In message , Bert
Coules writes
Apologies for the uninspiring subject line, but it does at least sum up
my question.

I'm constructing a large and quite high (thirty inches or so) raised
bed in my garden for planting: block walls enclose an area of bare
earth at ground level. The bed will be topped by a layer of soil, and
I've been advised to put in a bottom layer of hardcore "to assist
drainage".

What I don't quite understand is how a layer of rubble, broken-up
bricks and the like can have anything to do with drainage. If the
spaces between the bits remained clear and open, yes I can see that:
but surely the soil (or whatever I use as a "middle" layer) will simply
fill those gaps? OK, water will drain down through that material - but
it seems to me that it would do so even better and faster if the
hardcore wasn't there, getting in the way. Or is the hardcore itself,
rather than the spaces between the pieces, more permeable than I realise?

Tradition?

Probably functions fine if you use the membrane from your earlier post
to segregate soil from rubble. You can purchase proper geo-membrane
designed for this purpose.

Maybe the rubble is to provide a storage void like a soakaway to gain
time and to distribute local wetspots?



--
Tim Lamb