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Retaining wall drainage
Alan1991
Posts: 5
Hi,
I am in the process of building a small retaining wall. I have a load of pebbles which i have removed from the borders which will be binned/given away. But can i use the pebbles for drainage behind the wall? And can it be used for setting the posts into the hole before concrete is added?
I am planning on using 8x4 x2.4m sleepers using the same for the posts. Wall will be 4.8m wide and approx 6-700mm high. Is the size timber i am using overkill? Or could i use treated boards 8x2 for example?
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Hi Alan,
I'm just in the process of building a very similar structure although mine is to tidy-up and already existing bank. I am going three sleepers high giving me a total height of around 720mm. I have come up with what I think is a novel solution in that I'm using scaffold tubes as the posts so they won't rot. nI got these from eBay and you can select the length you need. I went for 1.2mm which gives me around 45cm in the ground. I have a local In-Excess DIY type store that I purchased zinc plated chain from and will be using that, cut into lengths of approx 200mm with coach bolts to fix to the sleepers. Because I have a slight slope then I have bedded the bottom sleepers on a layer of type 1 / limestone with 20mm or so plastic plumbing pipe laid in two positions under each sleeper to allow for drainage. At the back of the sleeper I have put a few inches of the same limestone (c20mm size) to allow the water to seep down through the soil, into the limestone then exit through the pipes / limestone bed under the sleepers.
I've used standard postcrete for each scaffold tube which, as you probably know, sets in just a few minutes so you have to be quick and have your levels / verticals right. You can certainly add some of the pebbles you have in each hole.
Like you, I looked at treated boards / scaffold boards etc. These are cheaper but then you would need to put posts (tubes) in the middle as well as the ends otherwise they will bow out under the pressure and weight of the soil. In addition they wouldn't last as long as sleepers. So I decided on sleepers for those reasons plus the aesthetics. I went for treated ex-German railway softwood sleepers as they are lighter than the hardwood ones so easier to manage plus cut which I need to do as I have a bend to negotiate.
Alan, it all depends on why you are building the wall as to what you use, if you are levelling a bank and need to retain the soil from moving then the sleepers are Ok. if just building raised beds then lighter wood will do. Yes you can use drainage though excess water would seep under the wall of sleepers, it does no harm putting some drainage in and yes you can use the pebble mix to put the posts in, saying that you may need take them out some day so just enough cement to hold it. Basically it all depends on why you are building the wall.
Frank.
Drainage behind the wall is a good idea
“It's still magic even if you know how it's done.”
Thanks for your replies and ideas. The wall is replacing a botch job by previous owners who tried using paving slabs to retain soil that also keeps the boundary fence foundations covered. So i want a more aesthetic solution.
I will probably go for the sleepers as they last longer. I can get tanalised treated sleepers for around £20 (200x100x2400). Glad i can put the pebbles to use.
Put a load of your pebbles behind the wall from the base of your sleepers right up to almost the top, making them wider at the bottom and sloping in as you go up. That keeps the water away from the wood but you can still plant close to the edge to trail over.
We have some substantial retaining walls. They were brick, which had cracked due to subsidence, so we replaced them with railway sleepers, sunk vertically about one metre underground. The sleepers were concreted at the base, and a water-permeable membrane put behind the sleepers, to give some added protection to them, and then we back-filled with gravel for drainage. Several years later they show no signs of movement. We kept the original brick walls, for extra stability, which you can see just behind the gravel in photo one. Some before and after pictures.
Thank you for the update, that looks a very impressive show to me and I do like the sleepers as you have used them. I would say you did not need our advice although sometimes an extra input gives some useful hints. Well done.
Frank.