garden drainage

[John S]

I recently moved into a new house that has a sloping back garden and heavy clay soil - so the drainage at the bottom of the garden is pratically non existant. To solve this, I have lifted the turf and dug a series of 12 trenches connected to a central larger trench - in a herringbone way - The side trenches are about 10" deep by about 8" wide and the central trench is approx 10-12" deep and 18" wide. I have filled these trenches with stones to a depth of approx 1-2". The central trench runs down to a sump - or soakaway - which is approx 4' deep and 3' x 3' wide. I have filled this will large and small stone stones up to the size of a brick. At the bottom of the sump there is a sand leval below the clay soil. Everything has been tested and the water seems to run right into the sump. I am nearly ready to put top soil down but am wondering how I cover the trenches so that the top soil can't get in and ruin the drainage. I have bought plastic corrigated sheeting - usually used for roofs - and thought that I could cut this and drill lots of holes in it to let the rain water through. My question is, if I use this, am I better cutting it to fit inside each trench on top of the stones or am I better laying it over the top of the trenches, securing it and covering it with the top soil??? I just don't want to put the top soil and turf down and find that this hasn't worked. Any advice would be much appreciated. Thanks.

[84-1093879891]

What you need is a permable geo-membrane. This will allow water to pass through but keep soiul (and roots) out of your drainage trenches.

See... Land Drainage for construction details and Geo-membranes for an explanation of what's what and why. :)

[John S]

Thanks for that Tony! I will check it out. Would I be better putting more stones into the trenches to fill them up more or would a couple of inches of stones (as I already have) at the bottom of the trench be enough? Would the geo-membrane be better being laid directly onto the stones inside the trench or could it be put across the gap at the top of the trench and secured?

[danensis]

As I understand it the geomebrane should line the trench, to stop soil getting in amongst the stones. Just dig all those stones out again!

[84-1093879891]

Ideally, the geo-membrane should line the entire trench to protect the filter medium from silting-up, but, if the trenches (and I'm not sure a 12" deep linear hole qualifies as a trench!) have already been backfilled, then laying a sheet of membrane over the top and tucking-in at the edges is a good second-best.

However....and it's a big however, as I said, I'm not sure you've gone deep enough with your digging. Bearing in mind that you need at least 150mm of soil cover if you want owt to grow over the land drains, your shallow trench-lets are barely deep enough to act as 'french drains'. If you want to do a proper job, you need to be at least 300-450mm deep, with a geo-membrane lining the trench, and proper pea-gravel as the infill medium. so, if you do decide to scutch out the "stones" you've already put in, and start again, you might want to consider a bit of extra digging. :)