I need to install a number of french drain channels in my garden.
The garden measures 50ft long by 20ft wide.
I'd like to get an understanding of how far apart each drain should be in order to improve the drainage (caused due to compacted soil) in my lawn?
might it be worth rotovating to uncompact the soil? you could mix in sand or gypsum to help drainage while you're at it.
but the topsoil (approx 10 inches) seems ok; its the sub layer of fill which is compacted. It was a new house build and the garden was raised 3 foot with hardcore fill.
Last year i spead 22 ton of Lough Neagh sand on the lawn but it has not taken the yellowish colour from the grass, so i'm thinkning a number of 3 foot deep French drains would help the sub layer drainage as well as allow the sub layer to spread and thus reduce compaction.
I'm just not sure how far apart the channels should be?
Also if the above sounds mad, please let me know.
I'd put the drains you show at 12' apart but would add herringbone side drains in between them to fully cover the area in between. Interleave them like your ammended dwg below comments about artistry skills at this hour of the morning not welcome.
Where is the water going to drain to I presume there is a fall? I've come across this many times bloody builders eh?
Where you still have access it is worth getting the ground mole drained to break through the compaction before you install drains. One side of my house was very badly compacted when I built (middle of winter of course). Just grassed it and left it at that but had to mole drain it after a couple of years as the pan was just holding the water never bothered putting drains in and it is now fine. Favourite saying of my missus "the vets dog is always sick"
yes there is a bit of a fall so the drains should help the water flow down the slope.
I'd not heard of mole draining before so will need to investigate this - any input on mole draining from members would be welcome.............
I think the problem is more of a "compaction" one and deep down i'm reluctant to start stripping back grass and topsoil to install french drains if its not needed and there is an alternative solution.
I think your plans are overengineered to resolve the surface problems. The problem is 3 feet of hardcore backfill which has probably provided a sealed base allowing no water to escape or very slowly. Why not simply excavate a series of trenches and backfill, this should allow water to escape.
A few land drains in the direction of the prevailing fall will accelerate run-off but if you use land drains, wrap in permeable fabric, this will prevent pipes from silting up.
IMO you require no pipework.
is the yellowish colour due to lack of fertility or drainage issues, do you think?
I'm sure the problem is drainage below the topsoil.
Lorry loads of hardcore fill were reversed over previous dumped fill over the first 3/4 of the lawn area. The result of which was that the sublayer was compacted down by the weight of these fully loaded machines, the topsoil was then spread straight on top of this compacted fill.
The reason i'm 100% sure is that in the last 1/4 of the lawn which was not compacted town by machinery (the last few lorry loads were piled high and spread out by the digger) the grass is lusk and green all year round.
Sonnenblumen, so that i'm clear are you suggesting that i should just open 12" trenches and fill with 1.5" clean stone and dont bother with piping at all? If so do you agree with Antiquo's suggestion that the trenches should be 12foot apart and would this be with or without the suggested herringbone side drains?
Thanks for the feedback so far folks.
any final thoughts/advice for me on this?
BG in my case the land I used to run all of the construction traffic on is fairly large plus easily accessible so I could get a 150hp tractor in and ploughed the whole area with a mole drainer. Its basically a blade with a round plug/shoe on the end with a wider plug on a link which opens a small dia hole. It goes down approx. 1m and as well as leaving a hole which acts as a drain the blade breaks up the pan and allows water to penetrate. I did this in the summer (albeit a couple of years after the compaction occurred) and never had an issue since.
However you do need room to operate a rarther large piece of machinery so re-looking at your overall dim's and unless you have open ended garden prob not going to fit.
So... dig a trench then fill it back in, use pipe but you don't need pipe?
If you are going to dig a trench in order to alleviate drainage issues it would be wiser and a much better long term solution to put pipes in while you have opened the trench.
Of course I could be wrong and the lads on the County Council lorry throwing spade fulls of cold tar into wet and muddy pot holes are doing a first class road repair which will last for years to come.
OP - I doubt you do not require pipes. As you seem to suggest there is a layer of compacted hardcore obstructing percolation, by stripping trenches in this hardcore layer, you can route water off site using gradient.
But if you want to pipe it go ahead. IF you use pipes be sure to wrap them in a permeable fabric to protect against silting over.