French Drain Design

[ultramods]

I have flat area in the garden that becomes a pond whenever it rains heavily.

 

I have dug a trench 15 metres long by 600mm deep. The trench is current level with no slope/gradient. The discharge/exit pipe is 400mm deep, this was put in by the builder, to get it deeper would mean adding 20 metres of additional pipework and digging up an existing lawn.

 

The perforated pipe have 360 degree perforations.

 

The perforated pipe and stone/gravel will be inside a non woven punched geotextile membrane.

 

As the exit point is not very deep, I am struggling to decide on the best option:

 

A: Fill the trench with the gravel to create a slope/gradient for the perforated pipe to sit on, then more gravel around and on top of the pipe, followed by 100mm of top soil.

 

 

B: Fill the trench with the clay I excavated to create a slope/gradient for the perforated pipe to sit on, then add gravel around and on top of the pipe, followed by 100mm of top soil.

 

? Fill the trench with the clay I excavated to create a 400mm deep level bed, then add gravel to create a slope/gradient for the perforated pipe to sit on, then add gravel around and on top of the pipe, followed by 100mm of top soil.

 

D:  Fill the trench with the clay I excavated to create a 400mm deep level bed for the perforated pipe to sit on, then add gravel around and on top of the pipe, followed by 100mm of top soil.

 

E: Put the perforated pipe at the bottom of the level 600mm deep trench then fill with gravel around and above pipe, followed by 100mm top soil.

 

And advice would be greatly appreciated.

 

Edited July 28, 2020 by ultramods

[Mr Punter]

I am not sure how wide the trench is or if you can get rid of the excavated clay, but A looks the best drainage option although you could reduce the fall to 50mm, so 20mm for each 6m length of pipe.

 

I have done a French drain under a lawn on clay soil and in dry spells the grass above really dries out.

[ultramods]

Thanks.

 

The trench is 400mm wide and I can get rid of the clay soil.

 

 

[ultramods]

  On 28/07/2020 at 15:21, Mr Punter said:

I am not sure how wide the trench is or if you can get rid of the excavated clay, but A looks the best drainage option although you could reduce the fall to 50mm, so 20mm for each 6m length of pipe.

 

I have done a French drain under a lawn on clay soil and in dry spells the grass above really dries out.

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Thanks.

 

The trench is 400mm wide and I can get rid of the clay soil.

[joe90]

If you have clay to loose (I just paid a fortune to have mine taken away) then use it, if not “A” is my bet (but will cost more in stone). Around here (heavy yellow clay) they don’t recommend using Drainage pipe as it can block, they recommend stone only. I have several French drains around my build (stone only) and they work well.

[jfb]

I'd get as much topsoil over the gravel as you can to avoid summer drying out as punter says.

I wouldn't use a membrane in clay soil - the membrane itself can clog up over time.

Not sure if I'd want to start putting clay back into the trench unless you have a year or two to wait for it to sink back down to its final level.

[joe90]

  On 29/07/2020 at 08:25, jfb said:

I'd get as much topsoil over the gravel as you can to avoid summer drying out as punter says.

I wouldn't use a membrane in clay soil - the membrane itself can clog up over time.

Not sure if I'd want to start putting clay back into the trench unless you have a year or two to wait for it to sink back down to its final level.

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+1  to those three points ?

[ultramods]

  On 29/07/2020 at 08:25, jfb said:

I'd get as much topsoil over the gravel as you can to avoid summer drying out as punter says.

I wouldn't use a membrane in clay soil - the membrane itself can clog up over time.

Not sure if I'd want to start putting clay back into the trench unless you have a year or two to wait for it to sink back down to its final level.

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What sort of depth of top soil would you recommend?

[joe90]

Whatever you can without compromising the gradient.