This week’s live Q&A was a by-request topic: retaining walls, angle of repose, and zone of influence. Specifically — how close can you build to a retaining wall before it starts putting extra load on it?
Thanks to Rob Dontis for the question. Here’s the rundown.
What a Retaining Wall Actually Does
Retaining walls hold up dirt on one side to create a flat (or flatter) area — a high side and a low side. In Queensland, who owns a retaining wall usually comes down to who it was built to benefit. That’s a whole separate conversation, especially with older walls, so we won’t get into it here.
What we will get into is the two things that decide how much load ends up on your wall: angle of repose and zone of influence.
Angle of Repose
Angle of repose is the angle at which soil naturally settles when it’s piled up under its own weight. Keep adding soil to the pile, and it keeps that same angle — it doesn’t get steeper.
This matters because it’s the soil sitting above that angle of repose line that actually loads up your retaining wall. As engineers, we treat that load a bit like a water load: it increases with depth, so it’s biggest at the bottom of the wall and smallest at the top. We model it as a triangular load.
When we design a wall, we’re looking at the total force and how it’s resisted — whether that’s a concrete slab, a deep footing, or a sleeper-and-post system with steel posts driven into the ground.
Why Drainage Matters So Much
Angle of repose isn’t fixed — it changes when soil gets wet.
Think about building a sandcastle. Sand straight out of the bucket holds its shape. Pour water on it, and it slumps. The same thing happens to the soil behind your retaining wall. Wet soil has a flatter, wider angle of repose, which means more soil ends up above that line — and more load on your wall.
That’s why drainage is such a big part of retaining wall design. We’re not just designing the wall as a structure; we’re designing it as a drain. A layer of rock or filter material behind the wall lets water escape, keeps the soil free-draining, and keeps that angle of repose (and the load on the wall) as low as possible. Skip the drainage and you’re asking the wall to carry a lot more than it needs to.
Zone of Influence — How Close Is Too Close?
This is the part Rob actually asked about.
Zone of influence covers what happens when there’s a structure, a person, or a vehicle on the high side, up on top of the wall. How close can that load be to the wall before it actually transfers into the wall?
The angle of repose gives us the answer. Anything sitting within that zone — inside the angle of repose line — is already loading the wall, so it has to be accounted for in the design. That includes people, vehicles, buildings, anything with weight. And even though that weight is pushing straight down, the way soil behaves means it translates into a horizontal load on the wall.
A simple way to picture it: if we’re using a 45° line (a 1:1 ratio, our default starting assumption for clay soils) and the wall is 1.8 m high, then anything closer than 1.8 m to the wall is inside the zone of influence and is loading the wall. Anything further away than that isn’t.
If there’s nothing currently in that zone but there’s a reasonable chance something could end up there — a car parking, someone walking, a future structure — we still design for it. You can’t always guarantee no one will ever build, drive, or walk in that space, so as structural engineers we allow for a surcharge load:
- Around 5 kPa (roughly 500 kg/m²) if vehicles could drive on the high side
- Around 2 kPa (roughly 200 kg/m²) for foot traffic only
That surcharge gets added on top of the soil load itself when we design the wall.
Can You Build on Top of a Retaining Wall?
In theory, yes — but the engineer needs to design for it. The wall, the footing, and the posts (if it’s a sleeper wall) all need to be sized to carry that extra load safely.
Got a Question for the Next Live Stream?
That covers angle of repose, zone of influence, and how close you can build to a retaining wall. If you’ve got a question you’d like answered on a future live stream, let us know — it gives us a chance to think it through properly before we roll the camera.
Get in touch with Cornell Engineers if you need a retaining wall assessed, designed, or checked before you build near one.
Video Transcript
Retaining Walls: Angle of Repose & Zone of Influence — Video Transcript
I’m Matt Cornell from Cornell Engineers. Welcome to your Saturday morning. Today we’re doing a by-request — a quick chat about retaining walls, zone of influence, and how close you can build to a retaining wall. We’ve actually gone back to the old style because you’re going to need to see my hands — the overhead camera. So welcome aboard. Let’s talk about retaining walls.
Retaining walls, as we all probably know, are used to hold up the dirt on one side to create a flat side, or a flatter side, on one side — a high side and a low side. In Queensland, the ownership of a retaining wall comes down to who the retaining wall was built to benefit. We’re not getting into that so much today, because that gets very complicated, especially when you’re talking about old retaining walls.
But let’s talk about zone of influence and how close — the questions that Rob Dontis actually raised for us were: retaining wall angle of repose, and zone of influence — how close can you build to a retaining wall? So let’s cut to the overhead and have a little chat about this.
Okay, so retaining walls, like we said, are typically walls. Sometimes they’re vertical, sometimes they’re less vertical, or built into the slope a little bit. But in this case, we’ll show the lower ground on this side, we’ll show the high ground on this side, and this here is our retaining wall. It’s holding the dirt, stopping it flowing down onto the low side.
So when we’re talking about angle of repose — I’m going to need another piece of paper already — angle of repose is the angle you get when soil gets dumped in a pile and it sort of settles down under its own weight. The more you build it up, the more you build it up, it still keeps that same shape.
