How to backfill trench with drainage?

[tgm]

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..... 

[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