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  1. #1
    Senior Member Rank = Hassai dc197's Avatar
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    New build - uneven base question

    Hello team
    Long time no see.

    Due to damage to my old pond I had to accelerate my planned "one day" rebuild, and that day is here.

    I've got a question about a problem with my excavation and how to remedy it please.

    The digger man I hired didn't create a very even base and one end had a dip that's quite noticeable, especially after rain when the hole gets water in. I don't want to get him back - significant cost for him and for a new grab lorry, and he was not very pleasant to work with. I don't want to manually dig out the soil to flatten it - lots of effort and would need another grab lorry.

    I've got diagrams below in which the dip is exaggerated for clarity, but in reality it's about 7cm difference. My question is about how to remedy that dip.

    I have a few options, each with pros and cons. Grateful for your advice, with a particular focus on the resulting structural integrity.
    I plan to lay ~5cm (when compacted) of type 1 MOT base and the pour a ~30cm reinforced slab (drawing inspiration from Feline's "over-engineered" build, but going further due to my clay soil).


    Option A - fill with soil spoil. Whack it down with a whacker. Then lay a uniform MOT base, and a uniform concrete slab.
    Pro: cheap, easy (soil is still on hand)
    Con: strength? gooiness of whacking wet clay.

    Option B - don't fill, lay uniform wavy 5cm MOT, and let the concrete fill
    Pro - easiest of all
    (The additional cost of extra concrete is not of concern in the big scheme.)

    Option C - fill with MOT, then pour flat concrete
    Con - I probably don't have enough MOT and am reluctant to buy another bag only to use part of it and have the other half of the bag sat on my lawn for years while I figure out what to do with it

    Option D - fill with broken bricks and soil (both spoil), whack, then lay even MOT and even concrete.
    Pro - easy, cheap
    Con - as per A?



    What are the strength (dis)advantages of each approach please?



    Thanks
    Pond base problem.png


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    Last edited by dc197; 09-07-2024 at 10:08 AM.

  2. #2
    Senior Member Rank = Adult Champion Twhitenosugar's Avatar
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    If it's out by around 7cm, I personally wouldn't worry too much about it.

    I'd just try and put a bit more mot down in that area if you can.

    Also, even if it's still not quite level, once the MOT is in, it will level itself out when you pour your concrete slab. You just need to ensure any shuttering you put in (if you plan to use any) is level.

    Sent from my Pixel 8 using Tapatalk
    13,000L fibreglassed raised pond with window

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  4. #3
    Member Rank = Nisai SteveW1's Avatar
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    I'm on heavy clay and the base of my slab is 1.2m below the surface. I think a subbase on heavy clay is not required-if I was you I would just use a nice wet mix and it will practically self level. Its the walls on clay ground that are important-clay shrinkage can leave a wall thats been backfilled with dirt unsupported. Only backfill with washed stone. Trying to compact type 1 on soggy clay with a wacker plate might just liquify it. Ideally the clay wants to be disturbed as little as possible.
    Last edited by SteveW1; 11-07-2024 at 09:57 PM.

  5. #4
    Senior Member Rank = Supreme Champion RS2OOO's Avatar
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    Most Builders merchants sell MOT in "handy bags" of around 25kg.

    I would go for these, or a cheaper option that a number of merchants sell is crushed concrete - fill your void with this (it is usually bigger than MOT but smaller than hardcore), tamp it level with the rest of the base and then put MOT on top as per the plan.

  6. #5
    Senior Member Rank = Hassai dc197's Avatar
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    Thank you
    Some useful things to consider there.

    Quote Originally Posted by SteveW1 View Post
    Only backfill with washed stone.
    I'm planning to backfill with more poured concrete. Expensive, but I agree that it's the walls that need most support (that's where my last pond failed).
    If money were no object, would you still recommend to backfill with stone rather than poured concrete? Could you explain why please?
    Thank you.
    Last edited by dc197; 12-07-2024 at 05:01 PM.

  7. #6
    Quote Originally Posted by dc197 View Post
    Hello team
    Long time no see.

    Due to damage to my old pond I had to accelerate my planned "one day" rebuild, and that day is here.

    I've got a question about a problem with my excavation and how to remedy it please.

    The digger man I hired didn't create a very even base and one end had a dip that's quite noticeable, especially after rain when the hole gets water in. I don't want to get him back - significant cost for him and for a new grab lorry, and he was not very pleasant to work with. I don't want to manually dig out the soil to flatten it - lots of effort and would need another grab lorry.

    I've got diagrams below in which the dip is exaggerated for clarity, but in reality it's about 7cm difference. My question is about how to remedy that dip.

    I have a few options, each with pros and cons. Grateful for your advice, with a particular focus on the resulting structural integrity.
    I plan to lay ~5cm (when compacted) of type 1 MOT base and the pour a ~30cm reinforced slab (drawing inspiration from Feline's "over-engineered" build, but going further due to my clay soil).


    Option A - fill with soil spoil. Whack it down with a whacker. Then lay a uniform MOT base, and a uniform concrete slab.
    Pro: cheap, easy (soil is still on hand)
    Con: strength? gooiness of whacking wet clay.

    Option B - don't fill, lay uniform wavy 5cm MOT, and let the concrete fill
    Pro - easiest of all
    (The additional cost of extra concrete is not of concern in the big scheme.)

    Option C - fill with MOT, then pour flat concrete
    Con - I probably don't have enough MOT and am reluctant to buy another bag only to use part of it and have the other half of the bag sat on my lawn for years while I figure out what to do with it

    Option D - fill with broken bricks and soil (both spoil), whack, then lay even MOT and even concrete.
    Pro - easy, cheap
    Con - as per A?



    What are the strength (dis)advantages of each approach please?



    Thanks
    Pond base problem.png
    Hi,

    To remedy the uneven base for your pond rebuild:


    Option A: Fill with soil spoil and compact it. Lay uniform MOT base and concrete slab.
    - Pros: Cheap, easy.
    - Cons: Potential issues with structural integrity due to the compacted wet clay.


    Option B: Lay wavy 5cm MOT base and let concrete fill the dip.
    - Pros: Easiest, minimal effort.
    - Cons: May compromise the strength of the slab.


    Option C: Fill with MOT and pour flat concrete.
    - Cons: Extra MOT needed, potential waste.


    Option D: Fill with broken bricks and soil, compact, lay even MOT and concrete.
    - Pros: Cheap, easy.
    - Cons: Similar concerns as Option A with soil stability.


    Recommendation: Option C offers the best structural integrity, despite needing more MOT. If strength is the priority, it's worth the extra material.

    Thanks

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