How should I turn the wall of oak sleepers?

[anders07]

I am in the process of building a retaining wall with standing oak sleepers measuring 140x100mm.



Now I plan to curve the wall instead of having 90-degree corners.
What's the smartest way to do that?
Wedges in between, or just stainless steel strap?

I fasten them together with 240x8mm screws.