What concrete to use for galvanised post concrete sleeper retaining wall?

[markphillips100]

Hi Community.  My Google foo and AI seem to give me a whole heap of competing answers so looking for a bit of clarity.  Note, I'm not a tradesman by any stretch.

I built a pine sleeper retaining wall about 15 years ago as a completely DIY project.  A few sleepers are looking a little worse for wear so figure it's coming end of life.  We want to concrete/pave the retained area so will be replacing the wall for longevity reasons with a concrete sleeper retaining wall with galvanised posts, and this time a Colorbond fence on top too.

The wall is no higher than 800mm at its maximum (4 sleepers) so I've opted for a 1.6m post (enough with a fence on top?).  This puts 800mm in the ground, plus I'll dig another 100mm below bottom, giving a hole depth of 900mm.  Diameters are roughly 450mm.

For the pine wall I mixed my own 1:2:3 concrete for the posts, and tried not to be too runny to give as much strength as possible.  The 200 x 75mm posts have not budged in 15 years so maybe I did something right.  When I go to break out the posts it will be a good assessment I guess.

1. Would it be reasonable to assume I should use the same mix for gal posts, some other ratio, or are premix concrete bags (just add water) suitable and sufficient?

2. Is my assessment of post length and hole depth appropriate given a fence on top?

[MitchellMc]

In terms of concrete choice @markphillips100, standard premixed concrete bags are absolutely suitable for galvanised steel posts and are what most DIYers and plenty of professionals use for concrete sleeper walls. You do not need to replicate a 1:2:3 site mix to achieve adequate strength for this application. Bagged premix is designed to hit a consistent compressive strength when mixed correctly and takes a lot of the guesswork out. The key is not the mix ratio but proper hole depth, diameter, compaction, and curing. Mixing it too dry actually reduces strength because the cement cannot hydrate properly, so aim for a workable mix rather than a stiff one. Quick-set concrete is also fine for galvanised posts and will not cause corrosion issues, provided the posts are hot-dip galvanised, which most sleeper posts are. Quick-set is convenient if you are working solo, but standard premix gives you more working time to plumb posts accurately. Either option is acceptable.

Your proposed post length and hole depth are generally reasonable for an 800mm high sleeper wall on its own. However, once you add a Colorbond fence on top, the equation changes. The wall height may be modest, but the fence introduces wind loading, which applies leverage to the posts rather than direct downward load. This is where councils and engineers start to take interest. Many councils require engineering once a fence is added above a retaining wall, even if the wall itself is under one metre. It’s not uncommon for engineers to specify deeper embedment, larger diameter holes, or even longer posts once wind load is factored in. Your 900mm hole depth and 450mm diameter is on the conservative side for the wall alone, which is good, but it may still be marginal for a wall-plus-fence scenario depending on soil type and exposure.

Before you commit, it would be wise to check your local council requirements. They may not require full engineering, but if they do, it is usually because of the fence rather than the wall height. If engineering is required, they may specify 2.0m posts instead of 1.6m, or deeper footings, which is far easier to do now than retrospectively. Given how well your original wall has performed, you are clearly doing things properly, so erring slightly on the side of caution here will pay off long term.

Please let me know if you have any questions.

Mitchell

[markphillips100]

Hi @MitchellMc , thanks for the reply.

Premix definitely sounds a lot less hassle so clearly the right choice.  Thanks for the info there.

As for post size, I had originally planned on a 2m post on the high sides to compensate for extra fence loading.  I downgraded as thought I was overdoing it as sleepers are 1.6m spaced so fence panelling would be 2 sheets instead of the usual 3, giving even more rigidity to the structure.  Maybe I should reconsider.

I'll check with council requirements to be certain.  Incidentally, I had 2 contractors not even mention doing an engineering report for them to do the same job.

[MikeTNZ]

Hi @markphillips100,

Just be aware that the galvanised steel that you use for this project needs to be reasonably thick-walled, as in 4.0-5.0mm, not the usual 2.5mm

wall thickness.

One other thing, concrete is incredibly corrosive (alkaline, not acidic), what I would recommend you do before installing any posts in the ground,

is covering the bottoms of the steel posts in some sort of membrane (such as  thick PVC or alkathene tape) above the level of the final level of the concrete before they are sunk into the ground, this will form a barrier between the steel and the concrete and ensure the longevity of the galvanising and hence the steel underneath it, I've seen installations where this has not happened and the steel has rusted out within a year or so after installation, this is very cheap insurance, if you want to do things once and not have to worry about it again. 

Cheers,

Mike T. 

[markphillips100]

Are these good enough @MikeTNZ ?

What about a bitumen paint as a barrier instead of PVC?

[MikeTNZ]

Hi @markphillips100,

Yes that would do nicely, provided the flat part of the "H" is against the wall, that is where the most structural strength is.

Bitumen paint, I think for the type of steel you are installing, would be the best option, it only has to go up to finished grade level (above the concrete).

All you are doing is creating a somewhat chemical barrier between the steel and the concrete to protect the steel from the concrete while it cures.

I would allow for 2 coats of this on each of the posts, you won't know until you have applied the first coat.

Best of luck with this project, I'd love to see pictures of how this turns out.

Cheers,

Mike T. 

[markphillips100]

Thanks @MikeTNZ .  I used bitumen for the bottom of the old pine sleeper posts too, and the back of a block wall, albeit the latter I also used PVC in addition to the gloopy stuff.  I was hoping it would be a good addition to gal posts too, so thanks again.