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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.

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This is off question, but the weight of your soil will be in the neighborhood of 100 lbs. per cubic foot, (or more when wet) so the load on your 2'x6' area will be 2' wide x 1.5' (high) x 6' (long) 100 lbs./CF = 1800lbs. Max span for 2x4s at 16"oc for 100 psf is approx. 3' span, so you should be OK. But what are you using as the floor of the planter? Plywoods will not be adequate. – Ashlar Mar 9 at 20:19
    
@Ashlar I'm not going to fill all the way to the top, let's say 1500-1800 lbs. I was going to use 2x6 or 1.5x6 cedar planks for the floor. – Jason C Mar 10 at 0:10
    
That will work! – Ashlar Mar 10 at 2:52
up vote 9 down vote accepted

I would build the support frame for under the boxes, just the frame (out of treated 2x4's). Then find your where you want the boxes to rest.

Then you scribe your 2x4's with a compass. You get your frame some what level, then set the compass (generally to the max gap, but only if you want the whole frame resting on the ground) and go around with the compass point on the ground and where the ground pushes up, the pencil will mark the board where it needs to be cut to 'relieve' the ground underneath.

I had to do this a bit with my dad's log home. All trim had to be scribed to get a nice fit with the logs.

Once you have your frame ready (and mark where the corners are on the concrete!) you can attach it to the bottom of your boxes.

enter image description here

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+1 This was going to be my suggestion but you beat me to it. No need for complicated 3D programs. Also, with 1000 lbs of soil on top, any small variations in the contour will probably be crushed out. – grfrazee Mar 9 at 15:47
    
Ya, you can get it pretty level when you know how to scribe and it's simple. – bowlturner Mar 9 at 15:52
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If you choose this option, establish the level frame as level as you can before scribing the contour; make certain that your compass will stay fixed at one setting for all all of the members in the frame; make a provision (weep holes) to allow water that makes its way under the planter to escape. – Ast Pace Mar 9 at 17:04
    
@AstPace all excellent advice – bowlturner Mar 9 at 17:04
    
+1 This works. I've done it to level a concrete floor which was 2-3" off level and uneven. – brhans Mar 9 at 20:16

I think your concerns with the adjustable height feet may be unfounded, especially if you only need to raise the boxes by 2-3". I doubt that they would collapse with sideways force if you source the heaviest-duty ones you can find.

Alternatively, it's pretty common when building things like this to pack the box on pieces of slate. Slate is easily broken up and stacked in thin layers to overcome unevenness like this and it's very good at supporting heavy loads i.e. it won't crumble.

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You could build a square or rectangular wooden frame, level to the top of this, using long spirit levels, filling any gaps below due to uneven ground, with, say, moist sand.

Then fill the frame completely with something like plaster of Paris, which being liquid originally, automatically forms a horizontal surface.

Remove wooden frame, and sand the fill (the plaster, or whatever you decide to use) has set, filling in any unsightly gaps in edges, for example, where the sanded filling was, with the same substance.

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Adjustable "threaded rod" type of feet are a stable solution that will support a lot of weight, and they're basically wobble-free at such small heights (even more so if a thousand pounds rests on them, consider that the Newtonian laws apply to 1000-pound objects in particular, these do not move easily). They do cost quite a bit of money, too.

However, the "technically correct" approach is a different one, and it can be done with the wood that you already have.

If you use three feet per box (not four), you are guaranteed that nothing can shake or wobble. A 2x4 will support virtually every weight if used as "foot".

If it's not necessary that the dirt boxes can also serve as dinner table, but "approximately level, as good as the naked eye can tell" is good enough (I daresay it is!), then you can even just cut the feet so they only approximately have the correct height. There is no real need to be perfectly accurate to the millimeter. As long as each box has 3 feet, it's all good. Even if it may be slightly off level, the box is always guaranteed not to wobble. There is no way a tripod will do that in a 3D world.

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However if the box has 3 feet but is any shape other than triangular, there will exist points outside of the enclosed footprint of the 3 feet of the box which may make the box unstable and prone to tipping when weight (such as that of a child climbing on the box) is placed over a point outside of the footprint. Imagine a rectangular box with two feet at one end, and one foot in the centre of the other end - if you put weight on one of the corners of the box at the end with one foot, it may tip. – WhatEvil Mar 10 at 0:04
    
@WhatEvil: That's right, for a very un-square rectangular box, that may be the case (depending on how much one exaggerates, it will be rather than may be). For anything in a "kind of normal shape" though, 50 pounds of child are no challenge to 1000 pounds of earth in a box, inertia-wise. It wouldn't move very much even if balanced on a tip (although I wouldn't recommend that for obvious reasons). – Damon Mar 10 at 8:28
    
Also note the mention of "wall behind boxes" in the Q. If you set up the boxes so the "two foot" side is on the front and the "one foot" side is towards the wall, then the wall will prevent the child from stepping on the "vulnerable point" but even if it manages to do that and carries another 100 pounds of load and jumps, the wall will prevent the box from toppling. If possible at all, it could only ever topple kind of diagonally-backwards, and there's the wall in its way. – Damon Mar 10 at 8:37

I would look into a sand base much like you would build for deck pavers. It self levels can support tremendous loads. If you were concerned with aesthetics, you could fashion some trim pre-treated shoe molding to fit at the bottom of the box to hide/contain the sand.

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