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Good Morning @Jas1983
85cm deep is a LOT, That takes it into the realm of something else needs to be sorted first.
So from the top of the concrete to the bottom of the fence pailinsg is 850mm?
You will need a retaining wall of some kind in this situation otherwise the fence will lean towards your neighbours withing a very short period of time.
Can you take a photo from the base of the fence down the fence line to show the distance better please?
I am heading towards a wall using these type of blocks Retaining Wall Blocks and you will also need a concrete footing for an 850mm drop.
Will wait for the photo and clarification.
Dave
Hi Dave
Fencer already put sleeper underneath fence.
Afternoon @Jas1983
Its not a matter of underneath the fence, its the height of material that may be against a fence that will push it over. if you have no soil then I am not sure what the 85cm is that you refer to?
Dave
Hi @Jas1983 , looking at your pic, the depth looks more like 200mm (perhaps185mm?). 85cm is almost a metre deep. If you wish to prevent weeds and save on the amount of pebbles needed you could either put down weedmat, then bluemetal (cheaper than pebbles), then cover that with a good layer (at least 50mm) of pebbles to make it look better. Otherwise, build up the channel with clean yellow or white sand, followed by weedmat, then a layer of pebbles . Best not to use garden soil to fill up the gap as this will allow weeds to grow.
thanks for reply. Already put sleeper . You can check in photos do I need to do anything with fence. Or just put weedmat, bluemetal and pebble. Will it stop washout underneath concrete. Ta
Do I also need put aggy pipe for water drainage. Or just put weed mat, blue stone or sand and pebbles. Thanks
Hello @Jas1983
If the soil drains very well and does not puddle at the surface, then there will be no need for agi-pipe. However, if the rainwater puddles, then it will be necessary to install agi- pipe under the pebbles. I've placed an image below to give you an idea of what it can look like once the pebbles are in place.
If you need further assistance, please let us know.
Eric

I agree with @EricL . That width of path is very unlikely to collect more water than the side channel will cope with through natural drainage. Which way does the land slope? If the runoff heads towards the far end (in your pics) opening, then water on the path and in the side channel will head that way too. Unless there is a heap of rain water entering the pathway from another source then you should be ok with it just running into the stones and soaking into the ground.
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