When researching about installing agricultural drains you will find some books recommend that you place a geotextile in the trench before the initial gravel and then wrap the geotextile over the gravel afterwards. Its something I used to do.
You can also buy agi pipe with a geotextile “sock’ around it like this:
The idea is that the geotextile prevents the gravel getting clogged with fine materials.
I DO NOT now recommend this as practical experience has shown that what happens is:
- If you wrap the gravel with geotextile the geotextile actually collects all the fine particles on its surface and gets clogged up stopping the water getting to the gravel and through into the pipe.
- If you wrap the pipe and put the sock around the pipe its only the very fine particles that get through the gravel but they then get caught in the ‘sock” in front of the slot blocking the pipe.
It far better to just go for a 10mm or smaller gravel surround. Any fine material that gets through the gravel will be that fine it will easily pass through the slot and get washed down the pipe.