On 22/11/14 08:36, Bert Coules wrote:
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?


limestone itself and to an extent brick will be permeable, and if you
mix with sand, the thing will not clog up with fine sand and clay.

I wouldn't worry too much, but do out drainage hoes in the base to avoid
waterlogging.


What the hardcore does for a decent time is to provide a space where
water can pool. thus relieving the soil above from total waterlogging.


--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll