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Welcome @S_Mack. It's great to have you join us.
You might like to check out @steveof2620's project Retaining wall with Whites Retain It system and @Adam_W's guide How to build a retaining wall to get you started.
Could you share a picture of this space? That way community members can get an idea of what you're working with and will be better equipped to offer suggestions.
Stevie
Hi @S_Mack,
Would you need to create an entire retaining wall, or could you dig down slightly at the fence line and then attach 200 x 50mm 3.0m H4 Treated Pine Sleepers at the base of the fence's posts with bugle screws? That should stop the soil from migrating under the fence and be much less effort than the retaining wall.
Mitchell
Hi @MitchellMc
Good day. I hope you are doing well.
I just bumped in to this post while looking for a solution to a somehow similar issue with my fence. There is a small gap between the ground and the wooden sleeper of our fence (roughly 30 to 40mm). This is only on some sections of the fence on my left and right and not all (probably the way the land is sloped). I am thinking and would like to try your suggestion of putting sleepers to fill the gap. But I am not sure how to do it. I can buy and cut the sleepers in Bunnings about 30mm high and cut them to length matching the length of my existing sleepers (post to post). But should I insert this sleeper underneath the existing sleeper? If yes, what is the best way to do it as I am not sure I can bend the sleeper easily to be inserted? Personally I would prefer this way as it would be neat and gives me a few mm of space between the fence and our apron/perimeter concrete to still put some plants/pebbles. Or what you meant by attaching sleepers is to screw the new sleepers to the existing ones side by side/flat to each other?
I appreciate any suggestion.
Thank you in advance.
Welcome to the Bunnings Workshop community @1smallstep. It's wonderful to have you join us and many thanks for your question about filling the gap under your fence.
What do you intend on placing between the concrete slab and the fence? When a building is constructed the slab height is normally positioned to allow for fill around it. I presume the bottom of those treated pine slabs is below the top of the concrete slab? The bottom of the treated Pine slabs would be covered over with whatever fill you are placing in this area. I've constructed a rendering below to illustrate better. This is further indicated by the fact that the top of the concrete footers are sitting above the current ground level. I'd say your fill should come right up to at least the bottom of the treated slab. Once you and your neighbour do this, there will be no need for an additional divider.
You're correct that you'll have difficulty getting a board into that position and I suspect that the slabs there were slid down from the top of the posts prior to the construction of the fence. Your only option now would be to connect to their face and have your new slab proud of the fence line.
Please let me know if you have any questions.
Mitchell
If the sleepers are both 50mm thick, Zenith 14 - 10 x 75mm Galvanised Type 17 Batten Bugle Head Timber Screws would be suitable. You might like to countersink them 10mm into the front sleeper, so a little more thread bites into that back sleepers face.
Please keep us updated on your project and reach out if you need further assistance or have questions.
Mitchell
I was trying to find some answers then i ran into this post.
My fence is constructed on top of a retaining wall. Image attached. Can i use the sleepers as retaining wall?
i am intending to fill our backyard but worried that the sleepers are not enough. Can anyone give their thoughts on this?
Hello @archiemorehate
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about the retaining wall.
Sleepers are the default material to use when making a basic retaining wall. They are made to hold back soil from a higher point in your property. A typical sleeper is 200mm x 50mm x 2400mm, supporting posts are placed in the ground to hold the sleeper in place. I suggest having a look at this guide - How To Build Retaining Wall to give you a better idea of how they are constructed.
Placing soil in front of the retaining wall is not going to compromise its stability. However, if the soil is very deep, I suggest thinking about installing drainage to prevent waterlogging to occur near the retaining wall.
Let me call on our experienced members @Dave-1 and @Jewelleryrescue for their recommendations.
If you have any other questions we can help with, please let us know.
Eric
Good Evening @archiemorehate
Love retaining walls I must admit 
Yours look solid and as @EricL states drainage is about the only thing to consider if you are back filling. So yes I would say you can.
A few questions to start with
How high are you thinking of raising the soil?
And with what type of material?
Last question is do you know where you storm water pipes are or you have free drainage away from properties?
And another question as I look at the photo, do you know what is on the other side of that retaining wall made of sleepers?
For drainage I would fill the area with soil and level it, then would go back and dig a trench and place a socked ag pipe in it ( Vinidex 100mm x 20m Socked Slotted Draincoil ) Now the one I have chosen is 100mm, I prefer the larger socked version as it wont clag up for awhile
As long as you have the end of the pipe exiting somewhere then it will drain the area, if you are unsure of where to place it, maybe along the fence line about a foot in? Depends on your landscaping. If you notice areas that pool with water now when it rains, then that would be a good place to start the pipe.
Dave
