Advice needed on French drain

[Omnibuswoman]

We paid a company to come and lay MOT/membrane and gravel, install a French drain around the house, and drains from the downpipes to the rain pond. Some of the work was to a good standard, but some we are having issues with. 
 

We discovered that some of the weed membrane hadn’t been placed under the gravel, leaving patches where weeds were starting to come up. This has caused us to do further exploration of the hidden aspects of the work. I have dug out a couple of areas of the French drain, and found no membrane or sock on the pipe, meaning it will become silted up. Can I get views on whether to get the company back to put this right, or just let it be and hope for the best??
 

There are no inspection chambers so no way of accessing the pipes other than by digging down into the stone, which is a mighty PITA!

photo attached of one of my test digs…

[Nickfromwales]

Simple question. What were you charged for, and what was delivered? 
 

Please state what was ‘in writing’ to help us help you. 😉

[jfb]

What sort of ground is it - clay or sandy?

if clay then it is sometimes not recommended to have the membrane as the membrane itself can become silted up.

[Omnibuswoman]

Thanks for your replies. This is the description given in the quote: 

 

 

This doesn’t describe the pipe having a sock or being wrapped. However, my understanding was that it is simply part of how a French drain is installed. We did discuss the pipe being wrapped in membrane when he came to quote for the job. 
 

The ground is shillet, which is a sandy/stony type of soil. 
 

[Nickfromwales]

Were you having issues with storm / rainwater runaway? 

[Omnibuswoman]

No, we didn’t have any problems with water, this was just part of the original plan for the house, to protect the foundation slab.
Shillet tends to have water sit on it a bit, then it will drain away given time. 

[Redbeard]

Well...

 

It would be good to have membrane 50mm or so below the top surface of the aggregate, not least so that (if the sheet of membrane is separate to that (which isn't!) under the main gravel area) you can every x years, lift up the membrane, dump the gravel (it's a bit coarse for 'gravel') in a bucket, washout the 'fines' which are your enemy in this instance and start again with a new sheet of membrane.

 

Whether it will clog up depends to some extent on the ingress of fines (you have no membrane at the moment to stop this), and on the extent to which it actually becomes a drain - the likely flow, which I read as potentially quite low (but I may be wrong).

 

With the addition of a membrane near the top, but without a 'sock' it will be no worse than many older FDs.

 

I had reason to query the efficacy of the membrane I bought, which is the typical (quite cheap) woven black stuff. It is almost completely impermeable, so if your 'sock' is made of that it could stop the water getting into your pipe! On the other hand I recently saw a test on Terram (other makes are, I am sure, available) permeable membrane, and it undoubtedly IS permeable! It is also a great deal more expensive, but I was glad it was being used for the (permeable paving) job in question. If the wrong membrane had been used then the 'reservoir' of no-fines coarse limestone below the blocks would have had a lot of difficulty working.

 

In your shoes I think I would to the 'over the top' membrane as above, find a couple of points where I could in future get into the pipe, and wait and see, including having a look at the outlet when it's persisting down to see if it's actually working as an FD at all.

[Alan Ambrose]

FWIW - there’s a membrane under our gravel grids and we still get weeds. Weeds are very tenacious.