how much concrete??

Ok, now I have my pallet wood stripped apart, I've got several cardboard boxes, and several bricks. I plan on putting about 3 bricks per box and six pallet planks per box. How deep should the concrete mix be so that the planks just don't pull out?

I'll quit asking questions after this one. I'm wearing out my fingers trying to do this right. Thanks for the assistance. Pictures to follow.

My guess would be 4 to 6'' and I would notch boards or put some nails in them to help hold the concrete.

I put 8 7foot tall and 2 in. square pieces of wood in a 5 gal. bucket and filled it half way and had to add a cinder block to it to make it sink.

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