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Foundations for extension - spec query...

Discussion in 'Building' started by spikeuk36, 10 Sep 2021.

  1. spikeuk36

    spikeuk36

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    Hi, I'm digging my own foundations for two story extension, I have quick 'blonde moment' question hoping someone might be able to clarify.

    Looking at our plans, foundations have to be 600mm wide, 850mm deep. ..and ''should be a minimum of 1000mm below ground level.'' (refer to yellow text in photo below)

    I'm digging a 1m deep trench 60cm wide, however the statement in yellow can be read to say the foundations themselves need to start a meter below ground level? - and then 850mm deep? is this correct? - or typo?/or sentence is repeating the depth in mm but not changed to 850mm? ... not sure how to interpret this passage...

    Full text:
    "Foundations: Load bearing walls are to bear onto mass concrete strip foundations 600mm wide and 850mm deep. or to local Authority approval. Foundations should be kept 100mm minimum away from boundaries and should be a minimum of 1000mm below ground level."



    1.PNG
     
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  3. RandomGrinch

    RandomGrinch

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    Theoretically it's a scale drawing!
    I measure that at around 1m below ground level! ;)
     
  4. cdbe

    cdbe

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    Dig down a metre, fill with 850mm concrete to finish 150mm below ground level. Obviously depending on what you find when you dig and whether the ground is sloping.
     
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  5. Notch7

    Notch7

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    The wording is wrong.

    A strip foundation is not the same as mass trench fill.


    In general 1000mm depth from ground level is just a guide - it’s sort of about the depth needed in decent soil.


    The process of trench foundations is: dig down to around a metre, if you reach good, load bearing ground at that level, get it approved by the building inspector. If you have trees nearby you may need to go deeper.

    Ignore the 850mm depth of concrete…the depth of concrete is from the bottom of your trench upto say 1 concrete block and 1 brick below ground level - typically to avoid a split course line up the top of your foundation with a brick course on the house (assuming it’s metric bricks).
     
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  6. spikeuk36

    spikeuk36

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    Perfect just as i'd hoped, thanks everyone wasn't looking forward to digging another meter deeper! :)
     
  7. johnny2007

    johnny2007

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    Or...
    You could simply build on a strip of 6 inches concrete as some "experienced" member here does all the time...
    Just kidding.
    Follow the plans and your extension will still be standing next century.
     
  8. SpecialK

    SpecialK

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    That is how my 1964 house is built. Its on clay and 1.2m deep! Poor brickies must have retired with bad backs... Its a perfectly acceptable method but no longer used as its a labour vs materials cost.

     
  9. mikeey84

    mikeey84

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    Don't get him started. Johnny doesn't believe Approved Document A
     
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  11. johnny2007

    johnny2007

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    It all depends on what soil you have.
    That's why there's not one size fits all.
    I have seen houses abroad without fundations because they are on solid rocks.
    That's why local authorities specify what kind of fundations they want.
    In my area you find solid ground at just over a foot and many houses are built with very shallow fundations, but in the same area, go near the streams and you only find soft soil until you reach 2 metres.
    That's why it's paramount to follow the approved plans.
     
  12. Londoner2

    Londoner2

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    I would do:

    1) 1.5m into ground.
    2) 1m wide

    Nothing will sink after this.
     
  13. TMPUK

    TMPUK

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    • What country are you really in?
    If you want to avoid lot of works and back pain as well than having a safe and secure foundation, have you thought about helical piles ?
     
  14. jeds

    jeds

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    Just for some clarity; if you find clay, you should dig minimum 900 deep. The regs were changed after the long hot summer of 1976 when many houses cracked because the clay dried and shrank. It was discovered that clay remains permanently saturated below 900mm.

    The actual minimum thickness of concrete is 150mm, but it is way more common to go 250/300mm. It depends on the condition of the clay but 600 wide and 250/300 thick will cover the vast majority of cases. As mentioned above, it is also common practice to just fill the trench up to about 250ish below ground level. The reason is nothing to do with structure, just that and extra bit of concrete is usually cheaper than getting down in a trench and laying blocks, especially if it ends up more than a metre deep.
     
  15. TMPUK

    TMPUK

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    Don't want to be too strict...if you look at the NHBC standards 2020, chapters 4.2 and 4.3, you will find some usual an safe recommendations about foundations in clay and other type of ground. https://nhbc-standards.co.uk/2020/
     
  16. mikeey84

    mikeey84

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    Safer too I imagine!
     
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