"Bert Coules" wrote in message
o.uk...
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?



Water in the upper soil layer drains into the hardcore
The voids in the hardcore acts as a store for water in heavy rain.
The water seeps away later nto the ground under the hardcore.

So the top lay neverg ets waterlogged.

Best if you put a porous membrane on top of the hard core to stop the soil
washing into it.
Old bulk bags cut up are ideal.