What is maximum distance between posts in the retaining wall with treated pine sleepers 200x75mm?

[jzacchary]

I want to build a retaining wall 600mm high, using treated pine sleepers 200x75mm and galvanised steel posts. I am not sure what length of the sleepers to go for, 2400mm or shorter, to end up with strong wall?

[MitchellMc]

Welcome to the Bunnings Workshop community @jzacchary. It's brilliant to have you join us, and many thanks for your question about building a retaining wall.

Before you start, it's best to check with your local council to ensure your selected height is within guidelines and that you don't require engineering.

You'll find that the supplier of the steel posts will have stipulations on their spacing, as you need to be aware of not only the timber but also how much each post can retain. You might like to look at this guide: How to build a raised garden bed for privacy. In this article, we've used a spacing of 1500mm, as specified by Ridgi, the post supplier. 2400mm is far too long a span as the sleepers will bow outwards, so I'd recommend 1500 or even 1200mm for a super strong wall.

Please let me know if you have any questions.

Mitchell
 

[jzacchary]

Dear MitchellMc,

Thank you for the prompt response and suggestions. I had thought that the 75mm-thick sleepers would provide a strong wall with spacing of 1800mm, but tested the water with the question mentioning 2400mm. Would you still insist on 1500mm, with the sleepers being 50% thicker than the usual 50mm ones?

With kind regards,

John

[Dave-1]

Evening @jzacchary 

I think what @MitchellMc is refering to isnt the strength of the sleepers but the span. The wider spans between posts make for a weaker wall. By having short spans you will have more posts (yeah it will cost more, but a lot less likely to have bowing in the wall after a year or two) 

If you change your mind and chosoe a Gabion wall, then yep can recomend a bunch of stuff for sure (I have used them as retaining walls) 

Dave

[jzacchary]

Dear Dave-1,
I find it hard to accept that the thickness of sleepers is irrelevant; wouldn't you expect that the wall made of 75mm sleepers can be equally strong with the span of 1.8m as the wall made of 50mm sleepers with the span of 1.2m?

[MitchellMc]

Hi @jzacchary,

Sorry, I should have been clearer. Post spacing has two major effects: it determines how much your sleepers can bow, and you're correct that a 75mm sleeper could span 1800mm without bowing. The second element which the post-spacing effects, is the ability of the wall to remain upright against the weight it retains. In that regard, the thickness of the sleeper is somewhat irrelevant as it could be made of the strongest material in the world, but if you don't have enough posts spaced at the required intervals, the entire wall could fall over.

However, we are not yet aware of what posts you are using or if the manufacturer has any stipulations on spacing. If you go with the Retain-iT 75mm system I don't believe the manufacture has any requirement on spacing, but I'd go with 1800mm as you typically get bowing at 2400mm with 75mm thick sleepers. 

Mitchell

[jzacchary]

Dear Mitchell,

We are now on the same page: I am indeed going to use Retain-IT 75mm posts and 200x75x1800mm treated pine sleepers. Thank you for your help, time and advice.

With kind regards,

John