How to build French drain to address concrete slab moisture issues?

[amapola]

Hi I would like to know if it possible to build a French drain as shown in the line described in the photo, my house is on a slope where there is clear path for the water flowing towards the house foundations and creating high moisture levels ( which I’ll address separately) however to address future drainage concerns I want to build a French drain and would like some advice on how to start this project and what materials will be needed.

thanks,

your DIY neighbour. 

 

 

[JacobZ]

Hi @amapola,

Welcome to the Bunnings Workshop community, it is wonderful to have you with us.

Considering the lay of the land, installing some kind of drainage system along the edge of the concrete apron definitely seems like a good idea.

A french drain is a good option if the water is beneath the soil, but with gradients like this, surface water will often be more of an issue. Have you had a chance to examine the slope during a heavy downpour yet? Is surface water a problem, or is subsurface water more of an issue?

If surface water is a bigger issue, then you would be better off installing drainage channel along the outside edge of the concrete and having a plumber connect it to your stormwater system.

If subsurface water is a bigger issue, then a french drain would be the way to go. Check out How to install garden drainage for some guidance on the process.

For a french drain, you would dig a trench around 300mm x 300mm, then line it with geotextile membrane. You would then fill the bottom 100mm of the trench with drainage gravel before sitting your socked agi pipe on the drainage gravel in the centre of the trench. You would then backfill around and above the agi pipe, before folding the geotextile membrane over the top like a burrito and backfilling on top to lock everything in place.

The french drain should follow a gradual gradient towards a drainage point. Typically, having the end connected to your stormwater system is the best option, although this is plumbing work that would require the assistance of a plumber.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

[AlanM52]

Hi @amapola,

I would use the Everhard drainage channel.

Here are some photos from my Workshop Drainage Project.

Cheers

[amapola]

Thanks Jacob for the detailed answer, I think I got an issue with both as my property has reactive soils, moisture is retained more than the standard range within the soils, is there a way to tackle surface water and groundwater at once with a drainage system? I’ll engage a plumber for connecting it to the stormwater. 

[amapola]

That looks like a really good design and cool project, but I’m just afraid it will not work as good due to to the the reactive soils moisture in tackling groundwater issues. 

[EricL]

Hello @amapola 

If you have reactive soil my best advice is to line the channel with geofabric and then place extra gravel at the bottom. Once the slotted agi-pipe is installed you can then cover it with another layer of geo fabric before placing the gravel at the top. the purpose of all that extra geo-fabric is to prevent your reactive soil from entering the agi-pipe which causes blockage. 

If you need further assistance, please let us know.

Eric

[amapola]

Hi Eric,

Thanks for the advice, so I have decided I will do a 450mm trench. I will have two Geofabric textiles (2m wide) wrapped around, so would you say 200mm of gravel placed at the bottom, then the socked agi pipe(40m), 200mm of gravel on top, and then want to add 50mm of decorative pebbles to maximise the drainage performance, does that sound reasonable? I’ll start the project this weekend depending on weather so will upload some photos, once completed. 

[EricL]

Hello @amapola 

Please remember to put geofabric on top of the socked agi drain before putting the gravel on top. But the rest of your specifications you mentioned will be more than adequate for the French drain.

Eric