Frost Heaving of Pilings

Freezing soil can create such great upward forces on telephone poles, bridge pilings and similar objects that they are literally lifted out of the ground. Several factors control the amount of frost heaving and the consequent lifting force on a piling.

Frost heaving is greatest in wet, fine-grained soils--clays and silts--since they undergo the greatest expansion of their volume as they freeze. The expansion is not caused only by the freezing of the water contained in the soil but also by the freezing of new water that migrates upward from below the frost line during and after the freezing of the soil.

The colder the ground surface, the greater is the transport of water upward. Freezing of this new water can create layers of pure ice within the top layers of the soil, even after the soil has become frozen. Consequently, frost heaving can continue right on through the winter if there is an adequate supply of water from below.

Russian experiments have shown that some clays, as they freeze, can nearly double in volume, especially if saturated with water rich in aluminum, iron or calcium. These elements help foster the upward migration of water as the soil freezes.

Upward frost heaving pressure on the bottom of a piling set in wet, fine-grained soil can easily be 60 pounds per square inch (4 kilograms per square centimeter). Similar forces act on the sides of the piling if the soil can form a bond to the piling as it freezes. If the temperature drops well below freezing, the upward forces become even larger.

Thus, a piling twenty inches (50 centimeters) in diameter set four feet (120 centimeters) into soil that freezes at least that deep will be lifted upward with a total force of approximately 200,000 pounds (90,000 kilograms). Ninety percent of this force is on the sides rather than the bottom of the piling. It is just about enough force to lift an engine of the Alaska Railroad. In one year, a piling may be lifted upward a foot (25 centimeters) or more, though in most instances the heaving is less. If conditions are favorable to continued heaving year after year--repeated freezing and thawing of the soil wherein the piling is set plus plenty of available ground water--the piling eventually will be lifted entirely out of the ground.