How to replace rotten retaining wall posts on a concrete base?

[GoodMate]

Some posts are showing clear signs of rotting, with material loss at the ground level, which I think require replacement. What is the right way of doing this? Thank you.These appear ok?This one requires urgent action.

[EricL]

Hello @GoodMate 

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

Generally speaking, support timber pieces will need to be attached to that section of the wall to prevent it from moving further while you repair the post. The concrete base will need to be broken in order to remove the old post footing. Once that piece has been removed you can then replace the entire post with a new one. Make sure to double check the size before purchasing the new timber post. My best advice is to paint the new footing with Gripset Betta 1L Waterproofing Membrane Bitumen Rubber to extend its service life.

However, if you feel that this is too involved, I recommend engaging the services of a builder to fix your retaining wall post.

Let me call on our experienced members @Dave-1, @Nailbag and @Noyade for their recommendations.

If you need further assistance, please let us know.

Eric

[Nailbag]

Hi @GoodMate 

There is a lot of weight behind that wall. So, before removing a rotting post, each sleeper level either side of the post to be replaced will need to be supported diagonally down and connected to the concrete ground. Then the concrete around the post needs to be cut large enough to support larger galvanised steel posts that are then concreted in to place.

I'm not sure I would risk doing this myself due to the supporting weight. Should the supports give way during the replacement, it could prove to be quite serious, not to mention dangerous.

I would recommend searching for a company that specialise in engineered retaining walls as a starting point.

Nailbag

 

[Dave-1]

Good Morning @GoodMate 

Nice tidy and straight retaining wall :0 Shame about thye posts rotting out 😕  And the answer I would be thinking of removing the rocky soil from behond and then redoing the posts, waterproofing them as @EricL has suggested. Trying to remove posts that have been concreted around and also look like they have been put in place properly would be hard to do individually let alone if they had nothing around them.

I would be thinking of a replacemenmt of the wall depending on how many posts are rotted. If the wall has 5 posts and two are rotting then Id be factoring in replacement efforts, 1 post, Id still remove the fill  to get rid of weight issues while working and remove that post. 7 posts and 2 are rotted... maybe try one post first and see, my concern is the amount of fill the wall is holding back so any weaking while removing will make the wall want to move.

How many posts out of how many pasts need replacing and are they next to each other in the run up the retaining wall?

Dave

[MitchellMc]

Hi @GoodMate 

I agree with @Nailbag that this is a job for professionals. The wall appears to be leaning slightly, which is an indication that the weight it's bearing is significant. The current posts can not be removed until the wall is properly braced in place and this isn't a DIY job.

I'd suggest reaching out to a professional, as your safety is the number one priority.

Mitchell

[GoodMate]

Hi,

Thank you all very much for your comprehensive advice. Yes, I understand the complexity of the issue, and the need for bracing the sleepers before attempting to weaken the footing of the post in concern. To answer the questions asked, I have (I guess) just over ten posts among which 3 or 4 (if taking an aggressive approach) can be replaced. Yes, it's really a shame that few ones are showing serious signs of decay while the retaining wall itself is in pretty good shape in general.

And yes, calling in a pro might be the best way forward. But I'm still considering the possibility of DIY. If I ask the question in different ways, I hope I can get more advice from you guys.

1. How, most likely, were the posts and retaining wall built when they were first done? Understanding this would help determine the right steps of replacing and the risks involved.

2. Specifically, in this case, how deep were the posts required, and most likely were, buried below the level of concrete?

3. How thick would the concrete slab possibly be surrounding the posts?

4.  As one option of repairing the retaining wall, is it easier to dig new holes at both sides of the affected ones and put in new timber or steel posts? Are there other options you can think of?

At one (tapered) end of the retaining wall, there is a shorter post which can also be replaced (though much less serious than the one shown). Due to the significantly lower level of risk involoved (less weight behind, and more open space for execution of the repair), I might start from that one and see how things go, then decide best course of action after that.

[Dave-1]

Afternoon @GoodMate 

You are on a path we have all been at some point. Its a hard path and really comes down to a solution for now or a long term solution. 

