I've searched around a bit, but couldn't find an answer to this one.
My back yard is essentially a concrete jungle with a pool in the
middle and it sorely needs some greenery. I have a large back left
corner (probably 20x30ft) that I'd like to be grass and plants.
Demo'ing the concrete is quite expensive, one quote (on the high side)
A friend mentioned putting in a large planter box ON TOP of the
concrete, and that this would be sufficient for grass and light plants
(no large trees or anything). Of course I'd want to put in some
drainage and even automatic sprinklers. I figured I'd use some nice
retaining wall blocks (e.g. Allan Block), perhaps 3 high (or ~12" in
height) so that it looks nice and finished. The back of the planter
box would be against a typical wooden fence that separates the yard
from the neighbors. I also thought I'd install some nice pressure
treated planks of wood along the bottom of the existing fence to the
same height as the planter box to protect the fence against moisture,
rot, etc. and make it a more permanent installation.
So that's my plan in a nutshell. I think with 12" of soil grass/sod
will be no problem. Small plants should be fine too. I believe the
retaining wall blocks are meant to be used on dirt, and that the
bottom of the first row is usually partially (a few inches?) below the
dirt, naturally this keeps the block in place. However on concrete
this is not an option. I'm concerned about the bottom row sliding out
since it would just be sitting on top of the concrete. I've
entertained ideas of drilling holes in the concrete and hammering in
some kind of anchor, such as rebar in the middle of the blocks to keep
them secured. Seems like it would be a lot of work though 'cause I'll
probably have 30-40ft of wall.
So, can I use retaining wall blocks on concrete? Do I need to secure
them, or will the weight of the blocks plus gravel inside them keep
them in place?