How a sleeper wall goes together
A sleeper retaining wall is a row of posts set in concrete with sleepers spanning between them. Concrete sleepers drop into galvanised steel H-section posts (or C-section at the ends) and stack until the wall is the right height. Timber sleepers are usually bolted to the face of timber posts. Either way the count is the same: posts at the sleeper length, sleepers equal to bays times courses, and a hole full of concrete under every post.
bays = wall length ÷ sleeper length (round up) posts = bays + 1 sleepers = bays × courses
Because sleepers come in 2.0 and 2.4 m lengths, the wall length rarely divides evenly and the last bay is cut. Concrete sleepers cut with a diamond blade and a lot of dust; timber cuts easily. If the wall length is your choice, make it a multiple of the sleeper length and save the cuts.
Post depth and length
The posts do the work. Everything the soil pushes against the wall ends up as a lever on the posts, so they need to be deep. The old rule for low garden walls in firm ground is that the post goes as far into the ground as the wall stands above it, with 600 mm as an absolute floor. A 600 mm wall on 1.2 m posts, an 800 mm wall on 1.6 m posts, a 1 m wall on 2 m posts. Post lengths come in stock sizes, so buy the next size up.
| Wall height | Courses (200 mm) | Post in ground | Post length | Bags per hole (350 mm) |
|---|---|---|---|---|
| 400 mm | 2 | 600 mm | 1.2 m | 7 |
| 600 mm | 3 | 600 mm | 1.2 m | 7 |
| 800 mm | 4 | 800 mm | 1.8 m | 9 |
| 1,000 mm | 5 | 1,000 mm | 2.4 m | 11 |
Above a metre, or wherever a driveway, pool, shed or house sits above the wall, the rule of thumb stops applying. The surcharge from that load changes the design and an engineer needs to size the posts, the embedment and the footing. In Victoria a retaining wall over 1 m normally needs a building permit, and lower walls near boundaries or structures can too. Ask the council before the excavator arrives.
Drainage
Water behind a retaining wall is the load nobody sees. Lay a slotted agricultural pipe along the base of the wall on the uphill side, falling to an outlet, then backfill 300 mm of drainage gravel behind the full height of the wall with geofabric between the gravel and the soil so the fines do not clog it. The calculator gives gravel, pipe and fabric quantities on that basis. Concrete sleepers have small gaps that weep; timber walls need the gaps or weep holes left open.
Concrete or timber sleepers
Concrete sleepers are heavier, last indefinitely and come in a range of finishes. Timber sleepers are lighter to handle and cheaper to buy, but even treated hardwood in contact with wet soil has a life measured in years rather than decades. For a wall you do not want to build twice, concrete with galvanised posts is the usual choice.
Worked example
A 12 m wall, 600 mm high, in 2.0 m concrete sleepers 200 mm high. Three courses, six bays, seven posts and 18 sleepers, plus two spares. Post embedment 600 mm, so 1.2 m posts. Each 350 × 600 hole takes 0.06 m³ of concrete, seven bags, so 49 bags in total or about 0.43 m³ of premix. Behind the wall, 2.4 m³ of drainage gravel, 13 m of ag pipe and about 16 m² of geofabric.
Common questions
How deep do retaining wall posts go?
As deep as the wall is high, minimum 600 mm, for low garden walls in firm ground. Over 1 m or under a driveway, get an engineer.
How far apart are the posts?
At the sleeper length: 2.0 m or 2.4 m centres.
Do I need a permit in Victoria?
Usually over 1 m, and sometimes lower near boundaries or buildings. Check with council.
How much concrete per post?
About seven 20 kg bags for a 350 × 600 hole, eleven or twelve for a 1 m deep hole.
Quantities and rules of thumb for low garden walls. Any wall over 1 m, any wall with a load above it, and any wall in soft or reactive soil needs an engineer's design, and most need a permit.