What material to tie two cinder block walls together?

Question

I am building a 24" high planter made from cinder block walls: Sketch below.

Though it looks like a normal planter, water is pumped in from a pond, and instead of dirt, various layers of increasingly smaller and smaller rocks are filled in. As such, the walls probably need to support a lot more weight than typical of a planter, as there is rocks and water, not just dirt.

For increased strength, I can run something strong from one wall to the other, since I believe pressure will be equal on both walls. The cinder blocks have holes, which could make a convenient attaching point, but their isn't much space between the bricks, insufficient to run chain. Additionally, I need something that won't rust and deteriorate much over time.

Is there some kind of product I can use for tying the walls good so they support the heavy weight of the rocks and water?

      _____________________________
     /                            /| 
    /            @  <--flowers   / | <--brick wall
   /            \|/             /  | 
  /              |             /   |
 /                            /    /
/___________________________ /    /
|                           |    /
|                           |   /
|                           |  /
|                           | /
|___________________________|/


       ____                            ____
       |  |                            |  |
       |  |oooooooooooooooooooooooooooo|  |   (slide view)
       |  |                            |  |
       |  |     (rocks + water)        |  | <-- three 8"x8"x16" cinder blocks tall
       |  |                            |  |
       |  |                            |  |
_______|  |____________________________|  |_______<-- ground level

Answer 1

The normal way would be reinforcing bar around insides of the blocks, then add concrete. This is called a "bond beam". Blocks with open ends are used. If you use enough reinforcing it can be very strong.

For your way stainless threaded rod, you probably need to go to a fastener specialist for that. (or stainless rod and you thread the ends, but threading stainless is hard)

or stainless, or aluminium flat bar and you drill the ends and put bolts through crosswise. aluminium will probably last 5-10 years, stainless much longer.

Answer 2

If you are firm in your use of cinder blocks, as a possible radical solution to your question, you could use pressure treated dimensional lumber (2x4, 2x6 etc. ) installed horizontally to wrap your finished planter prior to filling. At the least boards these would have to be screwed together with weather resistant screws on all corner connections. Additionally there are corner brackets made for the plank decking industry that could also be added for added strength. You could also add a top plate of wood (2x10 pressure treated for 8 inch block) to finish off your project and cover the holes in the block. If this board was screwed to the side board it would make the top level of the bed even stronger. I would also use a solid color stain to seal the lumber prior to installation to extend the life expectancy.

Answer 3

This sounds like maybe to ask on a koi/pond building forum. Cinder block walls for forms are commonly used for rectangular ponds, then lined and filled with water putting outward oressure. Blocks are laid in a brick pattern, vertical and horizontal rebar is used and the voids are filled with concrete. If you are worried about water leaking out just use an epdm liner material around the perimeter