Terram does it work? - Performance in tree pits and drains

[DavidB]

I am looking for some real experience (people who have dug up drains) in seeing the performance of terram in drainage, particularly in tree pits. A soil scientist mentioned to me that it often blocks up with silt (it would depend on the overlying soil) becomes like cardboard and stops the whole system from draining. This kind of thing will kill a tree if the object of installing the drainage layer is to prevent waterlogging in the tree pit. (this happens when you plant in naturally damp soils-the pit act like a sump).
It seems the old way of blinding with 100mm of sharp sand is better.
Has anyone here dug up 5-15year old drains and found the silt laden terram blocking drainage. How long is the the drain to be expected to last?
Thanks for your help.

[Tony McC]

I've been waiting to see if anyone else answers this thread first, but after 3 days, it seems no-one has experience of 'failed' membranes, which may be its own answer.

I've excavated, or more precisely, I've been present when membranes have been uncovered, and although there is occasional sign of some silting, I've never seen it bad enough to impede the function of the membrane.

The "oldest" membrane-wrapped land drain I've seen exposed was one we installed in 1987 at a site near Bury. In 2003 a new section of land drain was being addded to the existing installation. There was a claggy coating to the lowest part of the membrane, but the drain itself was still functioning remarkably well and there was almost no silt or sediment within the surround stone.

I've asked my contact at Terram if there's any evidence or case studies to back-up the claims made by your soil scientist.

[DavidB]

Its an interesting one.

I wonder what the long term effects are. Fabrics are often specified in tree pits. I have a natural suspicion of something that cannot return to nature over the long term after all a tree is going to be there for 300 years or more.