How to make sure water enters ag pipe?

[Cam1]

Hi, my backyard is on a big sandstone slab covered in about 2 foot of soil. After rain, the lower section of my yard is sodden for weeks. I assume I am getting a large amount of water running across the sandstone under the dirt and then it has no where to go and soaks the soil.

I want to fix the problem once and for all so I have dug out the dirt down to the sandstone. I have now cut a big channel into the sandstone to try and force any water moving along it into the channel and down hill. My plan was to put flat ag line in the trench and cover with aggregate and cloth. Further downhill of this, I was hoping to connect the Ag line into a pit and then plumb into storm water.

My question is, is how do I force the water into the ag pipe? My thoughts are that I will go to all this trouble and water won’t actually go into the ag pipe, it was just run along the bottom of the sandstone trench underneath it and miss the ag pipe and pit completely. 
I hope I’ve explained myself clearly. Thanks so much for your help.

 

[JacobZ]

Hi @Cam1,

Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Your plan is pretty much spot on and there's really nothing I can add that will help further. Going to the effort of cutting the channel into the sandstone is actually brilliant and goes beyond what I would expect a plumber to do in this situation.

Drainage is all about creating paths of least resistance. Water, whether above ground, or below it will always look for the easiest way to get where it is going. Agi pipe works by creating a path of least resistance through the soil. As water will try to find this path of least resistance, it will usually find its way to the agi pipe quite well.

Adding aggregate such as this Ki-Carma 20kg 10-20mm Drainage Gravel will certainly help direct the water towards the agi pipe, and the addition of Geotextile Membrane Drain Mat Fabric will stop the soil eroding, stopping the channels between the gravel from being blocked up.

You've already done a great job and everything you've said tells me you will have success with this project.

If you still have concerns, the only suggestion I can give is to have a drainage specialist carry out the work for you.

Allow me to tag some of our helpful members to see if they have anything they can add, @Dave-1, @TheProff, @Nailbag, @CSParnell.

Let me know if there's anything else I can assist with.

Jacob

[Dave-1]

Morning @Cam1 

Impressed with your cutting into the sandstone Nice work and agree with your thinking.  Id like to say howdy and a welcome you to the Bunnings Community page So many discussions on stormwater in here

With your concern about the water missing the ag pipe, (Had to check out what a flat ag pipe was as I hadnt seen one before) I had the same concerns with my stormwater piping and gravel around the pipe. I prefer a rounded socked pipe as I think structualy it will hold its shape better once gravel and soil is placed back over it. As to the water missing the pipe and traveling down the channel with the gravel, yes it will to a degree. But the quantity of water that has caused you to dig the trench and drain the area will actually use the pipe.

Stormwater rectification stage 1 

Stormwater rectification stage 2 

These are my stormwater projects. The gravel in the base of the trench to protect the pipe from being squashed down the track will also channel the water. I was concened about this as I thought why even bother with the pipe. This is for a solid pipe. The chanel has allowed the ground water to rechardge I think as a by product of the gravel.

Fixing water ingress issues in garage step 4, bottom of the page shows you on a small scale what you have in your yard. The socked pipe carrys the water very nicely into the hard PVC pipe and down to the stormwater pipe. All your slot in the sandstone will do is channel it to the easier path of the socked ag pipe. You could even do a slow S shape down the sandstone slot to help it gather water. I have been astounded at how the socked pipe plus gravel have kept that area dry and even today with the downpours we had overnight, its dry as in no pooled water at all.

For the second part of your question about running the ag pipe into the stormwater (depends on the state where you are by the sound of it) and your concern with the water effectivly guttering down the sandstone slot, you could do a small area such as did to collect the surface water and funnel it to the stormwater pipe if allowed. Those pavers on top of the gravel work a dream.

Dave

[Nailbag]

Hi @Cam1 I can't add anymore than what both @JacobZ and @Dave-1 have mentioned. If not already planed to make sure you install a stormwater pit at the junction for the ag pipe and stormwater.

regards, Nailbag