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Hi,
Your only concern will be the corners. This is where the load will be the greatest. You have to stack it "log home" style with each layer overlapping the one previous.
You can either use rebar that would slide in pre-drilled holes in the corners. But you'll have to have either a 48" drill bit and go real real straight, or drill each one as you stack it, and make sure you line it up perfectly.
The other option is "tie locker" screws. They come in lengths from 4" to 12" and they're simular to a lag bolt, but have a very aggressive thread for about 3" and then they're smooth shanked which really helps them suck your material together. I've used both ways building retaining walls and IMO the screws are far superior. With the rebar you only have the rebar as support where as with the screws you have all the friction between the lumber supporting the shank of the screw. Also, the rebar thickness required would be at least 1/2" (5/8" would be better) and even if you drill right in the middle of the material you're only left with a little less than an 1 1/2" on either side for support. (unless you use structural lumber which would be a true 4")
I built a pond simular to that out of landscaping ties (2.75 X 3.5) with the rounded sides. They have wayyy less structural strength but they're cheap as heck and I dug down 36" as well, so my weight was against the earth. Having said that, I used 10" tie lock screws so when I drove one in it was clamping down 3 ties at a time. I still put in 4 screws per tie so it's probably quite overbuilt. However, nothing has moved an inch to date. I predrilled each one with a thin bit, but I think the ties would split easier than proper 4X4's. On the ends just drive the screw down straight so you catch all three right at the corner and shouldn't be a problem.
I guess if you're really concerned a person could put 3" angle iron and case (vertically) each corner. Just drill some hole for screws and voila. Extra bomb proof.
Have fun!
Grant
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