BillSD, my pond isn't a see or even a lake. When ice is melting, I haven't seen drifting ice slabs. Everything happens slowly. So, it's not a danger. By the way, we don't have such thick ice as you. Like I said, ~0,50 - 0,60 m is considered to be a thick ice.
There are only 2 things to worry about:
1)ice could lift the whole system - I hope that concrete base is big and heavy enough to provide stability. The weight of it should be sufficient to avoid that.
2)concrete will start crumbling in water level. You know how does it happen. Water gets into concrete in autumn and after freezing its molecules increase their size, ruining concrete. But this happens slowly and concrete chimney can be repaired.
