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Can I fill a wet trench with concrete

Discussion in 'Building' started by ukdodger, 10 Jan 2014.

  1. ukdodger

    ukdodger

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    I dug a trench for a single brick wall before the rains came. The soil is heavy clay on top of chalk. It holds water quite well! I dont want to wait until the summer for it to dry out. Can I pour the concrete in while there is still water in it. At the moment there's about 1/4 inch of water at the bottom. The shuttering is in place. Thanks
     
  2. Jackrae

    Jackrae

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    why not dig a little sump in the trench, then as water drains into it, bail it out
     
  3. HERTS P&D

    HERTS P&D

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  4. ukdodger

    ukdodger

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    Thanks but draining it isnt the problem. The trench is small enough (about five feet) using a old saucepan will do it. But the soil is still waterlogged. When it drys out will it shrink and lower the foundation?
     
  5. Jackrae

    Jackrae

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    If it's heavy clay, as you state then the ground is "waterproof"
     
  6. munchingB

    munchingB

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    dont know what dimensions youre working with but digging sand in with the clay will improve drainage and change the shrinking characteristics of clay sub soil.
     
  7. ukdodger

    ukdodger

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    Thanks. That's a good idea.
     
  8. Nigel_Cro

    Nigel_Cro

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    Ok, so I might be way wrong here, but this rings alarm bells.

    If you dig sand in to the foot of the trench you are then effectively laying the foundations on made ground. Will you then need to properly compact it before pouring?
     
  9. ukdodger

    ukdodger

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    I hope not. The shuttering is in place now.
     
  10. BigIan

    BigIan

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    Yes digging sand is is not the best Idea, there cohld well be voids within the clay that may cause the footing to crack.
    The Idea of digging down to a untouched clay strata is that the firm clay can bear the weight of a building and as it is untouched these weight bearing characteristics should be uniform.

    If you mix sand with it you are intoducing air (voids) which will compress and you may find cracks forming.
    Mixing the sand with the clay wont improve the drainage properties unless you do it all theway through the clay to whatever strata is below what you would do is form a weaker disturbed clay layer that will retain water right where your footings are going to sit. Not a smart move.

    If you have already done this i suggest you dig it out and get back to a firm strata and start again.

    Back to your original post You want to remove as much free (pooled) water as you can frm the trench as too much water in the mix creates weak concrete.
    However tge surrounding soild being wet is good as tge water in tge concrete is needed to cure it properly (the stregthening of cement is due to the presence of water not drying as some believe)

    I hope that helps.
     
  11. ukdodger

    ukdodger

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    Thanks Ian. So are you saying I can in fact pour the concrete into a wet trench?
     
  12. BigIan

    BigIan

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    If the surrounding soil is moist there should be no issue just get rid of any pooling water in the trench.
     
  13. Nigel_Cro

    Nigel_Cro

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    Back in my Uni days, we used to cure our test blocks in water. I can't remember at what age we used to put them in the tank, but once in, they stayed there until they were 28 days old.

    As previously stated, water, or at least a damp environment is good for concrete curing, hence the reason wet sacking is put over fresh poured concrete in hot/dry conditions. If it dries too quickly, the heat generated will cause internal pressures to build up and create significant weaknesses to develop.
     
  14. ukdodger

    ukdodger

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    Thanks for all the replies. Very helpful indeed.
     
  15. JohnD

    JohnD

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    when the bottom of the trench is wet and soft you are supposed to dig it out to dry and firm.

    A spec I used to see for utility cabins said that if the bottom was damp after pumping out they would lay dry lean mix first and compress it well; it absorbs the moisture and gives a more firm surface that you can then pour you proper mix onto.
     
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