Protect well casing with concrete slab

[Rasmus]

I intend to pour a 3 ft. by 5 ft. concrete slab around my well casing. My initial plan was to pour 4 in. of concrete over a 3 in. gravel base, with a vapor barrier and rebar between the concrete slab and this base. However, the well is located in northern Minnesota, so I am concerned about the possibility of frost heave. If frost heave does occur with the slab, could it damage the well's cast-iron casing? And if so, should I allow space between the casing and the concrete slab? Should the space itself be filled with something, perhaps a polymer of some kind?

Thank you for considering the above questions.