On Saturday, November 22, 2014 10:11:57 AM UTC, fred wrote:
In article , 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?

I agree with you and I made the mistake in the past of making a shallow
soakaway with 40 odd mm hardcore without making sure that soil couldn't
migrate into it resulting in a dam rather than soakaway.

Hardcore is cheap however so if you buffer it from the soil then I think
you would be ok. This could be hardcore then pea shingle on top then a
soil blocking membrane. A full fill of pea shingle might be better
(again with membrane).

While I'm sure you do want free drainage, I'm also pretty sure you don't
want the beds to dry out too much either so there must be some kind of
balance to be had (IANAG).

The soil will be drier than the ground level garden simply because its raised. I wouldnt expect a wetness problem, thus no need for a solution. The rest of the ground doesnt need a hardcore & membrane layer, so why does the raised bed?

Weep holes in the sides avoid a lot of extra load, plus shed heavy downpours quicker.


NT