6-inch (150mm) concrete blocks willbe ample for these walls. Both the 18-inch and 36-inch flavours. Use a foundation trench 12-inces wide and 6-inches deep, fill with concrete as per our mixing concrete page where you can also work out volumes of materials required. https://www.diydoctor.org.uk/projects/mi ... ncrete.htm
Lay the blocks upright (no need to slant) but consider how you willfinish the top of the wall as you cannot lay turf onto the topof the block so you will need to build the block higher than the garden surface and put some sort of capping onto it. See our page on laying bricks and blocks where you will also find some information on laying coping stones https://www.diydoctor.org.uk/projects/la ... blocks.htm
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