A block retaining wall drains through clean gravel. Allan Block fills the hollow cores of its blocks and a band at least 12 in (30 cm) deep behind them, from the base to the top, with “wall rock”: stone ¼ to 1½ in with no sand, silt or clay in it. Walls over 4 ft, poorly drained sites and walls with a slope or paving above also get a drain pipe at the back of the base, vented to daylight at least every 50 ft. Surface water is kept away from the wall with berms and swales.
Why a retaining wall needs drainage
Soil that fills with water is heavier, and it pushes harder on the back of the wall. Allan Block’s manual says the soils must not get saturated during construction and the finished design must route water away from the back of the wall.
The gravel and the pipe behind the wall only deal with incidental water: what soaks into the soil just behind it. Allan Block is clear that they are not the main drainage path for water coming over the top or up from below. That is the job of the grading around the wall, and ground water needs an engineer.
The gravel behind the wall
Allan Block calls it wall rock and uses the same stone for the base, the cores and the band behind the blocks:
- Size: ideally ¼ to 1½ in (6 to 38 mm), angular or smooth, and compactable.
- Clean: no sand, silt or clay that would stop water draining through it. Mixes with fines, such as crusher run or stone dust, don’t meet that description.
- Where: in the hollow cores of every block and in a band at least 12 in deep behind them, from the base course to the top. Soil goes behind the gravel, compacted in lifts of no more than 8 in.
A 12 in band is 1 cu ft for every square foot of wall face, counting the buried course. For a wall 20 ft long with 40 in of block, that’s 20 × 1 × 3.33 = 67 cu ft, about 2.5 cu yd or 3½ tons at 1.4 tons a yard, before the cores and the base. The retaining wall calculator adds it up with the base trench for your wall.
When a retaining wall needs a drain pipe
Allan Block’s rules of thumb for a drain pipe, which it calls a toe drain:
- Walls taller than 4 ft (1.2 m)
- Reinforced (geogrid) walls
- Sites with poorly draining soils
- Walls alongside paved areas
- Walls with slopes above them
- Multi-tiered and terraced walls
Source: Allan Block, Residential Installation Manual, “When do I need drain pipes?”. It also lists all commercial and municipal walls.
Where the pipe goes and where the water comes out
- At the bottom, at the back: lay the pipe at the lowest possible point toward the back of the base trench, in the wall rock.
- Out to daylight: vent it to daylight or to a lower part of the site at least every 50 ft. Allan Block also runs it out through the wall face, with tee fittings every 30 to 50 ft. The drains can instead connect to a storm sewer.
- Keep it working: mark and protect each outlet so it isn’t damaged or plugged, and fit rodent screens to keep debris out. The drain must be protected from fine soil washing into it.
Keep surface water away from the wall
Before building, Allan Block reviews every source of concentrated water that could reach the wall: driveways, slopes above it, the grading of the site, water lines, roof downspouts, sump pump outlets and irrigation. Berms and swales above the wall send surface water around it instead of into the soil behind it.
While the wall is going up, cover the backfill and the wall at the end of each day when rain is forecast, as Allan Block advises. Soil that gets soaked during construction won’t compact properly.
Questions people ask
What kind of gravel goes behind a retaining wall?
Clean stone from ¼ to 1½ in, angular or smooth, with no sand, silt or clay mixed in (Allan Block). It has to let water through, so not crusher run or stone dust.
How much gravel goes behind a retaining wall?
A band at least 12 in deep from the base to the top, plus the block cores: about 1 cu ft per square foot of wall face, before the base.
Does a retaining wall need a drain pipe?
Allan Block puts one in walls over 4 ft, reinforced walls, poorly drained sites, walls beside paving or below a slope, and tiered walls.
Where should a retaining wall drain pipe outlet go?
To daylight or a lower part of the site, at least every 50 ft, or to a storm sewer. The outlets can run through the wall face.
Can I backfill a retaining wall with dirt?
Behind the 12 in of gravel, yes: soil compacted in lifts of 8 in or less. The gravel band itself must stay clean so it drains.
Sources
- Allan Block, Residential Installation Manual for Allan Block Retaining Walls: water management, drain pipes, wall rock, base trench, backfill and compaction
- FEMA, Appeal decision, Road Repair, Appanoose County, Iowa (1930-DR-IA): 1.4 tons per cubic yard of uncompacted gravel
Checked October 12, 2026. Metric values are converted. The gravel volume is our arithmetic.