I am building some 6' x 2' x 1.5' (depth) cedar planter boxes for specific locations on concrete outdoors. The boxes need to sit as close to the ground as possible, a few inches max.
I am building three of them.
The concrete is neither level nor flat (eyeballing it I would say it's up to a 2-3" height difference in some places), and throwing down a level concrete pad is not an option.
The challenge is that the boxes will probably each be holding about 1500-1800 lbs of soil. Additionally, they will have little kids climbing all over them, so it's important that they do not wobble and that any supports underneath do not collapse.
The other challenge is they sit against a wall. So I will only have direct access to one side once they are fully built.
Currently the basis for the design is pretty simple. I plan to build a pretty standard floor frame with 2x3 or 2x4 joists every 18"-ish, with the box built on top of it.
But I'm not sure how to build supports to level it. The shape of the ground is very complex and doesn't lend itself well to simple measurements and cuts. Also I don't want to paint myself into a corner and make it impossible to build, I need to construct it in such a way that I don't have to tear the whole thing apart if I don't get it level on the first try.
The boxes don't have to move once they are placed. They will be there until they rot.
How can I do this? How can I build a stable, level support for 1500+ lbs on an uneven concrete surface?
I am thinking about using those adjustable height feet with threaded rod, perhaps 8 of them around the perimeter of the base frame. Then I can level them before placing the box on top when I still have access to all the corners. But I don't have experience with these, and I'm concerned that they may wobble and collapse with sideways force.
I had also thought about cutting 2x4s to the contour of the ground and sitting everything on those. But I don't know how to transfer the ground contours to a cut pattern (Intel has those cool cheap 3D cameras out and I honestly considered grabbing one, but I don't have enough time to wait for one to arrive then develop software to generate contours), and when I run through this idea in my head, I see all sorts of opportunity for frustration.