if all you need to do is build sturdy walls four feet deep in the ground then you can dry stack concrete block and pour concrete in every other cell (the two holes in the normal block). Using wood increases flex and the issue of rot below ground but could be the next best alternative.
Remember, block just like wood is not a true measurement. I think they are a half inch shorter and taller so it adds up, especially over long runs. Start collecting used carpet to lay over the block now.
Edit: I saw Birdman talking about an above ground pond so I reread the intital post...And I agree with him. I was thinking you were going in the ground with this because I have 2 similar ponds but they are in the ground (with bottom drains).
Last edited by lukef; 02-09-2018 at 09:21 AM.
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