I have a question on French drains/percolation areas that there maybe a straightforward answer to, but I cannot get my head around and its this, why do you put more drainage stone Over the percolation/drainage pipe than below it ?
A french drain is, in essence a filter drain. The extra pea gravel above the slotted/perforated pipe assists in filtering silts that will be washed down from the topsoil with rainfall etc. Therfore reducing silt buildup in the land drain itself.