The Mechanisms and Effects of Frost Heave
Frost heave is the increase in volume experienced by soils when they freeze. Water moves to the upper horizons from below; when it freezes it forms segregated ice lenses which push apart the soil around them as they grow, causing the observed volume increase. Frost heave has a number of effects upon the soil and upon structures supported by the soil which make it an important process to understand.
During the freezing
of some soils, nearly pure ice forms in segregated lenses
parallel to soil isotherms (Hillel, 1980). The formation of these lenses causes frost heave, a phenomenon in which the surface of the soil is "heaved" vertically by as much as several tens of centimeters. The overall volume of the soil also increases greatly, and heave pressures of many atmospheres can build up (Mitchell, 1993). Frost heave often causes substantial soil disruption (cryoturbation) as well as damage to roads, fence posts, foundations, plants, and other structures within and on top of the soil. In this paper I will examine the mechanisms
and effects of frost heave.
Early studies of frost heave hypothesized that the observed volume growth in the soil was entirely due to the increase in volume that occurs when soil water changes to ice. Experiments by Taber in the 1960's, however, demonstrated that frost heave occurred even in soils saturated in benzene and nitrobenzene, liquids that contract when they freeze (Hillel, 1980). This finding led to a search for the new mechanism, the particulars of which are still being resolved.
Mitchell (1993) specifies three necessary conditions for ice segregation and frost heave to occur:
1. A frost susceptible soil,
2. Freezing temperature, and
3. A supply of water.
Frost heave begins when air with a sub-freezing temperature overlays soil with a temperature above freezing. At this point, a freezing isotherm begins moving through the soil. The exact temperature at which the soil water begins to freeze is determined by several factors, including amount of dissolved cations and particle surface force effects; regardless, ice typically begins to nucleate before the soil reaches -0.2 C (Smtih, 1985). Around the nucleated ice is a film of supercooled water which is gradually frozen and added to the ice mass and then replaced by water from nearby pores in the soil. Rather than freezing water in situ through the bottlenecks of the surrounding pores, which requires a great deal of energy, the ice tends to segregate, drawing water towards the nucleated region and pushing the soil away (Mitchell, 1993). Hillel (1980) writes that experiments by Beskow (1935) revealed that pore saturation had to be greater than 90% in the soil behind the freezing front for heaving to occur. This fact suggests that a great deal of water must move from lower horizons to the upper portion of the soil.
The movement of water from lower horizons to the newly-forming lens is the result of several mechanisms. First of all, the introduction of a temperature gradient in the soil causes water to move up towards the region of colder temperature (Dash, 1989). This movement occurs because the warmer water has a higher vapor pressure; that is, more vapor is forming and moving up from below than is moving down from above. This extra vapor soon arrives at the freezing front and changes from vapor to water to ice (Durbin, pers. comm.). The second cause of the movement of water from below is osmosis. Because the ice forms as a nearly pure phase, the unfrozen water around the ice develops high concentrations of ions. Water then moves from the area of lower ion concentration (below) to higher (the freezing front) to even the concentrations out. Thirdly, when water from the film at the edge of the ice freezes, tension at the ice-water interface "pulls" water from below to replenish the film (Mitchell, 1993).
Fed by the flow of water from lower in the soil, ice lenses from millimeters to centimeters thick form. The soil around the lenses becomes dessicated by the segregation of the water but does not lose an appreciable amount of volume; thus the combined volume of the lenses is almost exactly equal to the volume increase of the soil (Hillel, 1980). The growth of ice lenses is controlled by the supply of water and the rate of movement of the freezing front through the soil. If the freezing front moves too quickly, the soil water freezes before it has a chance to accumulate in lenses. This rate is affected by several factors, including the temperature differential between the air and the soil and the amount of snow and vegetation, both of which insulate the soil and thus impede the progress of the freezing isotherm. The latent heat of fusion produced when water becomes ice plays a critical role in slowing down the isotherm by warming the soil around the lens. This heat is often enough to cause temporary steady-state conditions during which the lens can continue to grow (Mitchell, 1993). When the soil around the lens becomes dessicated to the point that the remaining water is too tightly held by soil grains to be moved to the lens, the lens loses its water supply. Freezing, the heat source which creates the steady-state conditions, then ceases and the freezing front moves downward. Subsequent lenses often form deeper in the soil at a level at which there is again sufficient water to supply a growing lens. Because the thermal gradient decreases deeper in the soil, the freezing isotherm takes longer to move through the soil, leading to the formation of larger lenses at depth. These larger lenses cause a great amount of dessication of the surrounding soil, requiring successively greater distances between lenses until there is sufficient water supply. Thus, vertical distance between lenses also increases with depth.
This discussion of the mechanisms of frost heave suggests that certain soils are more susceptible to heaving than others. Fine-grained clays conduct water too slowly to supply a growing ice lens, while sandy soils, due to their large pore size, are poor upward conductors of water. Thus silts, which have moderate pore size, are best at providing a steady supply of water to growing lenses of ice and are most susceptible to frost heave (Mitchell, 1993).
