Agricultural drains (often called aggi drains) are used to drain waterlogged ground and also to keep the soil behind Retaining Walls dry.
In the old days agricultural drains used to be constructed out of short lengths of clay pipe butted together without proper joints. The water seeped into the pipe through these butt joints. If you are building on a demolition site you may still come across these pipes.
Nowadays agricultural drains come in two types:
- White slotted UPVC pipes with longitudinal slots which come in straight lengths (usually 6m long).
- Black corrugated UPVC pipes with lateral slots that come on coils of various lengths.
For domestic jobs the black corrugated coils are probably the easiest option.
You don’t need to be a plumber to lay agricultural drains in your garden. Some advice for laying the pipes are:
- The pipes should be laid at a gradient of no flatter than 1 in 100 (1cm in 1m) to the point to which the flow will be directed.
- When excavating a trench for agricultural drains you need to dig the trench at a width of at least twice the width of the pipe and at least 25mm deeper than the level of the pipe.
- Lay 25mm of 10mm gravel in the trench keeping it at the set gradient.
- Lay the pipe along the top of the gravel keeping it as straight as possible.
- Fill the trench with 10mm gravel to 100mm below ground level.
- Fill the last 100mm with topsoil.