Hi @Aabey,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Are you planning on having this agi pipe connected to your stormwater pipes?
If so, you will require the assistance of a suitably licenced plumber to make this connection. This is usually a good idea, so the water is taken away and is not simply moved to another area in the yard.
For a 100mm agi pipe, you should dig a trench that is approximately 300mm wide and 300mm deep. This size allows for adequate space around the pipe for gravel and ensures proper drainage.
I'd suggest Vinidex is the brand to go with for your project. You're going to need 2 x Vinidex 100mm x 20m Socked Slotted Draincoils and a Vinidex 100mm Draincoil Coupling to connect them.
For the gravel, you will need enough to cover the bottom of the trench and wrap around the pipe. Typically, you want about 50mm of gravel beneath the pipe and enough gravel to fill around the sides of the pipe. For a trench that is 25 meters long, you’ll need approximately 1 cubic metre of gravel. Gravel can be purchased in bags such as this Ki-Carma 20kg 10-20mm Drainage Gravel or ordered in bulk through your local store's Special Orders desk. Unfortunately, as suppliers are regional, I can't link you to a specific product page for bulk ordering, so you'll have to get in touch with your local store to confirm pricing and availability.
Instead of plastic, I'd suggest using this Geotextile Membrane Fabric to line your trench before putting the gravel and agi pipe in place. This will stop soil from getting into your french drain, but will allow water to pass through.
Before you start, check with Before You Dig Australia (BYDA) to ensure there are no underground utilities that you could hit. Additionally, it’s a good idea to have a slight slope in your trench to help facilitate drainage. Proper installation will help reduce water stagnation in your garden during the rainy season.
You might also like to check out How to install garden drainage for some further guidance.
Let me know if you have any further questions.
Jacob
