Land drain, to Terram or not to Terram

[88v8]

Our cottage was built into a slope, usual problem with damp walls.
Going to lower the ground level along the back, and install a land drain along the drip line. Flexible 4" perforated plastic in a 12" trench, laid and surrounded by pea shingle. Drystone retaining wall.

For most of the run, there will be no soil over the top, but inevitably leaves will drop in the trench. To prevent the drain from blocking over time, I'm inclined to wrap the pipe in a layer of Geocel or Terram, plus a further layer of textile over the top before putting on the final layer of shingle. Is this too much textile, will it overly inhibit water movement into the drain?
Don't want it blocking up, but not much use if the water can't get in in the first place.

For the final part of the run, not alongside the cottage, there will be plants on top, and here I intend to wrap the pipe in dpm to prevent it from blocking and also prevent it from absorbing water.

Ivor

 

[stuart45]

I always use this one for French drains. Allows a good flow rate.
http://www.wrekingeosynthetics.com/non-woven-geotextile-fabrics/index.asp
Wrap the trench before adding the shingle.

 

[88v8]

Thankyou. There are two options on this page, is it the NW or the SNW?

So, I line the bottom & side of the trench, but don't wrap the pipe? Or wrap the pipe as well?

Ivor

 

[mattice]

Just installed mine last weekend, geotex line trench with plenty of excess, layer of pea gravel, perforated pipe, 2/3 trench filled with pea gravel, Geotex fold over the top to enclose fully, topped with a material of your choice - no joints, no soil in the system. Managed to fit a rodding eye in one end to be sure. I think wrapping the pipe will inhibit the flow, if you're concerned I think they suggest washed gravel to reduce the possibility of silting the holes.

 

[stuart45]

I use the NW8. I don't wrap the pipe, although I know that some people do.

 

[Zebra]

For what it's worth Ivor, what I did with mine was to line the trench with geotextile, leaving enough of a flap at the top to fold over once it was filled. I didn't wrap the pipe but laid it in a layer of shingle, covered it over with shingle, and then used 20mm gravel to the top of the trench. Stopped filling just short of the top, folded the fabric over and tucked it in, then a shallow layer of pea shingle on the top. So the pipe is within a tube of shingle and gravel, and there's then no need to wrap the pipe separately as well.
I can't see a problem with wrapping the pipe as well - the geotextile is designed to let water through - unless the geotextile itself were to become blocked and silted, but then you'd have that problem wherever you put it. Bit of a belt and braces approach perhaps.
Opened mine up and took it all out again a year later - the French drain had not solved the damp problem - and it was still clean...

 

[88v8]

Thankyou all.
Can't get Wrekin locally in less than 100m rolls, but can get Terram T2000 so that's OK.
I'll go with wrapping the trench, not the pipe.
It takes a long time to determine whether one has solved, or at least improved, damp issues. I expect a year at least before our 2ft-thick wall dries out. Well it will never be completely dry as there are no foundations worth speaking off so the finished surface will be at internal floor level rather than the preferred 6" below, but the wall should eventually be drier.

I realise now I need to tweak the design of the drain a bit - the upper 25ft next to the house needs to take in water, but the bottom 17ft needs not to leak it out until it reaches the soakaway. I solved this on a smaller run by wrapping the lower portion in Densotape, but 17ft... hmmm. Could do the last 17ft in solid pipe, but then I can't wiggle it amongst the shrubs that are in the way.
Does anyone make flexible underground drain that's not perforated?

Ivor

 

[stuart45]

Does anyone make flexible underground drain that's not perforated?
Ivor

You can get non perforated.