Installing concrete sleepers involves setting steel posts into concrete footings, then sliding the sleepers horizontally into channels on the posts to form a retaining wall. The process is straightforward for walls under about 1.2 meters (4 feet), but taller walls typically require engineering approval and a building permit. Here’s how to plan and build a concrete sleeper retaining wall from the ground up.
Planning and Permits
Before you dig, check your local council’s requirements. Most jurisdictions require a building permit for any retaining wall that exceeds 4 feet (about 1.2 meters) in height, measured from the bottom of the footing to the top of the wall. Walls above that threshold also generally need structural plans and calculations prepared by a licensed engineer. Even if your wall falls under the permit threshold, it’s worth checking property boundaries and underground utility locations before starting work.
Decide on the total wall height early, because it determines everything else: the depth of your post footings, the diameter of each bored hole, and whether you need professional sign-off.
Tools and Materials You’ll Need
Concrete sleepers are heavy, often 70 to 90 kg each for a standard 2.4-meter panel, so plan for at least two people and ideally a small excavator or bobcat for larger projects. Gather the following before you start:
- Concrete sleepers in your chosen size (typically 2.4 m long, 200 mm high)
- Steel posts with H-shaped or C-shaped channels, galvanised or powder-coated for corrosion resistance
- Premixed or bagged concrete for footings (minimum 20 MPa strength for structural footings)
- Drainage pipe, perforated plastic with a minimum diameter of 100 mm (4 inches)
- Drainage gravel, clean crushed stone or pea gravel, free of soft limestone or shale
- Geotextile (geo-fabric) to line the back of the wall
- Stakes, string line, and a spirit level
- Post hole auger or borer (manual or powered)
- Shovel, wheelbarrow, and tape measure
Footing Size by Wall Height
Each steel post sits in a bored concrete footing, and the footing needs to get deeper as the wall gets taller. For standard 2.4-meter sleepers on level ground with no extra load on top (like a driveway or structure), use these as a guide:
- 0.6 m wall: 300 mm diameter hole, 600 mm deep
- 1.0 m wall: 450 mm diameter hole, 1.0 m deep
- 1.4 m wall: 450 mm diameter hole, 1.4 m deep
- 1.8 m wall: 450 mm diameter hole, 1.8 m deep
- 2.2 m wall: 450 mm diameter hole, 2.2 m deep
If there’s a load behind the wall (a driveway, heavy garden beds, or sloped terrain pushing extra pressure), those footings need to go deeper. A 1.0 m wall under load, for instance, jumps from a 1.0 m deep footing to 1.2 m. For a 1.8 m wall under load, the footing goes from 1.8 m to 2.0 m. This is where engineering calculations matter for taller walls.
Step 1: Mark Out the Wall Line
Drive timber stakes into the ground at each end of the planned wall and run a taut string line between them. This gives you a straight reference for placing your posts. Measure and mark where each post will go along the string. Post spacing matches your sleeper length, so for 2.4 m sleepers, you’ll set posts 2.4 m apart (centre to centre), plus one post at each end.
Step 2: Bore Holes and Set Posts
Using a post hole auger, bore each hole to the diameter and depth required for your wall height. Keep the holes plumb. If you’re working in clay or rocky soil, a powered auger saves significant effort.
Place each steel post into its hole and brace it temporarily with timber props. Use a spirit level to check that the post is perfectly vertical in both directions. This step is critical: if a post leans even slightly, the sleepers won’t slide in cleanly and the wall will look crooked.
Pour concrete into the holes around each post, filling to ground level or just below. Tap or vibrate the concrete to remove air pockets. Let the footings cure for at least 24 to 48 hours before loading them with sleepers. In cooler weather, allow longer.
Step 3: Slide Sleepers Into Place
Once the footings have set, start stacking sleepers from the bottom up. Each concrete sleeper slides horizontally into the channels on either side of the steel posts. Start with the lowest sleeper, resting it on or just below ground level. Use a spirit level to confirm it’s sitting flat and even. If the ground isn’t perfectly level, you may need to dig a shallow trench (about 200 mm wide) to partially bury the bottom sleeper so everything stays straight.
Stack each additional sleeper on top of the last, checking level as you go. The sleepers should sit snug against each other with minimal gaps. For taller walls, you’ll likely need a second person or a small crane to lift the upper panels into position safely.
Step 4: Install Drainage Behind the Wall
Drainage is the single most important factor in a retaining wall’s long-term survival. Water pressure building up behind the wall (called hydrostatic pressure) is the leading cause of retaining wall failure, so you need to give water a clear path out.
Start by lining the back face of the wall with geotextile fabric. This filter cloth allows water to pass through while preventing soil from washing through the gaps between sleepers and clogging your drainage. If your wall has multiple sections, overlap the fabric between sections by at least 300 mm (12 inches) to prevent soil seepage at the joints.
Lay a length of 100 mm perforated drainage pipe along the base of the wall behind the bottom sleeper, with the perforations facing downward. Angle the pipe slightly so it falls toward a discharge point at one or both ends of the wall. This could be a stormwater connection, a natural low point in the yard, or a pop-up emitter.
Backfill around and above the pipe with clean crushed stone or pea gravel to a minimum depth of 100 mm (4 inches). Avoid using soft or crumbly stone types like sandstone or shale, which break down over time and block drainage. The gravel layer should extend up the back of the wall as high as practical, ideally at least two-thirds of the wall height. Top the gravel with a final layer of geotextile before backfilling with the retained soil. This keeps dirt from migrating down into the gravel and reducing its drainage capacity over time.
Backfilling and Finishing
Once drainage is in place, backfill the retained side with soil in layers. Compact each layer lightly as you go rather than dumping it all in at once. Loose, uncompacted fill settles unevenly and can create voids that concentrate water pressure in one spot.
On the exposed face, you can leave the concrete sleepers as they are (most come in a smooth grey or textured finish) or paint them if you prefer a different look. The steel posts typically sit flush with or slightly behind the sleeper face, so they’re mostly hidden. If any post tops extend above the final sleeper, cap them with plastic post caps or cut them flush with a grinder.
Common Mistakes to Avoid
Skipping drainage is the biggest one. A wall that looks solid on day one can bow, crack, or collapse within a few years if water has no way out. The second most common error is underestimating footing depth. A footing that’s too shallow lets the post rock forward under soil pressure, and there’s no easy fix once the wall is built.
Using the wrong concrete strength for footings also causes problems. Standard general-purpose concrete (around 20 MPa) is the minimum for structural footings. Weaker mixes designed for non-structural work, like setting fence posts in stable soil, won’t hold up under the lateral force of retained earth.
Finally, don’t assume a wall under the permit height is automatically safe to build without thought. Even a 600 mm wall on a slope with clay soil can fail if drainage and footings aren’t done properly. The physics of soil and water pressure apply regardless of wall height.