Fines in mot type 1 sub base

Discussion in 'Builders' Talk' started by nick lane, Jul 27, 2020.

  1. nick lane

    nick lane New Member


    I'm in the process of using some mot type 1 as a sub base under a new patio. I've put 60mm down as first layer which I'll compact and then add to to make 100mm compacted.

    I've spread it out but before I compact first layer, I've noticed a couple of areas (each about 1 foot square) which don't seem to have much of the fines. I've got some spare sharp sand, can I add this in to these areas to balance it out to aid compaction? Total area for the patio is about 4m x 3m.

  2. I don't think you can replace the fines with sand, the fines are supposed to be of various particles sizes so that it compacts to a more solid mass.

    Can you mix up what you've laid down with the back of a rake?
  3. Bobby Dazzler

    Bobby Dazzler Active Member

    Assuming you're going to add another layer of sand anyway, to even out the base, just crack on. The sand will even out any lumps or hollows.
  4. stevie22

    stevie22 Screwfix Select

    Sand will be better than nothing
  5. nick lane

    nick lane New Member

    Thanks very much everyone. Might try some sand on one area see how it goes.

    Then if it doesn't work might have to rake it up and spread it a bit better. Was hoping to avoid that as I'd got it relatively level.
  6. Rob_bv

    Rob_bv Active Member

    Why a 60mm layer? Why not just hire a decent plate compactor and do the whole lot in one hit - you'll have better interlocking between the aggregate that way as well.
  7. This is contrary to what I have been told. Always been told to not do more than 70mm, as the top bit can lock in with voids underneath that can eventually move?

    To the OP, I had the same issues, I think were rain water had washed the fines to the bottom of the bad and I topped up with thin layer of sharp and compacted. Otherwise, the large bits bounce up off the compactor (I hand compacted, might be better with a whacker plate machine)
  8. Rob_bv

    Rob_bv Active Member

    People often overlook that the highest aggregate size allowed in type 1 is 32mm [just checked - 75-99% passing a 31.5mm seive]; the old rule of thumb is that a layer should be 3x the max aggregate size to allow for sufficient interlocking, so just shy of 100mm for type 1. If you want or need thinner layers, then you should consider a different material (for example a 20mm crusher run), otherwise you risk soft spots of little to no interlock.

    As for layer depth when compacting, that is dependent on what methods or plant your are using along with the necessary degree of compaction - most modern small vibrating plates will need 6 to 8 passes to compact a 100mm layer of material, whereas (for example) I am currently using a 15t vibrating roller on site to compact 10,000t of imported fill in 300mm layers.
  9. stevie22

    stevie22 Screwfix Select

    Crush often behaves better than "proper" type 1. We did a road some years ago and a roundabout done in crush stood up to heavy rain and traffic for 3 weeks while the limestone areas adjacent moved all over the place.

    Whatever the material it will compact better if it's a bit damp.
  10. Thanks for the explanation, that's a good rule of thumb to know :)
  11. unphased

    unphased Screwfix Select

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