What Goes Behind a Retaining Wall?
Retaining walls fail from water, not weight. Saturated soil behind a wall pushes with enormous force, and a wall built to hold back dry earth will not hold back wet earth.
Behind the wall goes clean crushed stone, not soil, with a pipe at the bottom to take the water somewhere. That is the whole principle.
The layers, bottom to top
| Layer | Material | Size |
|---|---|---|
| Base course | Compacted crushed stone | 6 in deep, twice the block depth wide |
| Drainage pipe | Perforated pipe in stone, holes down, falling to an outlet | 4 in |
| Backfill | Clean ¾ in washed crushed stone | 12 in behind the full height |
| Separator | Geotextile between stone and soil | — |
| Cap | Soil, graded to fall away from the wall | Last 6 in |
Why clean stone, and why washed
"Clean" and "washed" mean the fines have been rinsed out, so the stone is all one size with big voids between pieces. Water falls straight through to the pipe instead of being held against the wall.
Crusher run is the wrong material here for exactly the reason it is right under a driveway: the fines make it bind and hold water. Use angular washed stone — gravel vs crushed stone covers the distinction, and note the base course wants the fines in while the backfill wants them out.
The geotextile matters more than people think
Without a fabric layer between the drainage stone and the soil above and behind it, silt washes into the voids. Within a few years the stone is packed with fines, the drainage is gone, and you have built a wall with soil behind it after all.
It is the cheapest component and the one most often left out.
How much stone
For a wall 20 feet long and 3 feet high:
- Base course — 20 ft × 2 ft × 6 in = 0.7 cubic yards, about 1 ton
- Drainage backfill — 20 ft × 1 ft × 3 ft = 2.2 cubic yards, about 3.1 tons
- Total — roughly 4 tons
Stone is sold by the ton at about 1.4 tons per cubic yard; how much gravel do I need has the conversion. The base and the backfill are different products, so price them separately.
The things that actually cause failures
- Soil used as backfill. The single commonest cause. It holds water and swells.
- No outlet for the pipe. A drainage pipe that runs to nowhere is a trough. It must daylight at one end, or run to a drain.
- Base not compacted, or not buried. The first course should sit below finished grade — roughly one inch for every eight inches of wall height.
- No batter. Walls should lean back into the slope slightly. Most block systems build that in; with dry stone you set it yourself.
- Ground water ignored. A spring or a downspout discharging behind a wall will beat any backfill.
When to stop and call an engineer
Over about four feet, most jurisdictions require an engineered design and a permit, and the forces stop being intuitive. A wall holding up a driveway, a structure or a slope above it needs one at any height.
This is the unprofitable advice: if your wall is tall, holding something up, or you are unsure, the engineering costs far less than rebuilding. A failed retaining wall does not lean — it goes over.
For the base stone, the backfill and the soil that caps it, find suppliers near you on Topsoil.com.