Hardcore and the theory of drainage

[Bert Coules]

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?

[Tim Lamb]

In message <NpudnfIDpaGF1u3J...@brightview.co.uk>, Bert
Coules <ma...@bertcoules.co.uk> writes

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

[fred]

In article <NpudnfIDpaGF1u3J...@brightview.co.uk>, Bert
Coules <ma...@bertcoules.co.uk> writes

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).

--
fred
it's a ba-na-na . . . .

[newshound]

I don't think it is an uninspiring (or silly) question at all, I have
pondered the same thing in the context of stable yards (Type 1 on top of
hardcore) and also preventing field gateways from getting muddy in the
winter.

For flower beds I think it is largely the gaps that matter, not the
porosity of the bricks etc. in the same way that shingle or gravel is
good for trenches over land drains as well as soakaways. I agree with
Tim that a membrane between the soil and the hardcore (or gravel) is
necessary to slow down the rate of transport of fine material which
would fill the gaps.

If your "bare earth" is impermeable or tends to get saturated you might
want "weep gaps" in the block wall to help drain the hardcore zone,
otherwise the raised bed might fill up like a bucket.

If you have never come across it, the PavingExpert website is a mine of
useful information on more than paving.

[meow...@care2.com]

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

[alan_m]

On 22/11/2014 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".
>

Will you need any drainage for a raised bed of this height? Will it
ever become waterlogged? More likely it will be a lot drier than
planting at ground level. In the summer when you want more water a heavy
rainfall is unlikely to wet the soil by more than a few inches below the
surface.


--
mailto: news {at} admac {dot] myzen {dot} co {dot} uk

[The Natural Philosopher]

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

[Bert Coules]

Thanks to everyone for the replies. As usual with this sort of area, the
range of advice is extensive.

I have a source of old concrete panels which I can break up and use for
hardcore, but I'll have to bring in some smaller aggregate to use as a
filler material and to level off the top of the base layer - without that, I
suspect that any membrane, even the toughest, would be penetrated by the
edges of the concrete shards.

I've also been told that broken-up chunks of expanded polystyrene made a
good substitute for traditional hardcore (and would certainly be easier to
handle) but, as with something harder, I'm still not sure I completely
understand the way the drainage is supposed to work. However, any
moderately sizeable material would at least cut down on the amount of
smaller infill necessary, which I suppose would be a saving.

Newshound, particular thanks for the link to the Paving Expert site: what a
splendid resource.

Bert

[Terry Fields]

Bert Coules wrote:

> 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".

You might find that during summer the block walls will cause a loss of
water by evaporation through them; and a bed raised this high might
well have plenty of natural drainage anyway. You could well have more
of a drying out too fast problem rather than a too much water one.

I suggest you ask for comments in uk.rec.gardening.

--
Terry Fields

[stuart noble]

If your bed faces S/SW (prevailing weather) it will probably need good
drainage. I have a north facing bed that is almost always on the dry side.
I'm a great fan of vermiculite which, when mixed 1-6 with soil retains
water (and air) but is also self regulating drainage wise. A 20L bag was
about a tenner last time I looked

[harryagain]

"Bert Coules" <ma...@bertcoules.co.uk> wrote in message
news:NpudnfIDpaGF1u3J...@brightview.co.uk...

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.

[Bert Coules]

"harryagain" wrote:

> The voids in the hardcore acts as a store for water in heavy rain...

>
> Best if you put a porous membrane on top of the hard core to stop the soil
> washing into it.

If there are appreciable voids in the hardcore, won't the weight of the
topsoil onto the membrane cause it to be punctured on the hard edges? But
alternatively, if I try to smooth out the surface of the hardcore (say by
pouring on a smaller aggregate) won't that just sink down and fill the
voids, severely limiting the drainage?

[Bert Coules]

Stuart Noble wrote:

> I'm a great fan of vermiculite which, when mixed 1-6 with soil retains
> water (and air) but is also self regulating drainage wise.

Thanks for the suggestion. I'll investigate.

[harryagain]

"Bert Coules" <ma...@bertcoules.co.uk> wrote in message

news:UbCdnT-kIZnxYu3J...@brightview.co.uk...

Depends on the nature of the hardcore.
Ideally it should be topped off with pebbly stuff (no sharp edges) to
protect the membrane.
Available as "pea gravel" or "shingle" (dredged from the sea) at builders
merchants.
In practice any smaller stuff on top doesn't get far into the stuff below
and bulk bags are very tough.
Depending on the thickness.

All depends on how much of a perfectionist you are and what you want to
grow.

If the soil is light (ie not clay) you might need none of this.
Alternative method is just to mix sand in with the soil, you need a lot more
than you might think.

You may need weep holes in your retaining walls if the soil blow is clay,
reduces the forces on them.
Also good idea to paint inside of retaining walls with bitousmous paint,
stops frost damage and excessive drying out in drought conditions.

[Bert Coules]

Chris Hogg wrote:

> I think you're making a huge meal of this.

No doubt. But I enjoy looking into such things and I'm grateful for all the
comments and advice.

Bert

[johngi...@hotmail.co.uk]

[johngi...@hotmail.co.uk]

On Saturday, 22 November 2014 08:36:43 UTC, Bert Coules wrote:

[Rod Speed]

Nothing from you twice in a row.

Do you always get premature ejaculations ?

<johngi...@hotmail.co.uk> wrote in message
news:4920de70-79b3-4ec6...@googlegroups.com...

[Peeler]

On Tue, 18 Feb 2020 08:16:25 +1100, cantankerous trolling geezer Rodent
Speed, the auto-contradicting senile sociopath, blabbered, again:

<FLUSH senile trollshit>

08:16??????? ROFTLOL! IOW, you haven't slept ONE MINUTE last night because
this here is the ONLY place where you can talk to people without them being
able to get away from you, you cantankerous obnoxious auto-contradicting
sociopath! LOL

--
Norman Wells addressing trolling senile Rodent:
"Ah, the voice of scum speaks."
MID: <g4t0jt...@mid.individual.net>

[Brian Gaff (Sofa)]

Why resurrect such an old thread?

Brian

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This newsgroup posting comes to you directly from...
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Note this Signature is meaningless.!
<johngi...@hotmail.co.uk> wrote in message
news:cb7325bd-bda6-4583...@googlegroups.com...

[Osprey]

On Saturday, 22 November 2014 08:36:43 UTC, Bert Coules wrote:

I built 40m of rasid beds at my place .. filled in with 40mm clean stone, all fully wrapped in Terram, then soil on top.
At base of stone I also run in French dreain for full length ... open at ends (with mesh guard) water can then drain down through stones into pipe and out.

[harry]

On Saturday, 22 November 2014 08:36:43 UTC, Bert Coules wrote:

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

[Tim+]

Don’t you think that after 5 year Bert might have dealt with and/or lost
interest in the issue? You’re getting as daft as those poor souls over in
HOH.

Tim

--
Please don't feed the trolls