How to build a steel post timber sleeper retaining wall?

Hi, sorry if this is a repeat question and if so please point me in the direction of a previous answer!

I'm building a retaining wall 600mm high. I'm using timber sleepers 200x75mm cut to 1.5m lengths, and the galvanised steel H or C posts. As it is 600 high I have dug holes 700 deep allowing ½ in ½ out + 100 at the bottom.

I've read conflicting opinions on hole diameter. I used an Auger from Bunnings so the holes are 300mm diameter. As they are already 700mm deep surely that's enough for a 600 high wall? I've read about making the holes 450mm, is this correct?

Also for the 100mm at the base of each hole, should this be scoria or concrete please?

Lastly for a 600 high wall should the posts be vertical or should I tilt them backwards?

Thanks very much for your help!

(FYI I'm aware of the need for drainage and scoria backfill to further reduce load).

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Comments

EricL

Hello @RenoMike

Welcome to the Bunnings Workshop community. It's wonderful to have you join us, and thanks for sharing your question about your retaining wall.

For a 600mm high retaining wall, your existing post holes—300mm in diameter and 700mm deep—are generally sufficient for most residential applications, especially if you're using concrete and the wall isn't retaining a heavy surcharge like a driveway or slope directly behind it. The 300mm diameter is a common size used with standard augers like those available at Bunnings, and it's often suitable for walls up to around 1 meter, depending on soil type and wall design.

The suggestion for a 450mm diameter hole typically applies to taller or heavily loaded walls where more concrete and a wider footing are needed to counter lateral pressure from the soil. In residential settings with good drainage, stable soil, and a wall under 1 meter high, the extra diameter may be considered conservative or precautionary.

That said, soil type, drainage conditions, and local council guidelines all play a role. Clay soils or poor drainage might require deeper or wider footings to prevent movement. Since you're already at 700mm depth for a 600mm wall, you're giving the posts good anchoring.

For the base of each post hole, it's best to fill the full depth—including the bottom 100mm—with concrete rather than scoria. While scoria is excellent for drainage behind the retaining wall, it doesn’t provide structural support. A solid concrete base ensures the post is well anchored and helps prevent any settling over time, especially once lateral pressure from the soil builds up.

As for the post alignment, for a 600mm high retaining wall, setting the posts perfectly vertical is generally acceptable and widely done—especially if your drainage is well managed and you’re not retaining heavy loads behind the wall. However, a slight backward tilt (known as a "batter") of about 5–10mm per 100mm of wall height can help resist soil pressure and maintain a straighter line over time, particularly in clay soils or if the wall will be exposed to a lot of moisture. It's not mandatory for a wall of this height, but it can be a helpful precaution.

Just make sure your concrete sets properly before backfilling and always include adequate drainage behind the wall (such as agi pipe and scoria or gravel) to reduce water pressure, which is a common cause of wall failure. For peace of mind and compliance, it’s always best to check with your local council or a structural engineer—especially if you’re unsure about soil stability or load behind the wall.

Here is a handy step-by-step guide: How To Build Retaining Wall

If you need further assistance, please let us know.

Eric

RenoMike

https://www.workshop.bunnings.com.au/discussion/comment/52961#Comment_52961

That's fantastic Eric, thanks so much for the detailed reply!

Can I ask - the batter, is that 5-10mm per 100mm of wall height ABOVE the ground (ie. 60mm for 600mm above the ground), or is it 5-10mm for the entire post height of 1200mm including the 600mm below ground? Thanks again.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/88666#Comment_88666

Hi @RenoMike

Yes, that would be 5-10mm per 100mm of wall height ABOVE the ground. However, if you feel that 60mm is too much of a lean you can reduce it by half to 30mm. A great deal of it depends on the soil type that you have and the prevailing water run off that passes through that section of your property. But if your soil type is compact, stable and not made of clay then the 30mm batter will be more than sufficient to hold the retaining wall in place.

If you need further assistance, please let us know.

Eric

Midlife_Morgman

Hi @RenoMike, thank you for posting this question. I'm about to embark on a similar project and had the same question about a 300 or 450mm diameter hole for the posts. Thank you @EricL for your helpful reply.

Mike, out of interest what made you decide to go for timber sleepers instead of concrete. I was leaning towards going for concrete sleepers for my project and would be keen to know your reasoning

Cheers

EricL

https://www.workshop.bunnings.com.au/discussion/comment/54261#Comment_54261

Hello @Midlife_Morgman

Welcome to the Bunnings Workshop community. It's sensational to have you join us.

Let me tag @RenoMike as well to make them aware of your question. It would be interesting to know why Mike chose those particular materials for their retaining wall.

Eric

RenoMike

https://www.workshop.bunnings.com.au/discussion/comment/54261#Comment_54261

Hi, thanks for your question!

Basically aesthetics. I live in an area where timber retaining walls are really common. There is a surprising variety of looks whether people paint or stain the timber, and paint or leave the galvanised steel posts. There is one concrete sleeper retaining wall that doesn't look very good but that's probably more because it's all falling over (!) rather than the choice of sleeper. There is one I've seen that looks like it is made out of some kind of composite material in charcoal which looks amazing, I've never seen that before and wouldn't know where to get it.

If you're concerned about durability - we have an older ~11 years old retaining wall made of timber sleepers which has some rot. I read that it is beneficial to coat the timber sleepers with Bitumen like paint (I got Bondall Timber Protector permanent waterproofing paint from Bunnings). I painted this on the bottom of the bottom sleeper, and on the ends and back of all of the sleepers. For the top sleeper I painted it only half way up the back as I'm not sure I want to see it, but you're supposed to paint it up to 10cm above the soil line. Actually after I painted them I quite like the deep black colour that it made them! I've included a picture of where I'm up to (it's not finished yet) showing the timber sleepers with the galvanised posts, and the colour of the bottom sleeper. Actually I painted the front of the bottom sleeper too as the soil will be up against much of that.

Lastly I used 200mm x 75mm sleepers that I cut 1.5m in length (from 3m lengths). That was heavy enough but I could lift and position it myself. Concrete sleepers I can only imagine will be very heavy and there's quite a bit of trial and error positioning the first one to get it level and the correct height. I could do that myself with the timber, but I reckon you'll need some strong hands with the concrete.

Hope that helps!

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