How to improve backyard drainage?

[Skimmo24]

Hello experts!

Seeking some advice in relation to sorting out drainage issues in my enclosed courtyard area.

I was thinking of doing a french drain as per the photos attached (red line with yellow marks would be slotted pipe). This would connect to a storm drain located just outside of the enclosed area.

 

I would be looking at digging a 100mm deep trench, filling it with geotextile and drainage rocks before laying the slotted pipe. 

In relation to connecting this pipe to the stormwater drain, will be using a plumber to make sure this part is done correctly.

An alternative option I thought of was drilling weep holes through the enclosing walls to allow water to drain outwards. The enclosed area is slightly higher than the grass outside.

The entire courtyard will eventually be covered in synthetic turf.

Would this idea work? What other solution options do I have?

Thank you!

[JacobZ]

Hi @Skimmo24,

Thank you for your question about drainage in your courtyard area.

Your first solution of adding a French drain connected to your stormwater outlet is, in my opinion, the better of the two. Mainly because the water is being channelled to a system designed to carry it away to a suitable location whereas drilling weep holes in your retaining wall is just moving the issue to another location.

While this is the better of the two options, I would make two slight tweaks to your plan.

I'd suggest adding an additional French drain to the right side and the far end of your courtyard as I've shown below.

This way, all the important things are protected from groundwater and while you have the opportunity before turf goes down, it is worth futureproofing as much as possible.

The other tweak I would make to your plan is to dig your trenches deeper. Ideally, your French drains should be around 300mm deep, so they sit below the level of your home's foundation. The idea is they will capture any groundwater above them and any water that seeps through the earth to a level below them will dissipate into the earth well below the level of your house foundation.

Allow me to tag @TedBear, @JoeAzza and @Nailbag to see what they think.

You might also like to check out How to install garden drainage for some guidance.

Let me know if you have any questions or thoughts on my recommendations.

Jacob

[Skimmo24]

Thank you for your speedy response and suggestion to improve the plans!

French drain plans
In regards to installing the french drain, would you recommend wrapping the socked agi-pipes with geo-textile fabric into a burrito (fabric, gravel, pipe, gravel, fabric) or would it be reasonable to just lay gravel + socked agi-pipe + gravel only without the geotextile fabric given that the whole area will be covered in road base and blue metal dust prior to the laying of synthetic turf?

Geo fabric layout

Looking at the vinidex products, should I be looking at using 100mm or 65mm socked draincoil?

Regards,

Sean

[JacobZ]

No worries @Skimmo24, happy to help.

Your revised layout looks brilliant and is exactly what I'd recommend.

Best practice and my recommendation are to wrap the gravel in geo-textile fabric on all sides as this will prevent the surrounding soil from eroding and settling between your gravel.  If you only put the geo-textile fabric on top of the French drain like in example 2, the soil could still erode sideways and settle between the gravel. These types of drains work by creating a path of least resistance through the earth. Having finer particles settle between the gravel will block the channels between the gravel which means it may no longer be the path of least resistance, and the water will eventually find a new path of least resistance.

The size of the pipe is relative to the volume of water it is likely to have to manage. There's nothing wrong with the pipe being oversized, but if it is too small to manage the volume of water, it can cause some issues. The initial cost to use 100mm fittings may be higher, but I always try to plan for the upper limit of what is possible, and you'll likely find the cost will even out as the volume of gravel you'll need will be reduced.

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

Jacob

[Skimmo24]

Thank you!

I will do the burrito method and use the bigger socked pipe to futureproof the drainage.

In relation to the 90-degree bends that I have in the plans, can the socked agi-pipe bend 90 degrees without having to use a joint?

If I have to use a joint, is there a way to ensure that the socked agi-pipe stays in the joint? I was thinking of using some sort of duct tape to keep the agi pipe in the 90-degree joint if I cannot just bend the piping.

Thanks in advance

[JacobZ]

Hi @Skimmo24,

Agi-pipe is fairly flexible so you shouldn't have any issues with just curving the pipe around the bend. There's no real need to have an abrupt 90° bend so make it a gentle curve and you won't have any problems.

It would be worth laying the pipe out before digging your trenches to get a good idea of what the pipe is capable of and then dig accordingly.

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

Jacob

[JoeAzza]

Hi @Skimmo24  and @JacobZ 

I have been following this project, I just wanted to say that I have learned a lot  from you guys, enjoying the progress, can’t wait to see it finished, well done so far. 😀

[Skimmo24]

I have recently spoken to a plumber in regards to my current situation and they were of the opinion that a french drain would not be effective as the soil in this part of my yard is basically all clay.

Would a french drain be worth it even if there was an adverse effect to the grounds permeability due to the clay?

[MitchellMc]

Hi @Skimmo24,

I'm not entirely sure how clay soil would make a french drain less effective. The pipe goes at the bottom of a trench, gravel goes on top, water pools on the clay soil and runs into the gravel trap, then into the pipe and gets drained away. If anything, I think the clay soil could make the system all the more effective. However, I'm not a plumber, and I can't see the site firsthand.

They might mean that a French drain won't effectively capture subsurface water in clay soil, which is true as water doesn't permeate through clay. But we are not looking to capture subsurface water; we want to drain the surface water that is pooling.

Did they offer any alternative solutions? Perhaps you might want to speak with a professional that deals specifically with drainage.

Mitchell

[Skimmo24]

Unfortunately, they did not offer any solutions.

I have decided to just give it a go and have started to dig my trenches with slight deviations from the original plans above.

Thought i would post progress photos for those interested in whats going on.

Had to hire a 2m3 skip bin and by the end of trench digging, filled it to the brim. (For those thinking of hiring a skip bin, for reference, i dug a total of approximately 16m x 0.3m x 0.3m)

Will post more photos as the project progresses!