Why retaining walls lean, and when you need approval
Most retaining walls that lean or fall fail because of water, not weight. Why drainage behind the wall matters, and when a wall needs an engineer or approval.
The assumption we're testing
“It's only a small wall. Just stack a few sleepers.”
Water pushes walls over
A retaining wall holds back a slope of soil. That's hard enough when the soil is dry. When it rains, the soil behind the wall fills with water and gets much heavier. That water pushes on the back of the wall.
If the water has no way out, the push keeps building with every storm. The wall starts to lean, crack or bulge. That's why most walls that fail do so because of water, not weight.
Sydney clay makes it worse. Clay holds water for a long time, and it swells when it's wet.
What drainage behind a wall looks like
A properly built wall has drainage behind it that you never see. At the base sits an ag drain (a slotted pipe in a gravel trench). It collects the water and carries it away.
Above the pipe, the space behind the wall is backfilled (filled back in) with gravel, not soil. Water runs down through the gravel to the pipe instead of sitting against the wall. A layer of filter cloth stops soil from washing into the gravel and blocking it.
The pipe then has to go somewhere the water is allowed to go. A drain that doesn't lead anywhere just moves the puddle. See our drainage page for more on this.
Other reasons walls lean
A shallow footing is another common cause. The footing is the base the wall stands on. If it's too small for the soil and the height, the wall tips or sinks.
Timber sleepers rot where they touch wet soil. Posts that aren't set deep enough get pushed forward. And anything heavy near the top of the wall, like a car, a pool or a shed, adds to the load.
If your wall is already leaning, don't wait for a big storm. A leaning wall usually gets worse, not better.
When you need an engineer or approval
Taller walls need an engineer or council approval. Walls near a boundary, near the house, or holding up something heavy like a parking area or a pool may need them too.
The rules depend on the wall and where it sits, and they can change. So check with your council before you build. We check before we start, and we tell you if your wall needs an engineer or approval.
Before any digging, pipes and cables are located, so nobody hits a water main or a power line.
What to ask before you build
Ask anyone quoting on a wall these questions. What drainage goes behind it? What will the footing be? Where will the water go? Does this wall need an engineer or approval?
If the answers are vague, the wall probably will be too. See how we build retaining walls, or read our guide on hiring a landscaper.
Want it done once, and done right? We check the ground first, then plan the work.