Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
A wall of brick that is just one brick wide, stacked up the narrow way is not very strong, even when it is mortared. It is only 4" wide (most bricks are about 3-1/2" to 4" wide).
To make a retaining wall out of brick, but not use mortar stack the brick in a way that each brick points toward the center of the barrel, then put the next course slightly closer to the barrel and keep on stacking them that way.
Each course overlaps the other so that one brick sits on top of two.
To see an idea about this google 'shaker round stack' (it is about firewood).
Another way is to make a double row around the container, but include some courses lined up parallel to the barrel and some aiming in toward the center. Again, each course is slightly closer to the barrel than the course under it. And, again, overlap the bricks so that as much as possible one brick sits on two.
Another way is to use something else besides mortar that can hold them together, but is easier to remove. Expanding foam can do this. In either of the layouts I suggested there will be gaps because square (well, rectangle) bricks don't make circles really well. Fill those voids with foam. Careful, though, it really expands and can push the bricks out of alignment!
Another option is to make two boxes, one inside the other. The outer is only half the height of the barrel. This terracing will make each wall hold up only half the height of soil. I would still build this in either of the stronger manners suggested.
All the ideas here are intended that the enclosure will be planted. The evapo-transpiration from thriving plants will help cool the soil and the whole structure. If you choose a few plants that are hanging over the pond then there will be a bit of shade, too.