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Trench Fill or Strip Foundation?

Discussion in 'Building' started by Gravyspoon, 2 May 2020.

  1. Gravyspoon

    Gravyspoon

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    Hi all,

    I'm shortly going to embark on my rear extension project. Did my own designs and drawings and full plans approval granted by building control which I'm pretty chuffed about, but I'm by no means an expert!!

    The plan was, after digging a test hole before submitting plans and speaking with the BCO, to match the depth and type of the existing house (end of terrace built 1995 house). Trench depth is 1250mm below ground level, 250mm strip footing and then trench block and normal blocks up to ground level.

    Access is a pain as were the corner plot in a cul de sac and I've uploaded a picture for reference.

    The original plan was to have 1.7m3 of concrete delivered and my brother and dad would provide help barrowing it round the back of the house. Social distancing has put a spanner in the works for this. So I'm now considering trench filling up to below the last couple of block courses below DPC. This would mean just over 6m3 of concrete which I could get pumped.

    Costs for the original strip footing plan were £290 ready-mix, £350 in trench blocks and at least £350 (2 blokes at £175 a day) labour to lay the trench blocks plus about £60 quid sand and cement.

    Now, I need to call to get a price on Monday for the 6m3 and pump, but I can't think that the cost will be too much different.

    What's everyone's thoughts? Obviously need to check with BCO that changing the trench fill is acceptable, but does this seem like a sensible solution? No barrowing, less faff for the brick layer.

    Cheers!
     

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  3. bennymultifinish

    bennymultifinish

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    makes more sense, providing they allow.
     
  4. noseall

    noseall

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    TRENCH FILL ALL DAY AND TWICE ON SUNDAYS!

    Good enough?
     
  5. Gravyspoon

    Gravyspoon

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    A popular choice then! :) wish we have Mixamate round here, that looks like a very neat set up with the mixing and pump all done from one truck.
     
  6. noseall

    noseall

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    We are lucky to have a local family run business that provides both gauged concrete and a pump - something that the trade has been longing for, for decades.

    There are scenarios where strip foundations over rule mass fill, but they are rare.
     
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  8. Gravyspoon

    Gravyspoon

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    Hi all,

    Thanks for your comments. Just thought I'd update this. Having spoke with Building Control they've given the go ahead for trench filling too.

    Good thing about this is that they have said the trenches only need to be 450mm wide instead of 600mm for the strip footings so less to dig out by hand and less to get rid of, although less shoulder room at deeper depths. But this translates to less soil to dispose of, proportionally less concrete when comparing the two methods, less money on blocks, less bricky labour less sand and cement for mortar. Even factoring in the £400 for the pump hire, all being well, the cost to get to DPC level will now be about £200 less than planned, although I'm sure something will crop up to spend this money on!

    Thanks for the help and hope the above info is useful for someone in the same boat down the line!
     
  9. garyo

    garyo

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    Agree with all of the above, plus there is nothing more back breaking and miserable than laying blocks in a trench below ground level.
     
  10. garyo

    garyo

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    That's going to be the future if they've managed to do it without reducing the capacity of the truck too much. How many cubes can they carry?
     
  11. bsr

    bsr

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    Is it normal, that a trench fill can be narrower than a strip? Surely the limiting factor is the beating capacity of the soil below, which is the same in either case.
     
  12. John D v2.0

    John D v2.0

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    The bearing capacity of the soil can't be that limiting, we had most of the back upstairs of the house supported on a 600x600 square pad with a column built off it.
    I understood the width and flat bottom of foundations is more to do with stability, you don't want them rolling over in the ground.
    Any concerns and a structural engineer can calculate, as they did for our pad.
     
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