Frost heave affects soils greatly. Small lateral differences in snow cover, soil texture, vegetation, and topography can lead to differences in the amount of heave experienced by regions in the soil. Differential heaving causes layers to be displaced varying distances, leading either to the formation of wavy boundaries, or, in extreme cases, to the destruction of horizon boundaries altogether. At the surface, differential heaving often forms a pattern of circular bulges with depressions between them. These small bulges are better-drained than the depressions, and they thus retain their heat longer during cold spells. Frost heave then begins in the depressions first, causing lateral pressure towards the centers of the bulges. This pressure displaces more soil and pushes the bulges higher, forming hummocks, circular mounds roughly 1-2 meters in diameter and up to .5 meters high. Hummocks are the most common type of ground pattern caused by frost heave, and they are a tell-tale marker of a soil prone to heaving (Clark, 1988).
Frost heave can also lead to the movement of clasts within the soil. When a freezing front encounters a clast in the soil, it bonds the clast to the soil. As heaving progresses in the soil around the clast, a cavity is opened above the clast. The adfreeze strength of the bond between the frozen soil and the clast becomes great enough to lift the clast when the front is between 30 and 50% of the way down the clast. At this point, the clast begins to be heaved with the soil above it. A cavity opens in the unfrozen soil beneath the clast and is partially infilled with unfrozen soil that is perturbed by the upward movement of the clast. When the soil is thawed, the clast drops down, but does not return to its original position because of the partial infilling of the cavity beneath it. This process is repeated in every freeze-thaw cycle and leads to the progressive upward migration of clasts through the soil. The amount of vertical motion depends upon the heaving strain in the soil (i.e. the amount of vertical expansion experienced by the soil), the length of the clast not yet adfrozen to the soil when clast heave begins (which in turn depends upon clast size, the strength of the adfreeze bond, and the shear strength of the unfrozen soil beneath the clast), and the amount of infilling of the cavity beneath the heaved clast (Anderson, 1988). Experiments by Anderson (1988) have found that in the course of seven freeze-thaw cycles, a 9x4x4 cm clast can be heaved over 12 cm. Though these experiments were under idealized conditions, used a very large clast size, and had some errors, they do serve as a indication of the order of magnitude of upfreezing that can be expected. Upfreezing can cause many problems for farmers, in that it brings rocks to the surface of the soil and often separates plants from their roots (Hillel, 1980). It can also help to explain the presence of gravel-sized clasts in the middle of loess-derived soils. Archeologists have also begun to study upfreezing, fearing that it might cause artifacts to be stratigraphically displaced in the soil (Anderson, 1988).
The increase in soil volume caused by frost heave also has many important effects. Dash (1989) reports the Committee on Permafrost of the Polar Research Board's (1984) findings that the pressure exerted by ice lenses during their formation has been known to damage roads, displace foundations, and crack masonry and pipelines. Snow removal from roads promotes further damage, because the snow insulates the soil beneath the road and thus minimizes heaving (Hillel, 1980). Within the soil, lower horizons often undergo compaction due to the downward pressure caused by lens formation (Smith, 1985).
The final major effect of frost heave occurs during seasonal thawing. A great deal of water accumulates in the upper soil horizons when ice lenses form. During thawing, the upper portion of the soil melts first. Because the bottom layers are still frozen at this point, the melt water cannot drain. The soil becomes saturated and loses most of its strength. When soils supporting roads, fence posts, foundations, and other structures lose strength in this manner, the roads develop potholes while the fence posts and foundations can often become skewed (Hillel, 1980). Thawing areas on slopes are also susceptible to landsliding.
Frost heave can thus be seen to be an extremely dynamic and powerful process. Its many effects upon soil and the structures soil supports necessitate an understanding of the mechanisms which cause heaving. Although the water supply to most of Northfield's soils might not be sufficient to cause a great deal of frost heave, moderate amounts might be expected in this area. It would thus be interesting to explore the ways in which certain structures in the soils of this region might be explained through frost heaving.
Anderson, Suzanne Prestrud, 1988, The upfreezing process: Experiments with a single clast: Geological Society of America Bulletin, v. 100, p. 609-621.
Clark, M.J. (Ed.), 1988, Advances in Periglacial Geomorphology: Chichester, John Wiley & Sons, 481 p.
Dash, J.G., 1989, Thermonuclear Pressure in Surface Melting: Motivation for Frost Heave: Science, v. 246, p. 1591-1593.
Durbin, Steve, Personal Communication on 10/12/96.
Hillel, Daniel, 1980, Applications in Soil Physics: New York, Academic Press, Inc., 385 p.
Mitchell, James K., 1993, Fundamentals of Soil Behavior: New York, John Wiley & Sons, 543 p.
Smith, M.W., 1985, Observations of soil freezing and frost heave at Inuvik, Northwest Territories, Canada: Canadian Journal of Earth Science, v. 223, p. 283-290.