Depending on where it happens, frost heave is unsightly and possibly dangerous. Buildings erected in cold climates are prone to frost heave difficulties. In general, a cold weather blast from Mother Nature creates difficulties for anything on or in the ground. Knowing what causes frost heave and taking precautions to avoid it will keep your property looking its best.
What is Frost Heave?
The simple definition of frost heave is frozen ground expanding from the cold. It is caused by the freezing of the moisture in the soil. Water expands when it freezes and pushes everything around it to make room. The soil, and everything in it--including posts, rocks and plants--moves, shifts and pushes up.
Frost heave damages buildings in the worst way: from the ground up. Buildings require careful construction practices in frost zones to keep their foundations in place. Other frost heave problems are in decorative construction. That type of frost heave is more unsightly than dangerous, although an uneven path is a danger to those walking on it.
The Frost Line
The frost line is different in each area. The depth of the penetration of cold varies, but a safety line is 4 feet for any area where the temperatures drop below freezing for more than a few hours at a time. The frost line is most important in erecting buildings. Foundations and corner posts need to have a 4 foot depth in the ground to prevent them from heave. Secondary damage from frost heave occurs in the spring when frozen ground thaws. The structures that were pushed up now fall back into place. This is most destructive to concrete or other solid materials. The flexing causes cracks and breaks.
The two principle causes of frost heave are a general freezing of a water soaked soil, and frozen cells or pockets of water in shallow soil. The frozen pockets happen most in soils with moisture permeable materials like rocks or wood fillers.
Plant all foundation posts or walls at least four feet below the surface in cold climates. Provide a deep bed for all decorative structures such as walkways, paths, or stone borders. Tamp the bed firmly to make a solid, heavy base for decorative features so there are fewer areas for water pockets to form beneath the soil.