Land drain type

[deuce22]

Hi.

 

I am looking for some advice on what to use for land drains. I have a sloping plot and need to divert water into a watercourse in the centre of the plot. I'm wondering if it would be better to use perforated pipe (french drain) in multiple places or a bigger culvert at the top of the plot.

 

Any advice would be appreciated.

 

Thanks.

[joe90]

I was advised on our plot (clay) not to use perforated pipe as it clogs with clay but use a French drain with stone, membrane then topsoil.

[Stewpot]

7 minutes ago, joe90 said:

I was advised on our plot (clay) not to use perforated pipe as it clogs with clay but use a French drain with stone, membrane then topsoil.

 

I've been looking into this (speculatively, at the moment), and there seems to be no end of argument about what is the 'right' way to do land drains. A popular version (or at least my synthesis of it) is:

 

1) dig trench 450mm x 450mm+ deep[1]

2) line it with an unwoven ground fabric

3) 100mm gravel, leveled in the bottom of the trench

4) perforated pipe[2][3]

5) more gravel 100mm+ over the top of pipe

6) fold ground fabric over the top

7) replace top soil

 

[1] Some people go deeper and use more gravel - may depend upon how much water you're trying to drain out of the soil

[2] Some say the pipe should also be wrapped in a fabric sock, but few say use the sock without lining the trench as well

[3] If the pipe is only partially perforated, there is argument about whether the perforations face up or down

[Thedreamer]

1 hour ago, deuce22 said:

Hi.

 

I am looking for some advice on what to use for land drains. I have a sloping plot and need to divert water into a watercourse in the centre of the plot. I'm wondering if it would be better to use perforated pipe (french drain) in multiple places or a bigger culvert at the top of the plot.

 

Any advice would be appreciated.

 

Thanks.

 

I also have a sloping site.

 

The water mostly collects from an area of woodlands alongside our access, it passes into a culvert under the road and then runs down a ditch off the site.

 

Last week we had our groundworks contractor put in a big gravel drain to reduce the water that collects when the water table rises after heavy rain.

 

Don't under estimated the size of culvert you need to deal with the water and also that it can handle the weight of a concrete wagon.

 

 

Edited December 15, 2019 by Thedreamer

[deuce22]

20 hours ago, joe90 said:

I was advised on our plot (clay) not to use perforated pipe as it clogs with clay but use a French drain with stone, membrane then topsoil.

 

Hi joe90.

 

My understanding, is that a french drain is perforate pipe with stone. Are you saying that you can just fill the trench with stone and this is also called a french drain?

 

Thanks.

[Ed Davies]

14 minutes ago, deuce22 said:

My understanding, is that a french drain is perforate pipe with stone. Are you saying that you can just fill the trench with stone and this is also called a french drain?

 

A French drain can be either. I assume that originally they were just filled with gravel as, at the time they were popularised (1860s) by Henry Flagg French (hence the capitalized name), I don't think they had perforated pipe available in quantities to make its use economical. The key difference, I think, between a French drain and just a ditch filled with rocks is the use of some sort of filter to prevent silt getting in. Originally they put fine gravel on the outside and coarser gravel on the inside, nowadays we just use geotextile.

Edited December 16, 2019 by Ed Davies

[deuce22]

20 hours ago, Stewpot said:

 

I've been looking into this (speculatively, at the moment), and there seems to be no end of argument about what is the 'right' way to do land drains. A popular version (or at least my synthesis of it) is:

 

1) dig trench 450mm x 450mm+ deep[1]

2) line it with an unwoven ground fabric

3) 100mm gravel, leveled in the bottom of the trench

4) perforated pipe[2][3]

5) more gravel 100mm+ over the top of pipe

6) fold ground fabric over the top

7) replace top soil

 

[1] Some people go deeper and use more gravel - may depend upon how much water you're trying to drain out of the soil

[2] Some say the pipe should also be wrapped in a fabric sock, but few say use the sock without lining the trench as well

[3] If the pipe is only partially perforated, there is argument about whether the perforations face up or down

 

Hi Stewpot.

 

That looks like a thorough way of doing it. I want to get this done right first time, so I don't have to dig it all up if it blocks in the future.

[deuce22]

4 minutes ago, Ed Davies said:

 

A French drain can be either. I assume that originally they were just filled with gravel as, at the time they were popularised (1860s) by Henry Flagg French (hence the capitalized name), I don't think they had perforated pipe available in quantities to make it's use economical. The key difference, I think, between a French drain and just a ditch filled with rocks is the use of some sort of filter to prevent silt getting in. Originally they put fine gravel on the outside and coarser gravel on the inside, nowadays we just use geotextile.

 

Thanks Ed.

 

I'm thinking of doing it the way Stewpot explained. I have a run of about 50m across the back of the site and the ground keeps sloping up behind this. I have to put fencing across the whole length as well, but I'm concerned that the excess water may start washing away the fence posts. My plan was to put a drain on the neighbours ground about a metre away from the boundary fence. This will then remove a lot of water that would have soaked the ground around the footings of the posts. I would then put another drain about a metre away from the boundary fence on my side.

