· by Rick Kosta
It is the question everyone asks and the one nobody should answer with a number over the phone. How deep the posts go is not a standard. It is an output, and it falls out of four things about your block.
Here is what actually drives it, and why a wall built to a rule of thumb is the one that leans.
Height above the ground is the starting point
The taller the wall, the more soil is pushing on it, and the push grows faster than the height does. Double the retained height and you do not double the load, you roughly quadruple it. That is why a 400 millimetre garden edge and a 1.2 metre boundary wall are not the same job with different sleeper counts. They are different structures.
The old trade shorthand of burying a third of the post is a starting sketch, not a specification. It assumes good ground, no water behind the wall and nothing sitting on top. Change any of those and the number moves.
What the ground is actually made of
A post works by leaning against the soil around it. How much that soil resists is the whole game, and across Melbourne's north it varies more than people expect.
Through the northern suburbs you are generally on reactive clay over basalt. Clay holds water, swells in winter and shrinks back in summer, and soil that moves seasonally gives a post less to push against than stable ground does. Where basalt sits close to the surface, the opposite problem shows up: the hole stops before the design depth and the fix is a rock breaker, not a shorter post.
Fill is the one that catches people out on newer estates. A lot that was graded before titles were issued can have engineered fill on one side and cut ground on the other, and a post founded in fill is not founded in anything until it reaches competent material below it.
What sits above the wall
This is the factor most often left out of the conversation, and it changes the answer more than height does.
- Nothing but garden. The simplest case, and the one every rule of thumb assumes.
- A driveway or parking. A vehicle is a live load arriving at the top of the wall, and it has to be designed for even if it only happens occasionally.
- A slab, shed or pool. Now the wall is holding up something structural, and that changes both the design and who needs to sign off on it.
- A neighbour's ground. If the wall is retaining land that is not yours, the stakes are different and it stops being a landscaping decision.
Engineers call this surcharge. In plain terms: anything sitting on the soil the wall is holding gets added to what the wall is holding.
Drainage, which is really a loading question
Water behind a wall is not a maintenance issue, it is a load. Saturated clay is heavier than dry clay and it pushes harder, and on ground that does not drain the wall can be carrying that extra push for months at a time.
A wall with an aggregate zone, an ag line and a real outlet is designed for soil. A wall without them is designed for soil and then asked to hold water as well. That is the same wall doing a bigger job than it was built for, and it is the single most common reason walls fail here. If you want the longer version, we have written about why water sits in a yard and what actually fixes it.
Where the engineer comes in
For anything structural, post depth comes from an engineer's design, not from us and not from a table online. That is not us passing the buck. The engineer has the soil report, the wall geometry and the loads, and those three things are what the number is calculated from.
What we can tell you before that point is whether the block looks like it will be straightforward or difficult, what we have found digging nearby, and whether rock is likely to be in the way. That is usually enough to plan around.
What this means when you are getting quotes
If two quotes for the same wall are a long way apart, post depth and footing size are often where the difference is hiding. A quote built on a shallower post and a smaller footing will always be cheaper, and it will look identical from the front until the ground has been through a couple of wet winters.
Worth asking any builder: what depth are the posts going to, what is that based on, and what happens if you hit rock before you get there. The answers tell you a lot.
The short version
There is no single right depth. There is a right depth for your wall height, your ground, what sits above it and how the water is handled. Anyone who gives you a number before looking at the block is guessing, and you are the one who finds out whether they guessed well.
If you are planning a wall, have a look at how we build retaining walls across Melbourne, or read why some walls last and others lean. If rock is the worry, that is rock removal, and the drainage behind the wall is part of every job we build.
