Advice on Land Drainage

[graemefaulkner]

I'm looking to put in some land drainage in my back garden which is heavy clay and from august to april is water logged. I've tried putting in some small soakaways but these just end up full of water and will reduce over a couple of days of no rain.

Having done a lot of reading online it seems the best solution for me is to put in perforated pipe land drains with trenches dug in a shallow v shape to cover as much of the area as possible. One thing i'm not sure of is how deep i should actually dig it and how much soil i should put back on top?

The back garden slopes slightly towards the house and continues to slope down into the front garden so I was hoping to have one pipe along the lower point of the back lawn with pipes running into it (plan attached) however after starting to dig I've come across some concrete in the way, it looks like it's part of the extension foundation as its roughly the same width and juts out as if its to stop it falling away from the rest of the house. It's only 5 inches thick and about 1 inch below the patio stones so nothing like any example of foundation I've been able to find. Can anyone confirm if this is likely what it is? I've read you shouldn't run a land drain within 1m of and not lower than the foundations which is a bit of an issue since its so near the surface. I've not lifted any patio stones so it could be its nothing to do with the extension and is just there to support the patio although that seems a bit extreme.

I was originally looking at EZflow (https://www.ndspro.com/ezflow) as i thought this would make light work of the job however i've since read that these wouldnt suit my lawn since its such heavy clay and i'd end up needing to cover the whole garden in these pipes. However it got me thinking, why can't i just buy my own non-biodegradable polystyrene peanuts and use it instead of pea gravel, it is slightly cheaper per cubic meter and delivery is a lot easier. Has anyone done or considered this before and can you think of reasons why this would be a mistake?

I've attached a diagram of what i was planning, brown is perforated pipe, yellow is underground drainage pipe. Any feedback on it would be much appreciated.

 

[Lower]

You need to get the water away or through the clay. Laying that pipe in a trench in the clay isn't going to help the water drain away. The trench will just fill up with water that can't drain away.

 

[Ian H]

I would do it as you plan but not bother with the stuff you linked. I would not have so many branches either because the water will travel on top of the clay.

If you line your trench with a membrane, then use normal perforated 110mm pipe in gravel you will find it much easier to backfill to level.

 

[graemefaulkner]

As far as i know i'm not aloud to connect to them to the foul drain and the gutter drains start about 12 inches below the ground which i think is higher than the trenches can be so i think ill need to route the water to the front of the house where there's a surface water manhole.

 

[liam86]

You could run a main drain (say 150mm perforated pipe) from top left of your garden to bottom right then connecting with your solid pipe at a uniform 600mm deep; with 80mm perforated pipes running into it at right angles across the rest of the garden area. Back-fill with pea gravel or something similar as you suggest within a geotextile lining to keep soil out, wouldn't need to be too much stone. Would encourage the water to go where you want it too (away from the house and foundations) even if it runs down the bottom of your trenches. I guess you would need some silt chamber at the connection between the perforated and solid pipes.

 

[pilsbury]

Looks like a lot of hard work. How big is your garden?

Some other thoughts:

Depending on size and access - minidigger the lot out and replace with topsoil
Or minidigger a proportion out and lay proper bedding for fake grass/astroturf
Hard landscape it, slabs/stone/gravel/raised beds

 

[Doggit]

I've never come across that stuff before, but it certainly means that you wouldn't need the gravel, and there'd be less clay to dispose of afterwards, so if it works, (and the costs balance out) then it's worth thinking about, but I do some more research first. A normal land dran with gravel round it, will certainly drain the garden quicker, as there's more room for the water to get into before.

No you can't connect into the fould drain, so routing it to the front of the garden is the only sensible option; apart from installing a soakaway but how far down does the clay go. And whereas you can't go down under the foundations, you should be alright about 750mm away from them, as it'll soak in from the patio side, so doesn't need to be right up against it. but you might need to do a test hole to see how far the foundations go down; but in clay, you can reckon they go a fair way. You won't need as many draains going into the main one as you've marked.

Have a look at look at http://www.pavingexpert.com for any additional advice, as it's a pretty good site, but the pages for land drains don't seem to come up sensibly.