How to build a AGI or French drain?

[gwu2]

Hi, we're building a tennis court in our backyard but noticed some areas are still wet even on dry days. We're planning to install an AGI or French drain to manage the groundwater. The stormwater connection is at the front of the house, so groundwater collected at the low point will need to be pumped back to the front. We're also building a retaining wall—how far should the AGI drain be from the wall, and how deep beneath it should it go, considering we're digging a 300mm trench? Should the AGI surround the whole court, or is it enough to cover just the lower (west) side? The court will have a 1% slope towards the lowest point—would a pit work, or should we consider an underground water tank with a floating pump that activates as the water rises? Can I handle the AGI and pit installation myself and just get a plumber for the connection? Can we also install a roof barrier to stop the tree root at the back? Any advice would be appreciated. Thanks!

The wet area on the dry indicated ground waterAGI Drain to clear ground water

[JacobZ]

Hi @gwu2,

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

This is such an exciting project. I can envision many wonderful times spent with loved ones on your tennis court in the not-too-distant future.

With a project of this size, the drainage requirements are very high as an area this large can create a massive amount of runoff. As you are looking at a court that is 450sqm, assuming a downpour of 50mm in an hour, 450m2 × 50mm/h = 22,500 litres per hour. So, there is potential for a massive amount of runoff if you have a heavy downpour.

Designing a system that is capable of dealing with the potential volume of water is essential to maximise the life of your tennis court and minimise the risk of erosion. Unfortunately, due to the size of the project and the cost involved, this is likely something you should consult a drainage specialist for assistance with.

They will likely use large french drains around the full perimeter of the court, essentially creating an in-ground moat to capture any water that runs off or towards the court. These drains will likely run to a large tank that is capable of dealing with the "worst-case scenario" in terms of rainfall. The pit will also need to have a float pump that is suitable to move this "worst-case scenario" volume of water.

French drains usually run directly behind retaining walls, with a gap of around 150-200mm from the retaining wall being a good distance for the drain to be set back.

Using a root barrier such as this Jack 600mm x 5m Garden Root Barrier is definitely worth it to help prevent root growth that will crack your tennis court. It is likely worth running this around all sides that are in close proximity to trees.

Let me know if you have any further questions.

Jacob