How a retaining wall stays up
The soil behind a wall pushes it outwards, more strongly the deeper you go. A cantilever wall (a concrete block or concrete wall on an L-shaped footing) resists that push in three ways:
- Overturning: the soil sitting on the heel (the part of the footing under the soil) holds the wall down so it can’t tip forward. We check for a safety factor of 2.
- Sliding: friction under the footing stops the whole wall sliding forward. We check for a safety factor of 1.5. A “key” under the footing can add more.
- Bearing: the footing must spread the weight so the soil underneath isn’t overloaded, and the push should stay in the middle third of the footing.
The calculator finds the narrowest footing that passes all of them, relying on friction alone to stop sliding. On that basis the footing often comes out as wide as the wall is high, or wider. A shear key (a downstand under the footing) or soil in front of the toe can make it much narrower. Your engineer will design that.
Things that make walls fail
- Water. Without drainage behind the wall, water pressure can double the load. Use free-draining gravel, an ag pipe at the bottom and weep holes.
- Cars and buildings on top. A driveway, a house footing or a slope above the wall all add load.
- Clay. It pushes harder and grips less than sand or gravel.
Do I need an engineer?
In most Australian councils, walls over about 1 m high, walls near a boundary or building, and walls holding up a driveway need an engineer’s design and approval. The limit varies, so check with your council. The engineer also designs the reinforcement and the concrete, which this calculator doesn’t do.
Common questions
How wide should a retaining wall footing be?
It depends on the soil, the height and what’s on top. Relying on friction alone, a cantilever wall’s footing is often about as wide as the wall is high, and wider in clay or with a driveway on top. A shear key under the footing can reduce that a lot. The calculator above finds the narrowest base that passes the sliding, overturning and bearing checks.
Can I build a 1 metre retaining wall without an engineer?
It depends on your council. Many allow low garden walls (often under 0.9 to 1 m) without a permit, but walls near a boundary or a building, or holding up a driveway, usually need an engineer and approval even when they’re low.
Why do retaining walls lean or fall over?
Most often because water builds up behind them, the footing is too small, or extra load (a car, a shed, a slope) sits on top. Good drainage is as important as the size of the footing.
What is the heel of a retaining wall?
The part of the footing that sticks out behind the wall, under the soil. The soil’s weight on the heel is what stops the wall tipping over.