We all ask those questions of our projects btw. So lets go through the questions you have.

1. How, most likely, were the posts and retaining wall built when they were first done? Understanding this would help determine the right steps of replacing and the risks involved.

1A - The line of the wall would have been set using a stringline of some kind, the spacing would probarly have been worked out in reguards to the lengths of timber you could buy. The height, depends on what is above the wall you have now. 

2. Specifically, in this case, how deep were the posts required, and most likely were, buried below the level of concrete?

2A - A third of the overall height deep at least, also nothing less then 600mm. You dont want to go less and chance the same thing happening down the track after your effort to repair/replace the wall.

3. How thick would the concrete slab possibly be surrounding the posts?

A - How long is a piece of string? Taking a guess tho Id say at least 150mm. Tho it could vary all over depending on how level the ground was underneath and also if they used reo.

4.  As one option of repairing the retaining wall, is it easier to dig new holes at both sides of the affected ones and put in new timber or steel posts? Are there other options you can think of?

4 A - I would think not, you will need to cut/break up the concrete around the posts in question before even attempting to dig new holes, possibly finding out more of the post is rotted then you expect. The lateral force of the soil behind the wall I would not think of a steel post being added to it would solve the issue. (not knowing how thick and the makeup of your concrete slab as well) Thats a LOT of force being pushed against the wall and a post bolted, well it would be like holding back the Titanic i would suspect

At one (tapered) end of the retaining wall, there is a shorter post which can also be replaced (though much less serious than the one shown). Due to the significantly lower level of risk involoved (less weight behind, and more open space for execution of the repair), I might start from that one and see how things go, then decide best course of action after that.

A - A wiser move, at least you will be able to see the depth of the concrete slab at that point. If you end up replacing the retaining wall I would factor in drainage fo both sides, the rear underground and the front for surface water. Id also suggest A gabion styled wall as a replacement Take a wander through my bookmarks at the gabion walls I have used for retaining walls.

Dave

[EricL]

HI @GoodMate 

It's great that you've received excellent answers from @Dave-1. Just keep in mind that it's very likely that the posts were also concreted in aside from the slab that's holding it in place. Starting from the shorter end of your retaining wall sounds like a good idea. Plus, it will give you a general idea of how much concrete you'll need to break apart.

Please keep us updated with your progress, we look forward to seeing the start of your repair project.

If you need further assistance, please let us know.

Eric

[AlanM52]

My feeling is the only way to approach that is to remove enough of what's behind the retaining wall to create a narrow workspace to replace the timber  retaining wall. With personal safety in mind and factoring in my age (turn 78 in Nov) I would go ahead and make a start.

Off to the garden shed and grab the corded Ozito pole pruner.

Stage 1: Controlled spillover:

Starting around the middle of the wall cut a couple of posts adjacent to the bottom of the second board.

See what happens... cut out the top two boards.

Do another section... see what happens, it it's spilling over keep going to maybe provoke total collapse.

Stage 2: Move the soil.

If needed organise a working bee to move the soil onto the concrete side to create the narrow workspace.

Make sure there are enough spades and long handle shovels to go around.

Stage 3: Prepare post holes.

Use a power saw with concrete cutting blade to make three straight cuts around each post.

Do whatever is needed to remove the posts.

Stage 4: New Retaining Wall.

My preference is to use steel H-section retaining posts along with treated pine sleepers.

At this point it's good to have something like this:

Craftright Level Line Surface Set (5660294)

Clean out the post holes and start at one end.

Bang in the post to achieve the required height.

Keep checking for verticle.

Fill hole with water and pour in concrete mix.

Check again that the post is vertical.

Down tools for a while so the mix has partly set.

Enough to resist the pull of a tensioned string line.

Do the other end post use string line/level to get same height.

Take another break for that to partly set.

Continue with the rest.

Stage 5: Sleepers.

After they are installed consider a waterproof membrane before backfilling to prolong the life of the timber.

Cheers

[EricL]

Hello @AlanM52 

Let me tag @GoodMate to make them aware of your recommendations.

Eric