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Apologies for length, I just want you to understand my plan....if I win Lotto in the next week I'll have a professional rebuild the entire wall, but currently, no money in the pot for this, but I do want to stabilise this wall.
I have a 1m high log retaining wall. The cost of having a professional to rebuild is prohibitive.
I can't rebuild the wall, even in sections, as the risk of having the ground behind collapse is untenable. There is a large concrete patio topping the wall, the house is on piers, but I really don't want to disturb this ground at all. The timbers of the wall are actually in good condition.
Underneath the large concrete patio there is a significant void between concrete and level of dirt - it has dropped fairly uniformly by around 15cm ave. Once I have repaired the retaining wall, I am having closed cell foam treatment by a professional to fill, lift and stabilise the void.
I'd love your advice guys - you always understand where I'm coming from and have offered excellent guidance. As the wall is 30yrs old, and had some 'interesting' supports added by the ex around 20yrs ago, it doesn't seem to be moving. I'm pretty sure the timber uprights he put in weren't deep enough and he attempted to rectify the lean after 10yrs by adding posts and sort of wedging-in the back of the post. The ground is awful, dirt, sandstone boulders/rocks. The wall is mostly vertical, just everso 'slightly' leaning in areas. So I'm thinking it's better to leave existing wall undisturbed, but stabilise with steel H posts, 1 m deep in ground. If you look at image 2, I've marked where I plan to add the H posts (in blue) and where I'll trim the top of the posts (in orange).
1. Where there is a slight lean, if I put an H post in vertically, there will be a gap between posts and the wall at the bottom. What should I do to stabilise at these points?
2. I'm also sure the drainage is inadaquate behind the wall (I remember some ag pipe going in). Because there's the concrete patio topping the wall, I don't have access to dig out behind it and add drainage/geotextile fabric. I'm thinking perhaps seeing if I could get some vertical drains in where I can from the top, or/and digging under the base log of wall and placing ag drains running out? (see yellow markings).
3. Any techniques to help digging post holes through flipping sandstone rocks would also be valuable - I was going to hire a petrol post hole digger, but I think it's not suited to this position and ground, and it's a case of just attacking manually.(I will be building another retaining wall 2m from this one, 55cm from patio level to comply with local Council building code for retaining walls, so there won't be a huge drop, but plan on adding planters to top for additional safety).
Apologies for giving you a right tricky one.
Hi @ezzie,
Thanks for taking the time to lay out the situation so clearly—it's a complex setup, but your thinking around reinforcing rather than rebuilding is very sensible given the structural risks and limited access. The fact that the wall is around 30 years old, with added supports 20 years ago and still not showing signs of movement, does suggest it's relatively stable, especially considering the weight of the patio above. That said, it’s understandable to want to future-proof it, particularly before foam void-filling is done beneath the concrete.
Before going too far with reinforcement plans, though, it’s worth stepping back and considering whether anything actually needs to be done at all. You’ve noted the timbers are in good condition and that there’s no sign the wall is actively moving. There’s a very slight lean, but that alone doesn’t mean the wall is failing—especially if that lean has been unchanged for many years. In fact, it could just be a result of minor settling that occurred long ago. Often, a wall that’s lasted this long without obvious signs of distress is doing its job just fine. If the original posts were inadequately deep or poorly installed but the wall still hasn’t shifted significantly, that’s telling in itself.
Rather than going to the effort and expense of installing steel H-posts, your best bet at this point may be to bring in a structural engineer for an assessment. They’ll be able to confirm whether the lean is within acceptable tolerances, check that the original posts haven’t rotted, and let you know if any action is actually needed. It might turn out that the wall is entirely sound and doesn’t require reinforcement—just periodic monitoring. That peace of mind is valuable, especially before going ahead with the foam injection work under the patio.
If reinforcement is ultimately recommended and you go ahead with steel posts, installing them vertically against a leaning wall will leave a gap at the base. To stabilise the wall in that case, you could pack the gap using steel plate offcuts or hardwood wedges, driving them in tightly to form a snug connection. Adhesive like Sikaflex 11FC can help lock everything in place, and a couple of bolts through the H-post into the timber will add extra security, though you’ll want to be careful not to compromise the timber with too many fixings.
In terms of drainage, I completely understand the limitations imposed by the concrete patio. An option is to dig carefully under the bottom log of the wall in spots and run short sections of ag pipe out to daylight, helping to relieve hydrostatic pressure during heavy rain. Even a few functional outlets can make a difference in saturated ground.
