Mot sub base depth required

Discussion in 'Builders' Talk' started by Busybusybusy, Aug 6, 2021.

  1. Busybusybusy

    Busybusybusy New Member

    We’re building a new driveway. Need to excavate about half a meter of topsoil just to get to ground level, then sufficient depth for sub base and gravel etc. We are coastal and below the topsoil is pure sand. Driveway will be approx 6x30m so huge volumes involved. My question is how deep the mot type 1 sub base needs to be and whether it is sufficient on its own? Our groundwork guys reckon on 400mm mot type 1, then 100mm of a finer sub base, then 50mm gravel. Is this necessary due to pure sand base or overkill? Thanks in advance.
  2. jonathanc

    jonathanc Guest

    I guess he is experienced in that area so it sounds sensible to me
  3. kiaora

    kiaora Guest

    6x30m is a fair sized project, so it’s good to plan it, so it performs as required,

    it may seem over kill, but the cost of a Structual engineer, may pay you dividends, ?

    it’s not the sort of project that you want to fail after a year or so ?
    Do it once, etc….

    good luck
  4. stevie22

    stevie22 Screwfix Select

    Sand is a perfectly good fill material especially if you have a situation like yours where its filling a hole and therefore contained so you could use sand as the first layer but I have grave doubts as to whether you actually need to dig that depth out at all. There isn't likely to be much vegetable matter in the lower levels and that's the reason for removing topsoil because it decomposes and settles.

    For a drive which is having a gravel surface what's the worst that can happen? An odd hollow that you can top up/rake over is the answer. (unless you are intending to park HGVs or heavy plant on it)

    I would probably dig about 200 then wacker the ground. Dig out any soft spots and fill back to formation level wacking it well (in layers if necessary) . Lay a good filter cloth (Terram) then 100mm crushed concrete wacker well. I would then put in an edging of some description treated timber, concrete edgings, brick pavers: all sorts of options. Then another 100 of crush and wacker again.

    Depth of gravel depends on the size you intend to use: you don't want it too deep as it will be hard work to walk on.
    chillimonster likes this.
  5. Severntrent

    Severntrent Screwfix Select

    Agree with Steve more or less. 200 MoT and then gravel. Steve has a thing about crushed concrete but they always use MoT on roads and don't mess about with crushed gravel for a reason. No terram so gravel can actually bite into MoT and not go skidding off everywhere, similarly not to deep or you'll just end up make loads of grooves as you drive your cars over it.
    Why 200 MoT ? As stated sand is basically a good foundation material and using Type V soil from BR foundation design a as a guide gives a safe bearing capacity of 5 tonnes/m2 (which is about 3 times as much as what's required before settlement would occur). Assuming you are parking vehicles weighing around 2 tonnes with the more heavily loaded ones (front?) taking 0.75 tonnes and the area of tyre acting on the ground being .15 x 0.15m (0.0225m2 which equates to 33 tonnes/m2). Assuming this loading dissipates through the MoT at a 30 degree angle to the vertical, the loading at the sand/MoT interface would be decreased 10 fold hence 3.3 tonnes which is less than the 5 tonnes safe bearing capacity.
    Getting away from all the theoretical stuff when contractors rented a bit of farmland for storage purposes it was just a case of stripping off 300 topsoil and replaced with MoT, this would be taking delivery vehicles, vans, cars, JCBs etc.

    Your contractor must be used to building residential roads if he wants 500 thickness of sub base or your water table is high up
    chillimonster likes this.
  6. stevie22

    stevie22 Screwfix Select

    Stevie has a thing about crushed concrete for the very good reason that he has seen it outperform "proper" type 1 under a road under adverse conditions and at a far better price and a local source.

    Type 1 is essentially nothing more than a grading specification. Traditionally it was and is crushed limestone but you will also find granite and a variety of reclaimed materials. In particular you can buy crushed concrete to type 1 spec. For this sort of work straightforward crush is the way to go.

    Terram goes on the formation (ie under the type 1 not over it): as ST points out you want the gravel to bit in a bit.
  7. Busybusybusy

    Busybusybusy New Member

    Thank you for brilliantly explained and justified answers. Everything I had read suggested we were being quoted for twice as much as we needed so it’s nice to see the science behind it (I’m a biologist and hated engineering and mechanics!!) I will ask him for his justification as we are on a tight budget and having half as much topsoil to lose and half as much mot to purchase would help a lot.

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