I would like to hear more evidence of the threat on your project.
Some food for thought.
It is well known that water expands as it freezes. Just check your ice cube tray in your freezer sometime; if you still have one. On my farm (clay loam soil) it is very common for small tree seedlings planted in the fall, to be lifted out of the ground by the seasonal frost; sometimes an inch or more. Have you ever heard farmers say they felt like they were growing rocks when talking about some of there fields? It comes from the frost "heaving" up the stones which may be partially surfaced. If the frost gets deep enough to get under them it will heave them up a little each season due to the expantion of wet/frozen soil.
The same thing happens in the pond only if the piling is straight up and down there is not much lift just due to freezing because most of the expansion goes into a clamping action in the horizontal direction against the piling. However the result is sort of like floating your dock, or in my case, the bridge on top of the ice, sense the ice would have a solid hold on the pilings. Thus, any increase in the pond water elevation during the winter season which would raise the ice slab is likely to lift the structure with it. (Depending on the ice thickness)
In case you are thinking that the weight of the bridge could not be lifted or supported by an ice slab, I have driven my 1400 lb tractor on 4" if good ice with no problem. (I don't recommended it however).
