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AG behind my retaining wall - some qns

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  1. #1
    Senior Member
    Join Date
    Sep 2007
    Melbourne, Australia

    Default AG behind my retaining wall - some qns

    Hey all
    Finished the wall a month ago, have not put ag yet or backfilled it, am about to.
    My wall is about 30meters in length. I am unsure how I am going to ensure it stays on a decline enough for the water to drain out at one end. I am more than certain it will flatten out at some point.
    Anyone able to offer some advice? If i slope it up gradually all the way it will be half way up the wall at one end....


  2. #2
    4K Club Member
    Join Date
    Nov 2008


    You would need socked 90mm agg line ( slotted flexible pipe ) & 20mm aggregate stone, pipe laid at base of wall cover in aggregate by 300mm, no need for fall unless there is a natural sump say in the middle then you would have to have a drain at that point exiting the wall, a clean out point midway would be a good idea so a hose can be fed down it case of silting up of the line, ends clear to drain. Also if its a masonry wall you would need a thin layer of drainage material extending from the bottom to top of the wall to relieve any hydraulic pressure build up
    regards inter

  3. #3
    Old Chippy 6K
    Join Date
    Mar 2007


    If the wall varies a bit the old string line is the easiest way to ensure you get a slope all the way. Just run it full length between some pegs or some bricks or blocks use measuring or a line level to get sufficient slope (minimum 1:100 ie: 10mm per 1m) remembering that within limits the greater the slope the more water will be moved (gravity). I have used around 3:100 (30mm per 1m) mostly, but a little less or a little more isn't going to matter (professional drainers & plumbers - feel free to correct my experiential advice). Once you get the gravel bed slop right you can lay the socked aggy pipe in and away you go.

  4. #4
    Landscaper Planned LScape's Avatar
    Join Date
    May 2007
    Bundoora, Melbourne


    I find 90mm slotted pvc works the best, as it is stronger and straighter than flexible black pipe (we call it donkey dick). If it's a sleeper wall and it's all dead level you can use the sleepers as a guide as to fall you give it. Otherwise use a level, put some aggregate underneath 1st to lay the pipe on (with fall at one end) then backfill it.

    As long as it has fall it will always drain that way and keeps the weight of waterlogged soil and stone off the back of the wall.
    Planned Landscape Constructions

  5. #5
    Senior Member Eastie's Avatar
    Join Date
    Dec 2001
    Between a rock & a hard place (vic)


    1:500 will move water (same as a house gutter), so litterally only a couple of inches over 30m will drain ok but I'd be inclined to use 100mm slotted pipe. It's no good raising the pipe at the head end with agregate - too much free drain below the pipe and water will pool under the pipe, not in the pipe. As suggested run a string line to check the levels - a few mm here and there won't typically be a problem (do some searhing on why the water enters ag pipe to understand why it freely enters the pipe but is less inclined to seep out). If deviation is more like inch's then I'd suggest leveling it with a trecnhing shovel.

    An idea is starting at the head end with the ground dead level with the bottom of the wall, then digging the pipe trench progressively ever so slowly lower, it doesn't need tobe much, you aren't burying the whole pipe below the finished ground level!. So long as the drain point is low enough this gives you more fall to play with and also gives you a good capacity "gutter" that keeps the bottom of the wall dry. As stated above cover the earth below the pipe and the pipe itself (to a few inches above the pipe) in non-compacting crushed rock such as washed quarter minus, then cover the rest in scoria. If putting soil on top lay some geo fabric on top of the scoria to prevent the soil washing away. Quater minus is free-draining and with help filter sediment and scoria is extremely light weight and one of the most free draining backfills you can use. So long as the geo-sock on the pipe (if you use one) remains unclogged, you should never have a problem with water affecting the wall.

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