I tend to agree with the contractor, and think that ice lifting will be a non-issue.
Yellow Pine weighs somewhere around 37#-41# per cubic foot (roughly one 2x12x8). How many cubic feet of lumber will the bridge be made from?

You will have 10 sq. ft. of interface between the 4x6's and the ice if the ice is 18" thick.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).