I was thinking of using a 600mm wide/deep trench to make sure it's enough and I don't have to increase it at a later date. If the neighbour is not happy with me digging on his ground, the only other thing I can think of is to dig the post holes deeper and fill the bottom with gravel to help soak away the excess water.

I haven't come across this before, so it's all new to me.

[joe90]

31 minutes ago, deuce22 said:

 

Hi joe90.

 

My understanding, is that a french drain is perforate pipe with stone. Are you saying that you can just fill the trench with stone and this is also called a french drain?

 

Thanks.

 

This is what the locals do here, same as down the side of dual carriageways (before the grass covers it. ) also when the brick/blockwork was above ground level on my build they backfilled outside with 50mm stone and ran a drain out to a ditch, this stopped water collecting around the house as we have a very high water table. It’s been very successful.

[PeterW]

Not unusual to find French drains lined with old roof tiles - the gaps let water through - and with unjointed clay pipes through them. The trick is to get the pipe low into the bottom of the gravel but not so low that it fills with silt. 
 

Worth washing and grading the gravel too - they work better with 20mm around the pipes and larger stuff toward the top. Other options are to use Terram around the trench - using it as a sock around the pipes is really only useful for drainage fields. 

[deuce22]

8 minutes ago, joe90 said:

 

This is what the locals do here, same as down the side of dual carriageways (before the grass covers it. ) also when the brick/blockwork was above ground level on my build they backfilled outside with 50mm stone and ran a drain out to a ditch, this stopped water collecting around the house as we have a very high water table. It’s been very successful.

 

This is obviously the easier way to do it (dig a trench and fill with Chipping's). However, doing it the way Stewpot says and using a sock seems to be the most thorough way. What I don't want though is to over think it, do to much and it doesn't work properly.

[deuce22]

Just watched this and it worked great. Is this what would be recommended or is it overkill?

Thanks.

 

[PeterW]

Pipe isn’t deep enough and the gravel needs to be over the top more. 

[deuce22]

Ok, but that process is right?

 

This is what I'm thinking.

600mm wide/deep trench

Fabric

100mm chippings

P/pipe inside sock

200mm+ chippings over the top

Envelope with fabric

Topsoil.

[Temp]

The paving expert website also has a lot on drainage...

 

http://www.pavingexpert.com/

 

http://www.pavingexpert.com/drain03.htm#french

 

 

[deuce22]

Thanks Temp.

 

I'll have a look through.

[joe90]

I was told because of the nature of clay where I live it will smear on the sock membrane and clog it. I was told to use 50mm stone, not chipping as they are too small. This is all relevant to my subsoil and may not apply to other areas of the country.

[deuce22]

6 minutes ago, joe90 said:

I was told because of the nature of clay where I live it will smear on the sock membrane and clog it. I was told to use 50mm stone, not chipping as they are too small. This is all relevant to my subsoil and may not apply to other areas of the country.

 

Thanks.

 

I'll check it all out before making a start.

[Stewpot]

1 hour ago, deuce22 said:

Just watched this and it worked great. Is this what would be recommended or is it overkill?

Thanks.

 

 

I wonder if he'd like to come and spend 15 minutes digging my foundations?

[jfb]

If it was mine I would use membrane around the whole French drain if the ground is sandy. If clay I wouldn't bother with the membrane as it will clog up. Line up the perforated pipe so that there is a smooth (no perforations) channel right at the bottom. What size pipe are you thinking of using?

[deuce22]

14 hours ago, jfb said:

If it was mine I would use membrane around the whole French drain if the ground is sandy. If clay I wouldn't bother with the membrane as it will clog up. Line up the perforated pipe so that there is a smooth (no perforations) channel right at the bottom. What size pipe are you thinking of using?

 

I was thinking of digging it 600mm wide and deep, but have been told to do it 300 wide, 500 deep and use around 100mm P/pipe. I've been digging some of the ground on the site and it is just turning to sludge, I'm not really sure if using the sock and membrane will cause more problems.

[deuce22]

Just watched this for anyone interested. It sounds logical how he explains to do it, but then states how others are explaining it is wrong and will fail.

 

[Mr Punter]

A couple of points:

 

I have noticed that grass above a French drain can die off in summer.

 

I have also noticed that Terram membrane is not very free draining and the pores can get clogged with silty clay, making it  behave more like polythene.

Edited December 18, 2019 by Mr Punter

[joe90]

 I would put more soil than that over the stone or the grass will die off in dry weather as @Mr Punter says above. Plus I was told any membrane will clog if you have clay.

[deuce22]

I'm not to bothered about the finish after as it will be under a raised deck, It is more about how it is constructed. What I don't want is to have to be digging it up in a few years because it's blocked. It seems logical to put the pipe at the very bottom rather than raising it on stone and also to put a barrier between the stone and earth. However, I have no experience with this and it may not work as long as he states.

What would you use instead of the non woven fabric then?