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I'm making a raised garden bed that is about 2 cinder blocks high (with 1 extra cinder block below ground as footing) in a square shape 3.6 meter each side.

So far I made a footing of 5 cm of concrete, than laid a course of beam type cinder block (shaped like a U) with steel bar inside and filled with handmade concrete (this is below ground level). I already I laid the middle course of cinder block and now I must make the last course. I will probably finish the top with granite.

Now, the middle course should be solid filled with mortar or concrete? The top course should be made of regular cinder block or the U beam type? I have a lot of cinder block available so if possible I would like to use it instead of the beam type block.

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    I have made retaining walls out of standard cinder block and cement with a few pieces of rebar when I went over 4 high. Since you will be watering I would think you would want to fill the blocks with cement. Cement is stronger than mortar and if the water gets left on the wall will hold up at only 3 high. – Ed Beal Mar 2 '16 at 22:59
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The block will be much more durable and solid if the webs (cavities) are filled with concrete. I'm assuming the blocks are being set in mortar? and not dry set? Mortar can be used as a fill and since the height isn't much you shouldn't see any compromise in strength.

You can float the voids level, but a neater look would be to cap the top blocks with an appropriate and preferred material such as granite.

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The mass of the blocks provides sufficient resistance to overturning if you are going just two courses above grade. To avoid lateral slipping you don't need something with high strength. Usually you can just dry stack blocks for this purpose, especially since you will fill these raised beds with low-clay soil for planting. Just fill the blocks with fill dirt to add additional mass.

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