Frost Heave

Frost heave is the upward movement of soil caused by freezing groundwater expanding as it turns to ice. In Iowa, where frost lines can reach 42–48 inches, posts set at insufficient depth heave progressively — tilting, cracking footings, and eventually requiring replacement.

Posts should be embedded below the frost line — 42–48 inches minimum for gate posts set in concrete in central Iowa. Thrive drives line posts to 39–40 inches and uses concrete bell footings on gate posts. The flared bell footing shape resists frost forces more effectively than straight cylindrical footings.

Gate posts carry the weight and leverage forces of the gate in addition to frost pressure. This extra load, combined with the mechanical advantage that frost forces have on a straight post, makes gate posts significantly more vulnerable to heaving. Bell footings specifically address this by creating a mechanical interlock that resists the upward force.