How to replace rotted retaining walls?

Hello ,

I’ve read through a number of the retaining wall posts, and done some research online, but still not sure how to tackle these decaying walls.
The most immediate problem is the one in the picture with the grey building as that is holding back the pool!
I really want something that birds (predators mostly), lizards, frogs and insects can use for homes and hiding places. I’d also like it to have plants growing on it too.
As you can see there is significant decay. I’m keen for a relatively cost effective method that is not too difficult as I’m a beginner at this stuff!
thanks!

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Comments

JacobZ

Hi @Morv,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Replacing a retaining wall of this size is not a simple task and it is fairly likely you will need the assistance of a professional who is experienced with this kind of work. The basic method will be as follows.

Start by relieving the pressure on the back of the wall. You'll need to dig out at least 600mm behind the existing retaining wall down to the ground level at the bottom of the retaining wall. The volume of material that will need to be moved makes this a job that is best suited to an excavator. At this stage, some temporary shoring may need to be installed to prevent the soil from slipping.

Once the excavation and shoring have been done, you will need to concrete in posts of a suitable calibre for the height of the retaining wall. Ridgi makes a variety of sleeper uprights for different-sized sleepers and different wall heights up to 900mm high. For a wall 900mm high, you would need to use the 1.8m posts and have 900mm concreted into the ground.

Once your posts are in place, you can then install the sleepers. For a natural look like you are aiming for, timber sleepers are the way to go. For longevity, I'd suggest using 75mm thick sleepers or hardwood sleepers.

When the retaining wall has been reinstated, you will need to install drainage gravel and agi pipe along the back of the wall. This drainage will help reduce the hydrostatic pressure on the retaining wall and is essential for higher retaining walls.

After the drainage is installed, the soil can be backfilled to return the landscape to its original state.

This is not a small job so I would strongly suggest you consult a landscaper before making any decisions on what you'd like to do.

Allow me to tag @Nailbag, @Dave-1 and @Jewelleryrescue for their thoughts.

Jacob

Nailbag

Hi @Morv

My advice would be to engage an experienced landscaper to at least handle building the retaining wall near the pool shed. This is the one that needs to be installed correctly and with some urgency.

In following @JacobZ outline in how to replace the other walls, another approach where possible is to build new ones in front of the old. Then take away the old top level sleepers and backfill with soil to the new ones.

With this approach you could utilise the landscaper to dig the holes for the new posts and even cement some or all in position. This is the most different part of the project as the posts need to be accurately positioned.

Then at a more affordable time you can start dropping in the new sleepers in order of importance.

Nailbag.

TheHandySqirrel

Hi @Morv,

we had a similar journey. One retraining wall at the back of our property was completely rotten away, and we chose to hire a professional to replace it. Including some planting, that job cost us around $10k.

I then added and replaced other retaining walls myself. We have clay soil here in Sydney, so digging the post holes (with a post-hole digger) was the hardest part. But that tool is fantastic, and should help you just about to a depth of 900mm. After that you might need to make the hole more like an upside-down cone so you can operate the handles.

Hardwood sleepers look good indeed, but they are also darn expensive. If your budget is smaller, consider treated pine sleepers instead.

And if you don't want to excavate the entire 600mm behind the existing wall as Jacob suggested (and as I have done for projects too), get yourself an angle drill attachment. It saved me literally hours of prior manual Allan key fastening.

Good luck! And take one step at a time.

Dave-1

Good Evening @Morv

Thats a resonable sized job, definently doable but also wise to consult with an experienced person especially that end that retains the pool area.

I have built a resonable amount of Gabion styled retaing walls in my yard. Either for a fence line or to actually retain the soil.

Here is the one that holds the most soil back Large gabion retaining wall I have ones that are taller but they are configured differently due to the amount of clay I have on my block. They are very easy to build, you can use stones, broken concrete as I have, broken bricks or even whole ones stacked in the cages. I prefer the stones or concrete for reataining walls as they are heavier.

The only foundation you need is compacted earth, in your case You could scrape the top 5cm of earth off and probarly will find the soil solid. You could also build the wall in front fo your existing wall and then backfill and remove (please remove lol as I had to pull out old retaining walls as I was building mine) I would do it in sections and backfill as I go mainly to lesson the potential of slippage. I also used a course knit material behind the wall to stop fines filtering into the actuall wall, There is no drainage behind the cages as the cages themselves drain freely.

The wall should be two thirds wide as its height ish. Rem you are "retaining" all that soil from up the hill. Staggering steps up your hill would also work.and you could also stagger across the width so its not just a solid line of wall but something you could incorporate a path into.

Have a wander through my bookmarks and you will see a range of walls 🙂

Dave

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