LG, nice to see you here! Thumbs-up!
LG, I know except that I`m pretty sure that it´s not supposed to be
slippery stones.
I guess that "clean stones" means that there are no really small stones (no 0-fraction,
ingen nollfraktion). That´ll be that the fraction of the stones are (as an example) 10-19 mm (3/8"-3/4") compared to 0-19 mm.
The reasons there should be no 0-fraction are:
1. 0-fraction will block the perforated pipes.
2. 0-fraction will "draw" water up through capillary force. Not important in draining roads but in draining houses as capillary water will make the basement damp.
Nick, am I right with my guess about "clean stone"?
Do you use some kind of geotextile (wrong word?) around the stones in a drainage in GB?
We do to protect the stones from being clogged with mud. Or at least some of us do. There are always a few trying to earn as much as possible in a short time, those might skip the geotextile and even skip the clean stone. A few dollars less and many years of sevice life (wrong expression?) less.