As for digging post holes in ground littered with sandstone and rocks, a petrol auger likely won’t cope well. Manual tools—like a crowbar, trenching shovel, and if needed, a demolition hammer with a clay spade or chisel attachment—will give better results. Pre-soaking the area can also make manual digging easier, especially in clay-heavy or compacted soil.
It sounds like you’re putting a lot of careful thought into this, which is great to see. If the wall is stable and a structural engineer gives it the all-clear, you may find there’s little or no work required at all—saving your energy and budget for other improvements down the track.
Please let me know if you have any questions.
Mitchell
You guys are worth your weight in gold! Thank you for your feedback and advice. I won't get a structural engineer in as it'll cost a fortune, instead I can use the $ to stabilise the wall. As I'm spending a small fortune on the foam, it's not a bad idea to know it won't be moving further, even if in mm's. There's also a rock wall further along the length of the patio. My plan there is to spray Foam Fill between the gaps to slow soil washout.
Some (easier) questions for you if I can -
(1) what tool do I use to cut the tops off the old posts? Will a hand saw work...I don't want to be investing in tools as I'll be downsizing in the next few years, but will be doing more retaining wall work.
(2) Am I making the right choice with H steel posts, or could I save money and use more timber posts in this situation?
(3) Ground is hideous to dig, I think this is why my ex ended up not laying posts in the ground deep enough. If I run into this situation, would 1m above and 80cm below ground work, or I just have to hire a jackhammer?
(4) There's one horizontal log at the top showing a little surface wet decay - too risky to remove & replace, is there something I can coat it with to slow rotting process.
(5) I'd like to cover the awful previous work and the bent posts (see pics) 
(6) I'm going to have quite a bit of wood left over from projects (internal & external) Would you know of any community-type recycling/waste reduction centres that Bunnings supports that I could gift the timber, hardware etc, etc.
(7 What's this week's winning Lotto numbers? 
Hi @ezzie,
In terms of trimming the tops off the old posts, a handsaw will definitely get the job done, particularly if you're only tackling a few and don't mind putting in a bit of elbow grease. However, if you want a quicker and neater finish, especially on hardwood, purchasing or hiring a reciprocating saw or circular saw could make the job easier — and if you’ve got friends or neighbours with tools, it’s worth asking.
As for choosing between H-section steel posts or timber, steel posts will typically last longer and provide more structural stability — especially if the existing wall has started to lean. Timber could be a cheaper upfront option, but it may not offer the same durability, particularly in poor soil. Since you’re already investing in foam to protect against further movement, it makes sense to stabilise properly with steel posts so the effort isn’t undone down the track.
Digging through tough ground is definitely a challenge and sounds like it may have contributed to earlier issues with shallow post depth. If you can get to 800mm deep and have at least 1m above ground, that may be acceptable, depending on the height and load of the wall — but the deeper you can go, the better. If digging proves too difficult, hiring a jackhammer for a day could save you a lot of physical strain and improve the wall's long-term integrity.
Regarding the top horizontal log with signs of wet decay, you might be able to slow further deterioration by treating it with a wood hardener or preservative designed for outdoor use. These products soak into soft timber and help reinforce the structure while also repelling moisture. It won't reverse the decay, but it can buy you time if replacing it isn't practical right now.
For improving the look of the wall and hiding the bent posts, boxing them in could definitely work. You might consider using treated pine decking boards or fibre cement sheeting, both of which are affordable, rot-resistant, and relatively easy to work with. If you're not doing a full false wall, cladding just the front faces could help tidy things up without adding too much bulk or cost.
It’s also great that you’re thinking sustainably. If you have leftover timber and hardware, local Men’s Sheds, community centres, or even schools often appreciate donations of building materials.
And as for those winning Lotto numbers — if we ever figure them out, we’ll be sure to let you know!
Mitchell
Hello @ezzie
I don't think the Australian Handyman Supplies 50 x 50mm x 2.4m Galvanised Steel Fence Post is as strong as the H steel uprights. I suggest going to either the special orders desk or the trade specialist at your local store and asking them if they can order in the H steel uprights for you in the size and length that you need. Otherwise, I recommend speaking to an engineer and asking them for their recommendation on the size of the steel to use.
Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.
If you need further assistance, please let us know.
Eric
