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How to backfill trench with drainage?

Looking to backfill my trench but not 100% sure on best practice. It goes from 500mmm depth from one end to 1.6m depth the other over a 17m run. The trench I've dug is approx 600mm wide. I want to use two rows of agi socked or unsocked with 10mm aggregate around it and then wrapped with geotech fabric for the first 400mm and then top soil the trench. Is top soil a suitable backfill soil even though one end of the trench is 1.6m in depth? It was originally heavy clay soil. Do I need to add drainage? I've sealed the brick work etc which is stopping the water ingress underneath my home which is great.

Any advice would be appreciated.....

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Comments

JacobZ

Hi @tgm,

Thank you for your question about backfilling a drainage channel.

Your plan sounds extremely solid, and the choice of materials is great.

I do have one question though. Is there a reason for the 1.2m drop in height over the channel?

I ask because this is a fairly steep gradient for a drainpipe. Generally a gradient of 50:1, which would be a 340mm drop over 17m is ideal. Reducing this gradient slightly is likely a good idea to prevent sub-surface erosion from water running through your drainage gravel. You can use the topsoil to bring the height up slightly before adding your french drain. Just ensure you solidly compact the topsoil with a tamper in layers of around 200mm before adding the next layer.

Compact to a lesser extent in the top 100mm so the soil will better allow for root growth. This will ensure soil subsidence is minimal and if there is any you can just top it up a bit in the future.

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

Jacob

tgm

https://www.workshop.bunnings.com.au/discussion/comment/44724#Comment_44724

Thanks for your response.

The reason for the big drop in height is I wanted to remove the clay soil down to the footing which is where the mortar had eroded away over time causing a lot of water ingress underneath my home. Heavy rain caused a waterfall of water and I have been using bilge pumps in a pit to remove it as fast as possible until I could fix it from the other side of wall. So I dug the trench as low as I needed to. The footing steps down about 3 times approx. I dug it approx 600mm wide as I needed the room to work so I could pressure clean the bricks. Fill the mortar lines and holes with mortar then waterproof the wall. After water proofing the wall I used t-rex and glued 5mm thick coreflute as extra protection before I backfill trench. The gradient is approx

600mm over the 17m run. I can reduce the gradient to about 300mm with top soil.

Thanks for advice.

Is megaflow a suitable option?? It comes socked with geotech fabric.

What size is recommended for agi-pipe and on one end of the trench it will be buried approx 1m to 1.4m below the finished height. Will the agi-pipe cope with the weight? Does it have a weight limit so it doesn't get crushed and rendered useless?

Thanks,

Matt

EricL

https://www.workshop.bunnings.com.au/discussion/comment/84506#Comment_84506

Hello @tgm

That's a very serious looking trench and since you've gone past a meter in depth, I suggest getting some advice from a plumber regarding stormwater flow management. All that water coming from the agi pipe has to go somewhere and I suspect it will need to be linked to your storm drain. If it does need to be linked up, you'll need the services of a licensed plumber as it needs to be done properly and must comply with council regulations.

In regards to the size of the agi pipe I suggest going with 100mm, but going deeper might involve using PVC pipes with holes being drilled into them.

If you need further assistance, please let us know.

Eric

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