How to add steel uprights to leaning wood retaining wall?

ezzie

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.Blue = H posts Yellow = position of ag spot drains under wallOrange - trim topsRetaining wall...zoom in on interesting repairs!Patio above wall (to left)

MitchellMc

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