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Deeper retaining wall in front of existing one.

Discussion in 'Building' started by eb28, 15 May 2021.

  1. eb28

    eb28

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    Hi all,
    Hoping someone can give some advice please.
    I built a retaining wall a few years ago. Its 6m long and 600mm high. Made of 9" hollow blocks filled with concrete and rebar through to foundations. Usual drainage behind it and it's holding up well.
    Thing is, the mrs now wants to make the lower ground level, even lower. Don't ask why. I'm not even sure I know why, but I've agreed to do the work.

    So, I'm thinking of excavating in front of the wall and building a new, taller/deeper wall in front of it. Very crude picture attached. I will tie the walls together and fill the cavity with concrete. The front wall will be made of 6" blocks.
    Any obvious reasons why this is not a good idea, structurally? I was particularly thinking of the drainage (where I have marked the red star on the pic). Will water find it's way here and cause any long term issues.
    Thanks in advance

    retaining wall.png
     

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  3. greyam100

    greyam100

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    if the existing wall is actually retaining the material behind it, you are risking undermining it's foundation by excavating right in front of it. The point where your red star is, is right at the point of maximum bearing pressure at underside of foundation due to the overturning effect. Hence as you excavate, the supporting strata may start crumbling/collapsing if the wall actually has horizontal pressure acting on it from behind. As for the draiange, I guess you are just creating 1 wall but thicker, and keeping the existing drain to carry water away. Whether water finds its way between the two walls will depend how well you compact the infill concrete in the gap.

    And if you affect the integrity of the existing foundation, the new wall will need to be strong enough to retain the weight of the old wall which will then be unstable.

    Depending on it's height, you may need it engineered, so that it doesnt fall over and land on, say, a small child at some point. Or at least read up on some rules of thumb on foundation width to wall height ratios
     
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  4. eb28

    eb28

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    Many thanks for the reply. Yes, one concern I had was digging so close the existing foundation.
    I’m not really sure there is much of a horizontal pressure on it. When I initially excavated the area, it sat quite happily for a couple of weeks with no wall there at all. I suppose it is retaining the gravel for the drainage though.
     
  5. greyam100

    greyam100

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    yes. however earth has a habit of moving due to groundwater, freezing/thawing, creatures etc. At the end of the day you want to reduce the chance of creating more work for yourself so either just go very carefully in front of the found ar stay as far away from it as you can. Perhaps digging out the gravel will help minimise the weight acting on it. You could always brace the existing wall if you had anythng to brace it against, even a couple of posts sunk into the ground - just need to figure out how to remove said braces during/after building new wall
     
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  6. eb28

    eb28

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    I’ll tread carefully. There are a couple of places where I could put some braces.
    I could maybe do it in sections to reduce the chances of compromising the old foundations?
    then include some horizontal rebar in the new foundations to hold the sections together?
     
  7. IT Minion

    IT Minion

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    You couldn't cheat by having a raised flower bed in front of the wall? The Flowerbed being the same height, or higher than the current floor level, wide enough that the new Flowerbed footings won't disturb the retaining wall.
     
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  9. cdbe

    cdbe

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    Have you considered "underpinning" the existing wall in small sections? If you're not going much lower, dig out under the existing concrete and pour more under.
     
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  10. eb28

    eb28

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    Certainly an option, thanks for the idea
     
  11. ^woody^

    ^woody^

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    As long as the new foundations are lower than the previous, and suitably deep, you'll be OK.
     
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  12. eb28

    eb28

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    Im going about 450mm lower.
    Existing foundation is about 200mm deep, so I could underpin the extra 250mm (dirt next to the red star), in sections, maybe in the same pour as the cavity fill.
     
  13. itsnotplumb

    itsnotplumb

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    Be easier to get a new Mrs.

    I saw a video on a situation similar to this on utube a while ago by (i think) Stanley dirt monkey.
     
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