask
What gravel is best behind a retaining wall?
Clean, angular crushed stone, often 3/4-inch or #57, drains well and locks together.
How much drainage stone goes behind a retaining wall?
Many manufacturers call for at least about 12 inches behind the wall.
Do retaining walls need a drain pipe?
Most block walls benefit from a perforated drain pipe at the base, especially taller walls.
Can I backfill a retaining wall with dirt?
The zone right behind the wall should be drainage stone. Suitable soil can go beyond that zone.
Do I need fabric behind a retaining wall?
Geotextile between drainage stone and native soil helps prevent clogging.
Related guides
- #8 Stone: Size, Uses, and Weight per Yard
- Gravel Base Under a Concrete Slab: Depth and Tons
- When Separation Fabric Goes Under a Driveway Base
- Riprap Sizes and Weight: Classes, Tons, and Coverage
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