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What you need:
  • Concrete blocks
  • String
  • Rubber mallet
  • Shovel
  • Spirit level
  • Three wooden stakes
  • Plate compactor
The first step in building a circular retaining wall is marking off the area the wall will encompass. To do this you need a center point to start from. Assuming that the wall is being built around a mound of dirt, drive two stakes on opposite sides of the mound. Use a measuring tape to find the halfway point between the two outside stakes. Mark the center point by driving a third stake into the mound. Tie a length of string that is half the diameter of the proposed wall to the stake. Fasten a piece of rebar to the loose end of string. Now walk around the mound, using the re-bar that you tied to the string to mark the inside edge of the wall.
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Using the line you made with the rebar as a reference, dig away any excess soil left outside of the line. You want to remove any soil remaining above grade.
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Next, dig a 3" deep trench around the mound using a square-headed shovel.
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Fill the resulting trench with crushed gravel. Ideally you want to add enough gravel to the trench so that there is about a half an inch of gravel above grade. Gravel is used as a foundation because it does not expand and contract with freezing temperatures.
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Use a plate compactor to compact the gravel. Compacting the foundation will ensure that your wall does not settle over time.
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Cover the gravel foundation with an inch or two of sand, and run the compactor over the area again. The sand will be what you lay the actual blocks on.
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Make a new line in the sand, using the string and the rebar. Try to keep the string level while you scribe the line. The line will mark the outside of your wall.
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The blocks being used for this wall are preformed, interlocking concrete blocks called Roman Stackstones. Many styles and shapes are available. Place the first block on the inside of the line. Use a rubber mallet and a spirit level to level the block.
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Lay the second block, and level it using the first block as a reference point. Continue laying down blocks in this fashion until the first line of blocks is finished. If your first row is level, laying the rest of the blocks will be easy. Stagger the second row of blocks to the first.
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Keep adding rows of blocks until you are satisfied with the height of your wall. If you are using blocks that fit together using a tongue and groove design, they should come with caps for the top row. These caps have a groove on the bottom and a smooth top. Place the final row of caps on the previous row of blocks and the wall is finished.