Frost heaving (or a frost heave) results from ice forming beneath the surface of soil during freezing conditions in the atmosphere. The ice grows in the direction of heat loss (vertically toward the surface), starting at the freezing front or boundary in the soil. It requires a water supply to keep feeding the ice crystal growth; and the growing ice is restrained by overlying soil, which applies a load that limits its vertical growth and promotes the formation of a lens-shaped area of ice within the soil. Yet the force of one or more growing ice lenses is sufficient to lift a layer of soil, as much as 30 cm or more. The soil through which water passes to feed the formation of ice lenses must be sufficiently porous to allow capillary action, yet not so porous as to break capillary continuity. Such soil is referred to as "frost susceptible." The growth of ice lenses continually consumes the rising water at the freezing front.[1] [2] Differential frost heaving can crack pavements[3] and damage building foundations.[4]
Needle ice is essentially frost heaving that occurs at the beginning of the freezing season, before the freezing front has penetrated very far into the soil and there is no soil overburden to lift as a frost heave.[5]
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According to Beskow, Urban Hiäme described frost effects in soil in 1694.[5] Taber disproved the hypothesis that frost heaving results from molar volume expansion with freezing of water already present in the soil prior to the onset of subzero temperatures, i.e. with little contribution from migration of water within the soil.
Since the molar volume of water expands by about 9% as it changes phase from water to ice at its bulk freezing point, 9% would be the maximum expansion possible owing to molar volume expansion, and even then only if the ice were rigidly constrained laterally in the soil so that the entire volume expansion had to be taken up vertically. Ice is unusual among compounds because it increases in molar volume from its liquid state, water. Most compounds decrease in volume when changing phase from liquid to solid. Taber showed that the vertical displacement of soil in frost heaving can be significantly greater than that due to molar volume expansion.[1]
Taber demonstrated that liquid water flows towards the freeze line within soil. He showed that other liquids, such as benzene, which contracts when it freezes, also produce frost heave.[6] This ruled out molar volume changes as the dominant mechanism for vertical displacement of freezing soil. His experiments further demonstrated the development of ice lenses inside columns of soil that were frozen by cooling the upper surface only, thereby establishing a temperature gradient[7][8][9]
The dominant cause of soil displacement in frost heaving is the development of ice lenses. During frost heave, one or more soil-free ice lenses grow, and their growth displaces the soil above them. These grow with the continual addition of water from a groundwater source lower in the soil structure, below the freezing line in the soil. The presence of frost-susceptible soil with a pore structure that promotes capillary flow is essential to delivering water to the ice lenses, as they form.
Water can arrive at the forming ice lens at a temperature that is below the bulk freezing point, owing to the Gibbs-Thomson effect of confinement of liquids in pores. Very fine pores have a very high curvature, and this results in the liquid phase being thermodynamically stable in such media at temperatures sometimes several tens of degrees below the bulk freezing point.[10] Another water-transport effect is the preservation of a few atomic layers of liquid water on the surface of the ice lens, and between ice and soil particles. Faraday reported on the unfrozen layer of premelted water in 1860." [11] Ice premelts against its own vapor, and in contact with silica.[12]
The same intermolecular forces that cause premelting at surfaces contribute to frost heaving at the particle scale on the bottom side of the forming ice lens. When ice surrounds a fine soil particle as it premelts, the soil particle will be displaced downward towards the warm direction within the thermal gradient due to melting and refreezing of the thin film of water that surrounds the particle. The thickness of such a film is temperature dependent and is thinner on the colder side of the particle.
Water has a lower free energy when in bulk ice than when in the supercooled liquid state. Therefore, there is a continuous replenishment of water flowing from the warm side to the cold side of the particle, and continuous melting to re-establish the thicker film on the warm side. The particle migrates downwards toward the warmer soil in a process that Faraday called "'thermal regelation."[11] This effect purifies the ice lenses as they form by repelling fine soil particles. Thus a 10-nanometer film of unfrozen water around each micrometre-sized soil particle can move it 10 micrometres/day in a thermal gradient of as low as 1°C km−1.[12] As ice lenses grow, they lift the soil above, and segregate soil particles below, while drawing water to the freezing face of the ice lens via capillary action.
Frost heaving requires a frost-susceptible soil, a continual supply of water below (a water table) and freezing temperatures, penetrating into the soil. Frost-susceptible soils are those with pore sizes between particles and particle surface area that promote capillary flow. Silty and loamy soil types, which contain fine particles, are examples of frost-susceptible soils. Many agencies classify materials as being frost susceptible if 10 percent or more constituent particles pass through a 0.075 mm (No. 200) sieve or 3 percent or more pass through a 0.02 mm (No. 635) sieve. Chamberlain reported other, more direct methods for measuring frost susceptibility.[13]
Non-frost-susceptible soils may be too dense to promote water flow (low hydraulic conductivity) or too open in porosity to promote capillary flow. Examples include dense clays with a small pore size and therefore a low hydraulic conductivity and clean sands and gravels, which contain small amounts of fine particles and whose pore sizes are too open to promote capillary flow.[14]
Frost heaving creates raised-soil landforms in various geometries, including circles, polygons and stripes, which may be described as palsas in soils that are rich in organic matter, such as peat, or lithalsa[15] in more mineral-rich soils.[16] The stony lithalsa (heaved mounds) found on the archipelago of Svalbard are an example. Frost heaves occur in alpine regions, even near the equator, as illustrated by palsas on Mount Kenya.[17]
In Arctic permafrost regions, a related type of ground heaving over hundreds of years can create structures, as high as 60 metres, known as pingos, which are fed by an upwelling of ground water, instead of the capillary action that feeds the growth of frost heaves.
Polygonal forms apparently caused by frost heave have been observed in near-polar regions of Mars by the Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor and the 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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