A frost heave is a geological phenomenon where the ground swells upward due to the freezing of water within the soil. This swelling is common in cold climates, particularly when temperatures fluctuate around the freezing point. The process results in noticeable vertical movement of the ground surface. The mechanism is complex, relating specifically to the movement and accumulation of water in certain soil types, rather than simple expansion.
The Necessary Conditions for Formation
A frost heave requires the simultaneous presence of three specific environmental factors. The first is a temperature regime that includes a freeze-thaw cycle, where temperatures drop low enough for the freezing front to penetrate the soil surface. Sustained, fluctuating temperatures can worsen the effect over time.
The second condition is the availability of water, typically groundwater or saturated soil conditions, which supplies the moisture necessary for ice growth. Without a continuous supply of water, the phenomenon cannot progress. The third factor is a frost-susceptible soil type, specifically those containing a high percentage of fine particles, such as silt or clay. These fine-grained soils facilitate capillary action.
Soils composed mainly of coarse materials like clean sand and gravel are generally not susceptible because they drain water too freely. Their larger pore spaces prevent capillary forces from drawing water toward the freezing zone.
How Ice Lenses Cause Ground Lifting
The actual lifting of the ground is not caused by the nine percent volume expansion of water when it turns into ice. Instead, the upward force is driven by the formation and growth of structures called ice lenses. This process begins as the freezing front moves downward from the surface.
As the temperature gradient pushes the freezing front deeper, it draws unfrozen water from the soil below through capillary action. This action is the movement of liquid through narrow spaces against gravity. The fine pores in frost-susceptible soils create the suction necessary to pull water upward toward the sub-freezing temperatures.
When this water reaches the freezing front, it accumulates and freezes into a distinct, horizontal, lens-shaped layer of relatively pure ice. This layer, the ice lens, grows thicker by continuously drawing up more water from the unfrozen soil below. The growth of these lenses creates tremendous upward pressure, pushing the soil layer above it.
The continuous supply of water allows the ice lens to grow much larger than the initial volume of water contained in the frozen soil. Multiple ice lenses can form at different depths, increasing the total amount of heave and sometimes lifting the ground by a foot or more.
Observable Impacts of Frost Heaves
The force generated by growing ice lenses causes visible damage to both natural landscapes and human-built structures. A common impact is the distortion and cracking of roadways, sidewalks, and paved areas. This differential heaving, where one section lifts higher than an adjacent section, contributes to the formation of springtime potholes when the ice melts and leaves unsupported material.
Shallow foundations and concrete slabs, such as those under garages or porches, are particularly vulnerable to this upward pressure. The shifting soil beneath a foundation can cause the structure to crack, as movement is often uneven across the surface. Objects embedded in the ground, like fence posts, utility poles, and retaining walls, may be visibly lifted out of their original position.
In agricultural settings, frost heaves can severely affect plants, a phenomenon sometimes called “frost pull.” As the ground heaves, it can lift young plants or their root systems out of the soil, exposing them to the cold air and potentially killing them. The repeated cycle of freezing and thawing also disrupts the overall structure of the soil.