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Lowering area with clay soil


Just looking for advice on a bright idea I had to lower an area 9ft by 9ft and 1 ft deep at the end of our garden which was previously under a concrete slab.

Since digging the 'hole' average rainfall has took about three days to drain away but the nonstop rain we've had lately won't budge and i've been having to bail it out.

I was planning to put a couple of drain pipes under there to route the rainwater away before topsoiling and turfing, but not sure if this is necessary or worthwhile?

The claysoil is very swampy now and like quicksand! I'm hoping it will dry out one day but no doubt it will throw it down again soon and I'll be back to square one. Any tips on speeding up the drying out process or should i just write it off as a stupid idea?


Any advice appreciated!



  • Dave MorganDave Morgan Posts: 3,123

    It sounds like you have a serious drainage problem. The clay you have is probably compacted after having the patio on top so it'll be a devil to break up, which you can't do right now for obvious reasons. You'll need to break up the compaction before even thinking of laying a land drain. They are easy enough to install, but you have to do them properly or they won't work, plus you need to !make sure the water doesn't affect anyone else. You'll have to wait till it dries out, not completely, but sufficiently so that the work can be done. It may be worth getting some advice before taking the project on especially when it comes to where the excess water will drain too. After its in you can just top with topsoil and lay grass or sow seed. You'll have to check after laying the drains that they work. You can pick up the pipes from any good builders merchant plus the aggregate and sharp sand needed to make it effective.

  • Ste9Ste9 Posts: 4

    Thanks Dave, Hadn't thought of that being the cause.... So if clay soil doesn't normally need a land drain then hopefully digging wider rather than deeper and breaking up the soil should improve things, and i might not need the extra drainage?

    I've attached a quick layout so you can see whether our greenhouse is likely to be part of the problem? Maybe that's still likely to prevent drainage even after breaking up the soil everywhere else?



  • Dave MorganDave Morgan Posts: 3,123

    I don't know how long the concrete slab has been there Ste9 and I see you have a tree close by as well. Digging wider won't relieve compaction, you have to go deeper there's no alternative. It might be useful to know how big the tree is as well. The bigger they get the more water they take up. Clay doesn't always need land drains you're right but as i said the compaction isn't letting what's there drain naturally. Position of the GH makes no difference at all. It doesn't have to be a major exercise in engineering. Once you break up the top foot, and you can do that with a Rotavator you can pour pea shingle on and rotavated it in and that will help it drain better. Do you intend to grow anything there or is it for something else?

  • Ste9Ste9 Posts: 4

    I'd guess the slab was there well over 20 years! The tree's pretty old too, 7 / 8ft tall.

    The original plan was to lower the level by a foot just as a kids play area inc trampoline, so just turf needed. I've dug up a foot already but thats the level i'd like to keep it, so how much further down would you recommend digging up and rotavating so that the final turf is still a foot below the rest of the garden? The soil had got very yellow a foot down so i'm not sure what it will be like lower down. Presumably you need to be careful about going too low?!

    Thanks for all your advice!

  • Dave MorganDave Morgan Posts: 3,123

    If you're having a kids play area then why go with grass? Kids play area's need to be hard wearing and grass will quickly suffer with all the foot traffic leaving you with bare patches if any grass at all. You'd be better off laying a membrane and putting bark on top till they're old enough to not need the play area. It would save you money and a lot of work. You'd only have to create a border around the area to help contain the bark. 

    If the slab has been in place for that long the ground will be very compacted. But a Rotavator will break it up. But for now I'd go down the bark route. When they've grown up a bit natural forces will have loosened the clay a bit and grassing will be a lot less work.

  • Ste9Ste9 Posts: 4

    Don't worry Verdun, that's just my dodgy scaling. There would be plenty of room between trampoline and greenhouse. If it ever gets there!

    Kids play area is a bit of an exaggeration really, daughter is 9 so already is going to end up much less of a play area than when we planned it last summer. No problem with using alternatives to grass although i've heard cats like bark and we already have cat poo issues!

    I was thinking the more grass in the garden the better to help overall drainage. We've replaced a huge veg patch with and lots of paving slabs with grass which has improved things so was thinking that would be the ideal way forward.

    As i've already excavated and its sodden i assume there is nothing i can do until it dries out anyway? I've just covered it with tarpaulin hopefully a bit better than last time so it doesn't fill up again. If it ever dries out i'll get rotovating, i guess if it doesn't dry out by summer i'll just have to fill it back in and hope for the best!

  • Dave MorganDave Morgan Posts: 3,123

    Just a thought a pond? She's old enough now and they're easy enough and they are an endless fascination for kids.

  • LeifUKLeifUK Posts: 573

    It may well be compacted. Also you may be seeing water from the surrounding ground draining into the hole you have created. I dug a one foot deep test hole in my garden and come rain it filled up in minutes and took ages to empty. You could always hire a rotavator to break it up, perhaps dig in some sharp sand. Presumably you intend to fill up level and turf over? 

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