In article , dps
writes
While clay/sand/gravel isn't quite equivalent to cement (more properly
concrete) it is not the best way to improve a clay soil.

The gravel in concrete is not there to improve the appearance or provide
traction. It is there to add strength. Cement (the part of concrete that
hardens) has little strength in tension or shear by itself. It is very
strong in compression. Gravel adds to the shear strength. Steel
reinforcing adds to the tension strength.

Gravel or sand by themselves have good porosity and drain well. However,
if well mixed with clay, the clay will fill the porous spaces and
inhibit drainage. Since it is difficult to mix clay with gravel or sand
really well, there will be an improvement in drainage ability when they
are first mixed. However, with the passage of time (and water), the clay
will migrate into the spaces between the gravel/sand particles and it
will become impermeable again, so this solution is temporary (a couple
of, or maybe a few, years).

Organic material is better at improving clay soil for drainage for
several reasons. Initially, the mixture is just like the clay/sand
mixture, and provides spaces for the water to pass through. As the clay
washes into the spaces, the organic material decomposes, providing new
spaces. In this way, the clay/organic mix will remain permeable longer
than the clay/gravel mix. Of course this also means the organic material
has to be renewed regularly to keep the system permeable.

Also, the organic material will provide nutrients for plants (more so
than the clay/sand/gravel), and the action of roots in the soil is by
far the best way to keep the soil permeable.


Thank your for your explanation, clay is always a problem and we all
have our own ways of dealing with it, I find sand and gravel as well as
compost help, as I normally put down bark chips on soil to over winter,
that gets dug in when I tidy up during the spring. I don't have a huge
clay problem and in fact use the worst patch as a bog garden.. But in
other areas where I did have clay I have used all of the above to
improve the soil drainage, and I also have used in friends gardens where
clay was a real problem drainage pipes that are filled with sand or
gravel and placed down into a layer of soil through the layers of clay.
That however is a major garden undertaking and not something I would
want to do if its possible to avoid it.
When clay is dry it becomes cement like and cracks... the gravel and
sand help to stop that happening, by allowing water from lower down the
wick up into the clay layers.
We are all gardeners and we all have our own way of dealing with
problems, We use what works for us.. If it works then its something you
use again and remember. I have found sand and gravel plus compost works
in breaking up clay, I have also found ash from wood fires helps to, but
not everyone has ash laying around the garden these days, or can light
bonfires.
When coal and wood fires were normal before central heating ash would
be saved to use on the garden and it would be dug in to clay soil. Leaf
litter from trees helps but not all of us live near a forest.
Knowing what might be available to a gardener is part of gardening.
The best answer to clay is to dig it out, but you often cant do that...
especially if their are large amounts of clay and the cost of
replacement topsoil would be horrendous.

I am not a builder, so I will take your word on the concrete,


Shez wrote:
...clip...
Gravel is gravel. Tiny chips of stone.. They do use it in cement to give
a more natural appearance and better footing in the rain, but adding
sand doesn't make it cement.


--
Shez
Shez's Garden at http://www.oldcity.f2s.com/shez/