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.



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.