dps wrote:
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.

See. dps said the same thing I said only a lot better.

--

Travis in Shoreline (just North of Seattle) Washington
USDA Zone 8b
Sunset Zone 5