DIY Doctor

Newly installed French Drain NOT DRAINING! Help.

Postby watsonb01 » Tue Oct 13, 2020 8:59 am

Hi, I am new to this forum so hope I have posted correctly. Sorry this is quite a long question but I hope someone can help.

I am currently working on a large project at rear of my garden (wish I had never started).

At the back/bottom of garden there is a small stream. I have banked my side of the stream with a 12 metre long x 1 metre high Gabion Basket Wall. There is a double layer of geo-fabric between the garden soil and the Gabion Baskets to allow the water through but retain the soil in the garden.

3.5 metres back from the Gabion Basket Wall there is a newly built Retaining Wall. The wall has a french drainpipe behind it for drainage that exits to the stream on both sides. There are also weep holes along the bottom of the wall which allow through a fair amount of water, (even if it has been dry for days).

I am about to build a patio between the Retaining Wall and the Gabion Basket Wall as soon as I have installed appropriate edging. The top surface of the patio will be about 10cm above the height of the weep holes in the Retaining Wall and the edge of the patio will start 10cm back from the edge of the wall with the gap in between filled with chipstones. I therefore required a way to ensure the water from the weep holes drained away ok from the gap between the wall and patio.

So I decided to install a french drain just in front of the retaining Wall foundation that runs the length of the wall and terminates down at the Gabion Basket Wall hard up against the wall geo-fabric.

I dug a trench, ensuring at least a 3% fall from centre of wall to both termination points at the gabion Wall, laid fabric, filled with a few inches stone, laid a 100mm corrugated pipe (also encased in geo-fabric), and topped up with a few inches more stones then wrapped over fabric. Ran a hose on pipe it for at least half an hours and water drained away great (although not 100% sure it came out end of pipe at the gabions, but I assumed so at the time).

So to deal with weep hole water, my intention prior to laying patio was to cut out several 20mm wide/deep sloping channels in the wall foundation just in front of the weep holes and install short weep pipes that would run under the patio and terminate just above the newly installed french drain. The weep pipes would also be encased in geo-fabric to prevent silt blockage etc.

However before I dug out my first channel in front of one of the weep holes, prior to installing the weep pipe, I thought I would double check that the water would drain ok so poured some water on it. But the water did not drain away at all. In fact I dug back the earth a few inches down to the actual French Drain and poured water on it and it just sat there above the geo-fabric without draining through.

Confused a bit I decided to dig up the end of the french drain pipe at the termination point down at the Gabion Basket Wall. All looked ok but when I poured water in at that point it just sat there without passing through the geo-fabric and out into the stream.

The french pipe that is already fully encased in geo-fabric butts right up to the geo-fabric on the Gabion Baskets and I assumed it would just run down the pipe and filter through the fabric and out into the stream.

I have the pipe wrapped in geo-fabric. It is then layed in more geo-fabric and encased in 20mm washed gravel all round.

Could there be too much fabric to allow drainage.

I personally think it may slow the drainage a bit but I have tried running water through 8 layers of the fabric and it seems to filter through ok so a bit stumped.

Also why does the water i poured in at the Gabion termination point not filter out through the geo-fabric out through the baskets into the stream. It is only 2 layers thick. I read non-woven Geo-Fabric was much better for drainage that woven stuff.

I have attached some photos.

Any thoughts/advice would be appreciated.

Thanks.



Barry
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watsonb01
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Postby stoneyboy » Fri Oct 16, 2020 8:32 pm

Hi watsonb01
I don't think you have done anything wrong it's just that water takes ages to permeate through geotex fabric.
Regards S
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Postby watsonb01 » Fri Oct 16, 2020 9:16 pm

Hiya.

Thanks for your reply. I have found the cause of problem and have spent last 2 days fixing it. After I laid the french drain I ran a hose pipe on it at various points for 30 minutes and the water drained instantly. So i covered drain with an inch or 2 of earth followed by 2-4 inches of hardcore. When I dug it back out I found the earth was almost clay and that is what stopped the water draining. So dug out covering and replaced it with semi-washed hardcore and its draining great now. A lot of work but a lesson learned.

Cheers.
watsonb01
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