Solid clay and lawn problem.

edited 22 July 2016 at 11:26PM in Greenfingered MoneySaving
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lindos90lindos90 Forumite
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edited 22 July 2016 at 11:26PM in Greenfingered MoneySaving
We are in the process of doing our garden. It is quite small and had lots of odd levels. ( we live on a hill and the garden is higher on one side than the other and also higher at the back)

However we have come a cross a problem. White levelling out the main area where we want the lawn, it was solid clay.

We decided to carry on digging, and did a 'test hole' to see how deep it was, and it just keeps on going, it must be part of the subsoil rather than a layer.

I have googled 'clay soil' problems but I can only find advice about clay in soil rather than solid clay.

We plan to put some kind of french drain across the area leading to a soakaway, and then an overflow into the drain, which should help after heavy rain, however the thing that is stumping us, is how much soil do we need ontop of the clay, for grass to survive?

Just not sure how far to dig down all over before replacing the soil.

Hope someone can help :) Thank you x x


  • livetoclimblivetoclimb Forumite
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    Evening. I work as a consultant engineer in this field so hopefully I can give you some pointers... What area where do you live in? Various geological formations have clays as superficial deposits, and these can often stretch down many many metres. Typically clay subsoils will stiffen until they become weak rock, which will normally be where they are directly overlying the competent bedrock.

    Normally, I would recommend a bare minimum of 150mm of topsoil for any garden. However, the impermeable nature of clay and the high absorption properties of topsoil mean that when you're looking to lose quite a bit of water you may want closer to 300mm.

    You're unlikely to be able to construct a decent soakaway in clay (nowhere for the water to soak to), so I'd recommend running your French drain into a gutter or overflow pipe and that to a drain. alternatively build a pit full of gravel as a sort of attenuation pond, although once this is full it will become fairly redundant. Perhaps run a trench at the base of the topsoil at the low point of the garden to a drain, then put some perforated piping in and bury it in pea gravel. If you cover this in a layer of permeable membrane you can put the topsoil on top of it and it should act as a decent drainage system.

    You've generally got the right idea so far, just try and keep enough topsoil over the clay to allow water to travel. Make sure you use suitable topsoil too if you're bring it in from elsewhere, although generally the stuff you can buy at b&q will be fine. The more grass you have the better it should absorb water.

    Good luck!
  • edited 23 July 2016 at 7:34AM
    lindos90lindos90 Forumite
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    edited 23 July 2016 at 7:34AM
    Thank you for your reply Shay, we are in Derbyshire.

    The soakaway is on the lower side of the garden and itself is not in the 'solid clay' area. We have had some heavy rain over the last year, and although during one period the hole did fill half way up, it did drain over an hour or so, so we are hoping that will be enough for everyday rain. Connecting to the drain is hopefully belting and bracing.

    Yes the clay we are trying to dig out does almost feel like rock, I think it is very compacted.

    The area for the lawn is a circle, about 12 meters across. So far we have pick axed down about 5", so still a way to go then if we need to go to 12", recon thats another skip.

    The top soil we are using is from the garden, we removed it, sieved it and have it stored in bags to allow us to preposition a retaining wall. it looks decent stuff with no clay unlike the layer below.

    Very much appreciate your advice.
  • EctophileEctophile Forumite
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    From my experience, grass will survive well with as little as 100mm of soil, provided the ground isn't getting waterlogged. My subsoil is really heavy clay, but at least it never floods.

    But where the soil thinner than that, the grass really struggles to compete with the weeds.
    If it sticks, force it.
    If it breaks, well it wasn't working right anyway.
  • I_have_spokenI_have_spoken
    5.1K Posts
    I had my lawn laid on 7cm of grit to give drainage, the high water-table meant the clay soil underneath became a marsh.

    The only downside is it needs to be fertilised several times a year as the nutrients wash-out.
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