Frost heaving of soils

Frost heaving of soils arises due to the increase in volume of freezing moisture and the accumulation of ice (owing to water migration) at freezing. This process is widely developed both in the permafrost regions and in the regions of deep seasonal freezing of ground. The largest deformations due to heaving are observed in the freezing of an open system of highly permeable, usually sand-silty soils and saturated silty-sandy-clays at low rates of freezing and in the proximity of ground water (migration mechanism of heaving).

Often, heaving is also associated with freezing of coarse-grained soils in closed system conditions which arise in confined ground water-bearing horizons (intrusion mechanism of heaving). Usually, two kinds of heaving are distinguished, local and area. Area heaving is most often characterized by a high nonuniformity over the area as the magnitude of heaving within the limits of one landscape type can exceed by a factor of two the average value of heaving. Local heaving manifests itself more distinctly in the relief (mounds, hillocks, patches of heaving and other forms) which arises due to unsteady conditions of freezing, different water contents, soil composition and other factors of geological and geographical environments. In general, under natural conditions, heaving of soils may be associated with seasonal freezing of the seasonally freezing-thawing layer (seasonal heaving) and with perennial freezing of ground (long-term heaving).

Perennial heaving

Heaving of ground is the most widely developed cryogenic process, resulting in the formation of frost mounds differing in shape and size, on fine-grained soils and peat bogs. As a result of peat bog freezing, for example, in waterlogged depressions so-called 'inverse' relief is often formed rising for several metres over the surrounding surface. Local long-term heaving is always accompanied by formation of mounds reaching several metres in height and hundreds of metres in diameter. According to their formation local mounds are subdivided into segregation and intrusion forms, but a mixed intrusion-segregational formation is also possible.

The segregation-heaving type of mounds, referred to as palsas in the foreign literature, are formed as a result of soil moisture migration towards the freezing front under the influence of gradients of both temperature and moisture content. The local nature of their occurrence is confirmed by the fact that they are formed in particular places where, under the combined influence of a number of natural factors, more rapid and deep freezing of the soil occurs and where a lens of permafrost is beginning to form. It is here that migrating moisture begins to arrive from the surrounding unfrozen soil, causing heaving of the ground and elevation of the surface at the site. In winter snow is blown off the elevated surface; thus, the temperature of the ground is lowered, while the thickness of the lens of permafrost continues to increase relative to the surrounding sites, leading accordingly, to the enlargement of the mound. The growth rate of such mounds in the north of West Siberia during the initial stage reaches 10-30cmyr_1, then, with the bigger permafrost core and enlarged mound the growth rate is reduced to l-2cmyr_1. The height of such mounds can reach 20m with a horizontal area ranging from several tens of metres to hundreds of metres dictated by their age and conditions of formation.

As a rule, long-term mounds of heaving of the segregation type accompany new formation of permafrost on unfrozen sites on flood-plains and limnotic basins (Fig.5.1a); often chains of mounds arise along the banks of lakes, over shallow water and on boggy areas.

It is often the case that mounds are confined to sites of peat deposits. This is associated with the presence in peat of a great amount of moisture which means that the thermal conductivity of the frozen peat is higher than that of unfrozen peat, and convex sites covered with peats are cooled more in winter than they are warmed in summer. Therefore, freezing is more intense in peats than in mineral soil which promotes development of local heaving and formation of peat frost mounds.

Long-term frost mounds of the intrusion type usually arise through injection of water (or fluidized soil) under the influence of the hydrostatic pressure that develops in closed systems in the course of freezing. They are mainly associated with freezing of sublacustrine (flooded) closed taliks surrounded on all sides by permafrost. In the North and in Central Yakutia they are called bulgunnyakh, or pingos in the foreign literature. Freezing of sublacustrine taliks is usually caused by shallowing or emptying of lakes. As a result the sublacustrine talik begins to freeze on each side, thus decreasing in size. The enclosed water is subject to hydrostatic pressure. Owing to this pressure the frozen roof warps in the thinnest place, forming a frost mound with a core of injection ice or of an ice and soil mixture (see Fig. 5.1b). Since freezing of a water-bearing talik is a long-term process, intrusion of water into the growing bulgunnyakh occurs many times. Along with this, the segregation type of ice formation can take place, leading to the complex structure of bulgunnyakhs.

The size of bulgunnyakhs is dependent on the amount of water in the closed taliks; they can be 30-60 m high, and 100-200 m horizontally. Often, another type of injection mound, the hydrolaccolith, arises when water is injected under the influence of the hydrodynamic pressure of subpermafrost and supra-permafrost water. Hydrolaccoliths are confined to sources (places of discharge) of ground water or to closed taliks, including those in flood-plains mainly in the southern zone of permafrost.

Seasonal heaving of ground

Such heaving accompanies seasonal freezing of nonpermafrost soils and freezing of the seasonally thawing layer over permafrost. The average heaving of a seasonally thawing layer is usually a factor of 1.5-2 less than

Cryogenic geological processes and phenomena a

Fig. 5.1. Perennial frost mounds (photo G.I. Dubikov): a - migration-heaving peat mound, b - degrading heave mound (pingo).

