On Saturday, 22 November 2014 08:36:43 UTC, 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?

Works best if you put coarse stuff in the bottom & then successive layers of finer stuff and then a permeable membrane & then you soil.

A lot depends on what you want to grow, Some stuff likes it wet.
If it's something that likes it dry, mix sand with your soil if it's aclay soil.
Leave "weep holes" in the walls of your raised bed or they may collapse with the weight of water.

https://en.wikipedia.org/wiki/Weep