That’s important to us because it’s the soil that lands on top of that angle of repose that actually helps put load on our retaining wall. So if our retaining wall was built here, it’s this soil above that line that is actually putting a horizontal force onto our retaining wall.
As engineers, we look at this force from the soil in two ways. It’s sort of like a hydraulic load, like a water load — the higher the wall gets, the bigger the load gets at the bottom, and the load actually gets smaller towards the top. We show it as a triangular load, as engineers.
When we’re designing a retaining wall, we’re more interested in the overall forces on the retaining wall and how it’s resisted — whether there’s a concrete slab underneath, or a deep footing built into the ground. That was the sleeper retaining wall with a deep footing built into the ground, or the concrete slab, which is what we’d normally use for a concrete masonry retaining wall. There’s also timber retaining walls, which are a bit like the sleeper retaining wall with the deep footing built into the ground. But this wall is essentially just holding back that soil that is above that angle of repose.
The angle of repose changes when the soil gets wet. So remember the last time you went to the beach and you built a sandcastle. Sand sitting on the beach, straight out of the bucket, the sand stays up nice and straight. As soon as you pour some water on it, it slumps.
So imagine you did that to the soil behind your retaining wall — you poured water in behind the retaining wall. The water does the same thing to the soil behind the retaining wall, unless it’s rock, as it does to the sand on the beach. It makes it slump.
So when we talk about retaining walls, we talk a lot about drainage. We talk about the force of the soil on the retaining wall that we’re designing as a structure, but we’re also talking about it as a drain. We don’t want water building up behind the retaining wall, because that extra water — apart from the fact that it can also put load on the retaining wall itself — also makes the soil slump a lot quicker, and puts a lot more load on the retaining wall.
If we had the choice, we’re going to design a retaining wall to be as economical as possible. So we include drainage — a layer of rocks or filter material that lets the water out of the soil, so that the soil is free-draining and the angle of repose stays nice and vertical. Angle of repose for wet soil would be much flatter, and there’s a lot more weight being applied to the retaining wall as a result. So that’s angle of repose.
Zone of influence isn’t really covered there. Let’s go back to our nice, neat retaining wall diagram. Our zone of influence is more about when we have a structure, or someone standing on top of the wall, or someone driving on top of the wall on the high side of the wall. How close can they be to that wall without it putting load on the wall? Or alternatively, if it’s close to the wall, how much of that load actually gets transmitted into the wall?
It has to be designed by the engineer, with the footing and the strength of the wall itself, the strength of the steel posts. If it’s a concrete sleeper retaining wall with steel posts and concrete sleepers between it, how much of that load has to be covered by the retaining wall. Ideally, the further you put the load away, the better.
The angle of repose tells us that anything within that line of soil — where that soil on top of the angle of repose is already putting load onto the retaining wall — has to be designed for it. That extra weight from people, cars, and buildings on top of the retaining wall also applies a load to that soil. It might look like they’d just be pushing straight down, but because of the way soil behaves, that load also translates into a horizontal load on our retaining wall.
So the answer to “how far away does something have to be before it no longer applies load onto the retaining wall” becomes a little bit simpler once we understand this zone of influence line. Say it’s a 45-degree line — a one-in-one line — and the wall is 1.8 m high. That line of soil that’s affected, that puts load onto the retaining wall, is obviously 1.8 m out. So if you’ve got a 1.8 m high retaining wall and you’re closer than 1.8 m with your building, your vehicles, or your big heavy people walking across the top of the retaining wall, those loads are being resisted by the retaining wall. If you’re further away than that line, they’re not — assuming a 45-degree line, which is our basic starting point if we don’t have any other information. We’ll typically assume it’s a clay soil, and we’ll assume about a 45-degree line.
If you’ve got nothing in that zone right now, we’re still going to design your retaining wall for that surcharge load, because you can’t necessarily stop someone building there, or driving there, or someone walking across the top. We’re going to have to allow for it in our design as structural engineers.
So we allow maybe 5 kPa — 500 kg a square metre — if someone can drive on that high side of the wall. A little bit less if it’s only a footpath — maybe 2 kPa, 200 kg a square metre, which is still a fair bit. And that’s the extra load, on top of the weight of the soil, being applied to the design of the retaining wall.
So, I think we’ve just about covered all the questions we had from Rob Dontis. Retaining wall angle of repose — we’ve covered it. Zone of influence — we’ve covered it. And how close can you build to a retaining wall — we’ve covered that too. In theory, you can build on top of a retaining wall, don’t forget — it just means the engineer has to take that into account when they do their design.
I’m Cornell from Cornell Engineers. I hope this has been useful. Stay tuned, we’ll have some more live streams every now and then — you know what, there’s a lot less editing involved in setting up a couple of cameras and having a chat to you live than there is when I record with multiple cameras and put it all together. This is way easier for me.
So, if you like this, and you’ve got a couple of questions you’d like me to answer in the next live stream or in future live streams, let me know. That’ll give me a chance to have a think about what we’re going to say — we’ll roll the camera and I’ll answer your questions.
I’m Cornell from Cornell Engineers in Queensland, Australia. Hope you have a good weekend. I’ll catch you later.