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1m high retaining wall on very steep slope.

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  1. #1
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    Default 1m high retaining wall on very steep slope.

    Hi guys,

    I am building a 1 meter high retaining wall using H channel and 75mm pine sleepers in front of my house. I live in the hills on a steep block of around 30 degree slope. My plan is to buy 2.1 H channel and dig 1.2m deep holes, 400mm diameter. I'll determine the concrete pre mix slump so we can just push the channel into the concrete until the top reaches the 1m high string line. I would like to ask the experienced wall builders a few question if I may.

    1. What would the post spacing need to be? Is 1.5m OK?
    2. How do I specify the concrete to be delivered, as I want to fill up the holes then push in the H channel.
    3. Is 75mm thick sleepers going to be OK or would 100mm be better?
    4. I plan to use landscape fabric all around the backfill scoria and use an un socked drainage pipe. The pipe and scoria would be encapsulated in the fabric. Is this method OK?
    5. Anyone had experience pushing in the posts, how hard is it to do?

    Cheers in advance

  2. #2
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    Default 1m high retaining wall on very steep slope.

    Closer post spacing = better

    Swap pine sleepers for concrete ones if you can. Pine rots. You donít want to be doing this again if something other than a garden bed is going on top.

    For a steep slope 2 x retention height would be the starting embedment (of the piles/ posts) for a post and panel wall like this, so your starting figures are close but if you want it to stay a long time look at putting in some tie backs of some form at each post. Or drill the post holes deeper.

    Simple threaded rods buried horizontally and bolted to a buried plate works for a tie back (called dead man sometimes). Put the plate 2m back from the wall or more (on the high side of course)

    If there is a structure below or above get an engineer to help. $ for a design is soooo cheap compared to fixing broken buildings.

  3. #3
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    If you think you can push steel post one meter down into fresh concrete, think again. May be with the help of a large excavator.
    If you think you can fill a series of holes and push the post in once the concrete truck has left, you are dreaming.

    Concrete does not set immediately, yet the aggregate settles and pushing steel through it is not possible. If you think that you don't only need to push it down, you need it to line up, the strategy you propose is not feasible.
    Place the steel in the holes, fill with water and pour dry concrete mix in the water. adjust for alignment and vertical, and go to the next post.
    Science is never settled,
    it advances one funeral at the time.
    Max Planck

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    With that size steel you can probably fill 1/3 of the hole with concrete, sink and position the steel accurately and then fill the rest of the hole completing the pour before the first pour starts to go off, keeping an eye on the steel that you aren't moving it too much, but slight adjustments will be possible if it starts off well placed in the first third

    I've sunk 2.4m long 100x100x4 SHS into 600mm of concrete, but the second one was a bit 'touch and go' as to whether I could make it sink the last 20mm

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    The depth and diameter of the holes is dependent on the foundation material & the steepness of the slope, the posts will sink easily into 80mm slump concrete , you will probably have more of a problem accurately locating the post where it is supposed to be the base, a better way is to stand the post first then brace it.
    inter

  6. #6
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    Thanks for the replies. Ok so seems like no one likes the idea of pushing in the posts. It was mostly to save time bracing, also it's much easier to quickly fill an empty hole while the truck is waiting, rather than work around posts and bracings. There is a guy on Youtube who pushes his H channels into the concrete. He then places a spacer to get the right distance for the next post. Seems to be a very quick process. To be honest though it would be WAY less stress to have every post in the perfect position before pouring. Do you guys use solid plastic or landscaping fabric behind the sleepers?

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    How many posts?

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    There will be around 10 posts.

  9. #9
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    The last biggish DIY wall I'd done was ~20 posts, 40m long, posts 2m apart to fit the concrete sleepers ranging in height 400mm to 1200mm down a slope in country where the soil turns to slop in the winter and rock in the summer.
    The wall was definitely over engineered, but a small increase in DIY cost was worth it (for me) compared to paying 4 times the total cost for a crew to construct.
    The holes were 400/450mm round and 1.5 deeper than the wall was high and posts just double the height length...so that meant all posts were suspended in the hole.

    - hard to know what spacing is right for your situation as it all comes down to the engineering specific to your site.
    - 75mm should be fine but I'd definitely use builders plastic behind to keep any moisture off them regardless if they are concrete or wood
    - definitely use geotextile to wrap the backfill. Backfill 30% min with the scoria. Drainage is a preference thing, but for walls with no step down, my preference has always been 90mm slotted PVC pipe and I usually have a couple of inspection/access points either at the end and/or a T to a riser capped in case of any blockage issues.....unlikely but easier new than some point down the track.
    - for 10 posts, unless I was doing 1 at a time, I'd not be game to fill in 10 holes and push the posts in as you go on top of ensuring the posts are to the correct height and not constantly sinking, plumb, braced if required, correct distance apart etc etc.....unless there were 3 or 4 experienced hands on the day.

    The way I approached my job as I was on my own (no help on the day of the pour), was to suspend and fix them in place ready for the pour. Most never moved except 2 of the 1200mm (2400mm in total) posts and required a crowbar to move them back into position asap. all my holes had mini concrete truck access and were filled via the chute. ...Just need to be careful when filling that you arnt too aggressive as there is a lot of force with the concrete pouring into the holes and risks moving the posts.

    As per pics, I'd generally placed 2 2x4's each side of the hole as securing plates, laminated together with 75mm nails and used 10" nails driven into the hard ground to hold into place (3rd pic)
    I'd measured how far the post had to sit in the hole and clamped two 2x4's to the post...bought a box of clamps for the job. Rotary laser level was very handy to get everything spot on with each drop in levels.
    Once in place, used the 75mm framing nailer to nail them onto the the plates.
    then clamped 2 braces at the top back to 400mm star pickets.
    I'd left the string line in place to easily gauge if the posts had moved during the pour from the horizontal line and kept the wooden rod (2000mm + 20mm) to ensure my distances apart remained consistent to avoid cutting any concrete sleepers due to errors.

    ...its was rock solid and made the pour very quick and final checks easy
    Prep for this method is many, many times longer than the pour on the day which took 30min each ( 2 lots of 10 posts) for 2 mini concrete trucks spaced a couple of hours apart but made the day of the pour mostly stress and trouble free.

    If your planning to use treated pine sleepers, then it is slightly easier. Rather than being exactly 1500mm + 20mm (if that's the sleeper length your planning to use), You can always cut the sleepers to fit if there are errors in the distance between each post or movement during the pour. The only advice I'd offer on sleeper length is make them a standard length that fits concrete sleepers as it will potentially make someone's life much easier at some "distant" point in the future when they are replaced.

    I'd installed the posts in in March 2017 and didn't order/install the sleepers until September 2017. The winter rains had washed the upper section away in a couple spots and was thankful I'd over bored those holes/footings for zero impact.

    ..hopefully the pics give you some idea....but there are many ways to perform the same job particularly if you've got a few helping hands

    Attached Thumbnails Attached Thumbnails retaining1.jpg   retaining2.jpg   retaining3.jpg   retaining4.jpg   retaining5.jpg  

    retaining6.jpg   retaining7.jpg  

  10. #10
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    Bart thankyou for that comprehensive reply, although now I don't feel like doing it myself, lol. That is so much prep work, so many clamps, so much timber, so much aching back, so much time. You are a true pro at building walls mate, can you come and do mine I think i'll have to archive your post and use it as a reference on how to properly construct retaining walls. Perhaps I mis calculated the effort required, especially considering the steep slope, and having to barrow in the concrete. Everything sounds easy in my head but the truth is retaining walls are hard to build properly. You need a medal for that wall.

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