hi, the sleepers are interlocked on the corners and long bolts in the corners and 2 on each side.
the weight of the sleepers hold them and there is no bowing after 8 months.
i used a router on the sleepers and painted.
the sleepers are 8x4

i love the pond out of wood, great idea. what is the size of the wood? how is it ancored to the ground, do you have bolts through them. have you noticed any bowing in the wood. i like how you have made the wood look like blockwork.