Fig. 5.1. Perennial frost mounds (photo G.I. Dubikov): a - migration-heaving peat mound, b - degrading heave mound (pingo).

that of the seasonally freezing layer. This is associated with the fact that the seasonally freezing layer is most often an open system and its freezing is accompanied by an active migration of moisture. Freezing of the seasonally thawed layer is more similar to that of a system of closed type because moisture migration out of the underlying frozen soil does not take place, with only vertical migrational redistribution within the layer and lateral to it - necessarily with nonuniform freezing. As shown by observations of the annual cycles of deformation, on freezing the ground surface rises in relation to an immobile bench mark, due to soil heaving as the thickness of the freezing layer increases, and reaching maximum elevation at the moment of maximum freezing. With the beginning of thawing the surface again sinks due to thawing of layers of segregation ice until it reaches the zero elevation of the bench mark. Such movement of the ground surface is also called hydrothermal.

Seasonal heaving of soils over an area is characterized by a high nonuniformity. An extreme case of expression of this nonuniformity is the formation of one-year (or seasonal) migrational frost mounds which exist only during the cold season of the year and disappear during thawing of the frozen layer. The width of these mounds can reach a few metres, and the uplift above the surrounding surface about 0.2-1.5 m. Intrusion seasonal frost mounds are usually associated with nonuniform freezing of soil of the seasonally thawing layer, resulting in development of hydrostatic pressure in the water moving above the permafrost. Such a phenomenon is most often encountered at the foot of slopes.

Manifestations of the seasonal heaving process are also observed in fine soil with inclusions of larger rock debris (rock waste, pebbles, blocks, boulders, etc.). Thermal conductivity of rock debris is higher than that of fine soil; accordingly, fine soil beneath the rock fragments is chilled more intensely and water begins its migration primarily towards the front of freezing there. When water is transformed into ice it uplifts and pushes out such rock debris. At seasonal thawing a rock fragment does not manage to return to its place as this space is already occupied, partially or completely, by water and fine soil, while the rock fragment itself is held by the surrounding ground. Multiple repetition of the cycle results in heaving (freezing out) of rock material with sorting (redistribution) of debris within the seasonally freezing layer: the upper part of the section is enriched with coarse material, on the ground surface. This is the way 'boulder fields' and 'boulder streams' are formed.

Development of this process in soil masses where freezing is accompanied by the formation of diagenetic fractures or frost cracks, with freezing along their walls, results in part of the coarse material being shifted with the formation of 'stone polygons' and 'stone rings' over the surface (Fig. 5.2a). On gently sloping hillsides in paragenesis with other slope processes 'stone polygons' are formed elongated downslope, and at gradients over 8-10 C stone stripes appear (see Fig. 5.2b).

When piles, poles or pipelines are placed into the seasonally thawing or seasonally freezing layer, they are gradually heaved (frozen out). As a result of the annual repeating of this process poles are pushed out, become less stable, tilt and, in the long run, fall (Fig. 5.3). Heaving of poles leads to disturbance of communication lines, while piles pushed out of the foundations of different structures cause their deformation.

Fig. 5.2. Cryogenic sorting of fragments within the seasonally thawing layer: a - stone rings in Spitsbergen (photo A. Jahn), b - stone stripes on a gently sloping hillside in the Sopky range in the Urals (photo G.I. Dubikov).
Frost Heave Rocks Farm Images

Fig. 5.3. Diagram of heaving (freezing out) of a pole extruded from a seasonally thawing layer composed of wet fine-grained deposits (from I.D. Belokrylov). I-III - stages of extrusion of the pole in an annual cycle, IV- falling of the pole out of the seasonally thawing layer after a series of years; 1 - thawed soil of the seasonally thawing layer, 2 - perennially frozen soil, 3 - frozen soil of the seasonally frozen layer, 4 - cavity, which is formed by withdrawal of the pole by the freezing to it of the soil of the seasonally thawing layer, and is filled with ice or ice-rich soil, 5 - cavity filled with water saturated soil at thawing, 6 - boundary of perennially frozen soil. Ah - pushing out of the pole; H - soil heaving in an annual cycle.

Fig. 5.3. Diagram of heaving (freezing out) of a pole extruded from a seasonally thawing layer composed of wet fine-grained deposits (from I.D. Belokrylov). I-III - stages of extrusion of the pole in an annual cycle, IV- falling of the pole out of the seasonally thawing layer after a series of years; 1 - thawed soil of the seasonally thawing layer, 2 - perennially frozen soil, 3 - frozen soil of the seasonally frozen layer, 4 - cavity, which is formed by withdrawal of the pole by the freezing to it of the soil of the seasonally thawing layer, and is filled with ice or ice-rich soil, 5 - cavity filled with water saturated soil at thawing, 6 - boundary of perennially frozen soil. Ah - pushing out of the pole; H - soil heaving in an annual cycle.

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Responses

  • everett
    Why does the ground heave when frozen?
    6 months ago
  • petros
    Does frost heave uncompact soil?
    20 days ago

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