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How much crushed rock for garden bed trench with agi pipe?

PatDIY_Dad

Hi guys

This trench will be a garden bed and when it rains we noticed the water just pools on the crappy clay at the bottom. So going to install an agi pipe and discharge it on the other side of the cladding wall as there is an empty paddock there

The agi pipe will also serve some drainage from the front of the lawn ( where you cant see in the photo)

I googled you need 100mm of 20mm crush rock bottom / side and top.. Seems alot for just drainage. Or is google giving over engineering advice? I thought 50mm for drainage is enough

It also said use "clean" crushrock. Where there is no dust in it. Ive never ever see any crushed rock without some dust at least at the bottom of the pile.

edit. - This install video says to use 50mm base later of 10-13mm crush rock

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EricL

Hello @PatDIY_Dad

I agree, I believe that 50mm of https://www.bunnings.com.au/ki-carma-20kg-10-20mm-drainage-gravel_p3460052 should work sufficiently. My only other recommendation is to add https://www.bunnings.com.au/pillar-1-8-x-10m-grey-geotextile-drainage-membrane-mat_p0126613 to filter out fine particulates that would settle on the bottom of the agi pipe. The only clean crushed rock I am familiar with is the drainage gravel. If you need to order it in bulk, please speak to the gardening specialist and they will arrange the order.

Just a friendly reminder that any plumbing work needs to be completed by a licensed professional for safety and compliance. Any changes that involve moving or reconfiguring pipework would be considered regulated plumbing work and would need to be carried out by a licensed plumber.

Let me call on our experienced members @Dave-1, @TedBear, @Noyade and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

Nailbag

Hi @PatDIY_Dad

Just to clarify @EricL comment around the requirement to use a licensed plumber. This is only for the connection to the stormwater from the agg-pipe system you’re running.

Nailbag

PatDIY_Dad

@EricL

Thanks for the info

The run is 12m and using a 65mm agi pipe

Any idea how much crushed crock id need for that? Google tells me 0.3m3 but it's been wrong before

Also with the geo fabric. Is is necessary when using a agi pipe with a fabric sock already on it?

If yes I'm unsure how to install the fabric bit if it's underneath or on-top of the pipe

And if I use it for the width of the garden bed or just wide enough to cover the pipe

EricL

Hello @PatDIY_Dad

Can you please provide us with a little bit more information? How deep and wide is the channel? Will you be adding soil to the side or will you be filling it with drainage gravel all the way? Once we have that information, we'll be able to calculate the amount of gravel you'll need.

The geofabric is placed over the socked agi pipe, you can trim it to size to the width of the garden bed. The geo fabric adds an extra layer of filtration over the existing sock of the agi pipe. So, even if you happen to get gravel that has fine dirt with it the geofabric will block that out.

Eric

PatDIY_Dad

@EricL

The length is 12m , 500mm deep, 370mm wide

Yes we will be putting garden soil for plants along it

EricL

Hello @PatDIY_Dad

Thank you for that extra information. Your trench is 12 metres long and 320 mm wide. You are installing a 65 mm agi pipe at the bottom and want the drainage gravel to finish 50 mm above the top of the pipe. This gives a total drainage zone of 115 mm, calculated as 65 mm + 50 mm = 115 mm.

To work out the space occupied by this drainage zone, convert the measurements to metres and multiply them together: 12 m × 0.32 m × 0.115 m = 0.442. This is the total space occupied by both the gravel and the agi pipe.

The 65 mm agi pipe takes up approximately 0.040 of this space over the 12-metre length. Deducting this gives 0.442−0.040 = approximately 0.402 of space that needs to be filled with gravel.

Using an approximate drainage gravel weight of 1,600 kg (approximate 1 cubic meter) for that full unit of space, the calculation is 0.402 × 1,600 kg = approximately 643 kg of gravel.

The Ki-Carma 10–20 mm Drainage Gravel comes in 20 kg bags, so 643 kg ÷ 20 kg = approximately 32 bags. I would allow a little extra for spreading, settling and variations in the trench, so 35 bags × 20 kg = 700 kg would be a practical amount to purchase.

For the installation sequence, I suggest lining the bottom and sides of the trench with geotextile fabric, adding a 30mm gravel bed, then laying the 65 mm agi pipe with a slight fall toward the outlet. Surround the pipe with gravel and continue until you have approximately 50 mm of gravel above the pipe. Fold the geotextile over the gravel to completely separate it from the soil, then add your garden soil, plants and mulch on top. The proposed sequence is therefore, from bottom to top starting with geotextile, gravel, agi pipe surrounded by gravel, geotextile, then soil, plants and mulch.

If you need further assistance, please let us know.

Eric

Dave-1

Good Morning @PatDIY_Dad

I have always messed up on my calculations of how much gravel to order :) I would go @EricL 's calculation route for sure.

If you havnt bought the 65mm agi pipe you could purchase one that is already socked to save the hasel, 30mm of 8mm gravel is what I have used and covered whatever pipe by 20-30mm has worked well for me. I have a paved area with the 100mm agi pipe socked underneath with 8mm gravel (Its what I had over ordered lol) and the pavers have stayed dry and not moved at all.

Dave

PatDIY_Dad

@EricL Thanks for that advice

I would prefer to buy the crushed rock from a soil place as its immensely cheaper than buying bags

32 bags $14 each = $448

1/2 cubic meter of crushed rock ( more than I need ) = $36

I would of thought you would just put the fabric on top of the pipe not under it. As its a filter to stop dirt and dust going into the pipe

Dave-1

Morning @PatDIY_Dad

The fines from the gravel, soil, clay as it drys out will filter all around, I always worry that the agi pipe due to the ribs will clag up. The sock seems to be best to stop the extra fine fines from getting into it, Having the fabric below will help stop fines rising from under the pipe is what @EricL may be meaning. As long as you can get to the end of the pipe and shove a hose up there to flush it out if it starts to clog it wont become a problem.

Dave

JacobZ

Hi @PatDIY_Dad,

Your geotextile is acting as a filter for the stones just as much as it is for the pipe. When the stones are kept free from soil, they also act as a drainage channel that you want to keep clear. Wrapping them in geotextile helps prevent soil from settling between the stones so the open spaces remain available for water to flow through.

Typically, you'd line the bottom and sides of the trench with the geotextile, add your stones and pipe, then fold the geotextile over the top. This keeps the two materials separated, preventing the stones from gradually sinking into the soil beneath while also limiting soil from migrating into the drainage aggregate.

Wrapping the geotextile around the stones also helps keep it contained and anchored in place, rather than leaving an exposed edge that could potentially work its way out of the ground over time.

Jacob

MitchellMc

Hi @PatDIY_Dad

Just to mention, you don't necessarily need to purchase the drainage gravel in individual bags. You can also speak with the Special Orders team at your local Bunnings store about arranging a bulk order, which may be a more practical option for the quantity you require.

Mitchell