Retaining Wall Base Gravel: Type, Depth, and Tons
Retaining wall base gravel is the compacted leveling pad that a segmental block wall sits on. Most block makers call for about 6 inches of crushed stone with fines, compacted, in a trench about 24 inches wide, with the first course of block partly buried. That works out to roughly 0.33 cubic yards, or about 0.45 to 0.5 tons, for every 10 feet of wall. Behind the wall you need a separate order of clean drainage stone and a perforated pipe.
Volume and tonnage calculator →Two different gravels
| Where | Material | Why |
|---|---|---|
| Leveling pad under blocks | Crushed stone with fines (dense grade, crusher run, 3/4 minus) | Compacts hard and flat |
| Behind and between blocks | Clean angular stone, often 3/4 inch, no fines | Drains water away |
Mixing them up is a common mistake. Clean stone does not compact into a firm pad, and stone with fines does not drain well behind the wall.
Pad size and depth
A typical spec for walls up to about 4 feet:
- Trench width: about 24 inches, or the block depth plus 12 inches.
- Pad depth: 6 inches of compacted base; some makers allow 4 inches for short garden walls.
- Block burial: the first course is buried at least partly, often about 1 inch for every 8 inches of wall height, with a 6-inch minimum.
Taller walls, walls near driveways, and walls on slopes often need geogrid and an engineered design.
How much base gravel
| Wall length | Pad 24 in x 6 in | Approx. tons |
|---|---|---|
| 10 ft | 0.37 cu yd | 0.5 to 0.55 |
| 25 ft | 0.93 cu yd | 1.3 to 1.4 |
| 50 ft | 1.85 cu yd | 2.6 to 2.8 |
| 100 ft | 3.70 cu yd | 5.2 to 5.6 |
These use about 1.4 to 1.5 tons per cubic yard for compacted crushed stone. Use the volume and tonnage calculator for other sizes.
How much drainage stone
A common detail puts 12 inches of clean stone directly behind the blocks, filling the block cores too. For a 3-foot-tall wall, that is about 3 cubic feet per foot of wall, or about 1.1 cubic yards per 10 feet. A 4-inch perforated drain pipe at the base, wrapped or in a sock where soils are fine, carries water to a daylight outlet. See crushed stone for retaining wall backfill.
Compacting the pad
- Dig to firm, undisturbed soil and compact the trench bottom.
- Add base in lifts of about 2 to 3 inches.
- Dampen slightly and compact each lift with a plate compactor.
- Screed the top level front to back and along the length.
- Check with a level on every block of the first course.
Our page on compacting gravel base in lifts has more detail.
Bottom line
Use about 6 inches of compacted crushed stone with fines in a trench about 24 inches wide as the base for a segmental retaining wall, roughly half a ton per 10 feet. Add clean drainage stone and a perforated pipe behind the wall. Level the first course carefully, because every course above follows it.
Questions people ask
What gravel is best for a retaining wall base?
Crushed stone with fines, such as dense grade or 3/4 minus, which compacts into a firm, flat pad.
How deep should the gravel base be for a retaining wall?
About 6 inches compacted for most segmental walls, plus burial of the first course.
How much gravel do I need for a 50-foot retaining wall?
About 1.85 cubic yards, or 2.6 to 2.8 tons, for a 24 by 6 inch pad. Drainage stone is extra.
Can I use pea gravel under a retaining wall?
It is a poor choice for the pad because it does not lock together. Use angular crushed stone.
Do retaining walls need drainage stone?
Yes. Clean stone behind the blocks and a perforated pipe help relieve water pressure.
Related guides
- How Many Retaining Wall Blocks Do I Need? Quick Math
- Retaining Wall Block Weight: Sizes, Pallets, and Loads
- Crushed Stone for Retaining Wall Backfill
- Compacting a Driveway Gravel Base in Lifts
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