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How to stop erosion under retaining wall?

Getting Established

How to stop erosion under retaining wall?

Hi community,

I have a two tier concrete sleeper retaining wall. It’s built up on my side however, on the verge side you can see slight erosion has started to occur underneath the sleeper. The verge side has been built up to the bottom sleeper (increasing gradient).

I have tried filling the gaps with soil and grass seed but it has never taken. I’ve run out of ideas on how to fix this before it gets any worse. Any suggestions are appreciated! 



Bunnings Team Member
Bunnings Team Member

Re: How to stop erosion under retaining wall?

Hi @callum2510


Thank you for your questions and welcome to the Bunnings Workshop community, it is fantastic to have you with us.


This is a bit of a tricky one because it really should be a 3-tiered retaining wall with an extra sleeper below the two that are there. 


My suggestion is to backfill the void with drainage gravel and then install a timber plinth such as this 150 x 25mm 4.8m Plinth H4 Treated Pine Sawn Wet along the front of the retaining wall to hold it in place. This plinth can be set in place with 400mm Plinth Pegs.


The plinth board will hold the gravel in place. The drainage gravel will allow the water to flow through, while stopping the soil from eroding further.


Allow me to tag some of our helpful members to see if they have any ideas, @Dave-1, @Jewelleryrescue, @TedBear.


Let me know what you think and if you have any further questions.




Re: How to stop erosion under retaining wall?

Hi @callum2510 


I think you want to keep the clean lines of your fence. 

Other wise a Jacobs  board even  will work for a bunch of years over the bottom fence gap.


I think your problem  here is surface flowing water flow re erosion  cheap solution I might try cutting cheap pool noodles in half and  jamming it under the timbers to stop water flow and  therefore dirt erosion. Geo cloth would be the best material packed under. After the  gap is  filled  you will be  able to replace the missing dirt for grass to  take right up to the fence.  / finer gravel or  Coarse crushed  granite 10mm pressed into and under the timber could work too the idea is to slow the free flowing water. The gravel will gradually trap  erroded dirt and form a better  block over  a month or two.  As your grass grows  thicker at the edge as its getting good water it should help stop erosion too once the soil held.


I have  used concrete joint foam 100mm wide behind the timber plinth on the up hill side to good effect but I had access to both sides of the fence. I also used  gravel  to good effect.


@JacobZ  is right  re  letting water flow the pool noodles may tempary back  some storm water up but water will always find a way and sink in or flow  under in a  more controlled way.  Typically retaining walls have ag  drains under them to avoid water  issues but only a  2 timber plinth retaining wall is not really needed.


I think pouring crushed  granite, or small gravel along the fence and packing it in under the timber plinth will last the longest the life of the  fence. Adding some soil  later as per Jacobs original suggestion. 

Home Improvement Guru

Re: How to stop erosion under retaining wall?

Evening @callum2510 

@JacobZ has taken the words out of my mouth. My suggestion was exactly the same (well I was just going to suggest slimber sleepers lol) But as to the gravel and the reasoning that pretty much matches up.


Have you had any subsidance on the high side yet? Once you have put in place a fix you may need to run a fine coat of soil along the fence line on the high side to keep the grass there flat. (just in case you get some dips in the soil)



Getting Established

Re: How to stop erosion under retaining wall?

@Dave-1 @Jewelleryrescue @JacobZ 

Thank you all for the prompt responses and advice! Adding a minor plinth sounds like the best option while packing in the voids with some finer rock. I'll give this a crack over the next few weeks and report back. Cheers!

Getting Established

Re: How to stop erosion under retaining wall?

No subsidence that I can see yet. It's a good point you bring up and something I'll action in the fix. Thanks for the recommendation 

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