I'm putting in a low planter retaining wall along the fences in my back yard
and want to know what the best material for a base is. These are the small
locking retaining bricks and I'll go about four courses high for a total of
about 12 to 14 inches. I need to put a base under them and will use rock,
but I don't know whether to use river rock (pea gravel) or larger crushed
rock. I'm putting a train track along the top, so it's important for these
to be very stable.

Also, should I add some fill sand to whichever rock is used for the base to
help lock it place, or will this interfere with drainage? Thanks in
advance.

- Magnusfarce