Definitions

frost-heave

Frost heaving

Frost heaving (or frost heave) occurs when soil expands and contracts due to freezing and thawing. This process can damage plant roots through breaking or desiccation, cause cracks in pavement, and damage the foundations of buildings, even below the frost line. Moist, fine-grained soil at certain temperatures is most susceptible to frost heaving.

Cause

Originally, frost heaving was thought to occur due simply to the freezing of water in soil. However, the vertical displacement of soil in frost heaving can be significantly greater than the expansion that occurs when ice freezes. In the 1960s, frost heaving was demonstrated in soil saturated in benzene and nitrobenzene, which contract when they freeze.

Frost creep, an effect of frost heave, involves a freeze-thaw action allowing mass movement down slope. The soil or sediment is frozen and in the process moved upward perpendicular to the slope. When thaw occurs the sediment moves downwards thus mass movement occurs.

The current understanding is that certain soil particles have a high affinity for liquid water. As the liquid water around them freezes, these soils draw in liquid water from the unfrozen soils around them. If the air temperature is below freezing but relatively stable, the heat of fusion from the water that freezes can cause the temperature gradient in the soil to remain constant. The soil at the point where freezing is occurring continues to draw in liquid water from the soils below it, which then freezes and builds up into an "ice lens". Depending on the soil's affinity for moisture and amount of moisture available, a significant amount of soil displacement can result.

The earliest known documentation of frost heaving came in the 1600s.

Susceptible soil types

Three conditions are generally necessary for frost heaving to occur:

  • freezing temperatures
  • a supply of water
  • a soil that has:
    • the ability to conduct water
    • a high affinity for water
    • saturation (i.e. the pore spaces are filled with water)

Silty and loamy soil types are susceptible to frost heaving. The affinity of a soil for water is generally related to the surface area of the particles that it is composed of. Clays have a high ratio of surface area to volume and have a high affinity for water. Larger particles like sand have a lower ratio of surface area to volume and therefore a low affinity for water.

Conversely, the hydraulic conductivity of a soil is related to the pore size. Soils composed of very small particles like clay have small pores and therefore low hydraulic conductivity. Soils composed of larger particles like sand have larger pores and a higher hydraulic conductivity.

The offsetting nature of these two requirements mean that clayey and sandy soils are less conducive to frost heaving than silt, which has a moderate pore size and moisture affinity.

Frost creep: Soil locomotion due to frost heave

Frost creep, an effect of frost heave, involves a freeze-thaw action allowing mass movement down slope. The soil or sediment is frozen and in the process moved upward perpendicular to the slope. When thaw occurs the sediment moves downwards thus mass movement, or locomotion, occurs.

Structures created by frost heaving

In Arctic regions, frost heaving for hundreds of years can create structures, known as pingos, as high as 60 metres. Frost heaving is also responsible for creating stones in unique shapes such as circles, polygons and stripes. A notable example is the remarkably circular stones of the islands of Spitsbergen.

Polygonal forms caused by frost heave have been observed in near-polar regions of Mars by the high-resolution HiRISE camera on the Mars Reconnaissance Orbiter. In May 2008 the Mars Phoenix lander touched down on such a polygonal frost-heave landscape and quickly discovered ice a few centimetres below the surface.

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