MOT and Clay Soil????

Discussion in 'Builders' Talk' started by SparkyAnt, May 6, 2021.

  1. SparkyAnt

    SparkyAnt Member

    Hi there,
    After some great advice earlier this week (thanks again rogerk101 & stevie22) on suitable foundations for my workshop build Ive got another question... it might be a stupid one but it’s been playing on my mind...

    My soil contains a lot of clay, so by using MOT in the base of the concrete pads I’m about to pour for the workshop foundations wouldn’t I just be making the drainage worse? ...water sitting under the concrete pad

    I know with clay I’m at risk of expansion and contraction with the seasons/weather anyway so wouldn’t want to make it worse.

    Compacted MOT is strong and a good base but it doesn’t let much water at all drain through ( so I’ve read)

    Not sure if I’m overthinking things here ;(
    Is MOT “the” best base for poured concrete in clay soil or would another material be better?

    The way I planned to do it is just 100mm tamped MOT then concrete up to a level top.
    (All holes are 400-500mm deep) pads are 600x400 x18 of them over a 4.8x3.6m base

    cheer guys
  2. stevie22

    stevie22 Screwfix Select

    The only stupid question is the one you don't ask.

    Water is everywhere in the ground below the water table filling up all the little gaps between the soil particles. It's the water that largely controls soil properties: walk down the beach dry sand (loose) near the water damp sand (hard) in the water (looser). All down to water.

    So if you put in type 1 and beat it down you'll be fine. Myself I wouldn't bother in a pad situation: it means another material and good compaction so I'd just use concrete. It doesn't need to be very strong: 8 or even 9 to 1 would do. Keep it nice and stiff
  3. Rob_bv

    Rob_bv Active Member

    It'll be fine - most foundations throughout the UK have a similar build-up. Just as an aside, 100mm isn't enough for MOT Type 1 due to the maximum grain size; either go for 150mm or go with a 20mm crusher run (which may also be cheaper). MOT Type 1 is a vastly overrated medium!
    SparkyAnt likes this.
  4. SparkyAnt

    SparkyAnt Member

    Cheers mate, managed to get it done today with the break in the weather! All covered up now in prep for tomorrow’s deluge!lol
  5. rogerk101

    rogerk101 Screwfix Select

    Many people use a blinding layer when working on clay. It's just a non-structural relatively thin layer of dryish concrete mix, the only purpose of which is to stop the clay mixing with the structural concrete when that's poured. Some literally just put the concrete blinding layer on dry and let it soak in the water from the surrounding clay to set, while others make a stiff dry mix, which then needs a bit of gentle work to stop the clay working its way into the blinding. I've used both, but prefer the first because it's the one that least disturbs the clay. Let the blinding set for a day if you can, but try to do the pouring of the structural concrete done as soon as possible after blinding so that the top of the blinding is still uncontaminated by crumbling sides or leaching clay.

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