The freezing (thawing) of soils is accompanied by complicated physico-chemical processes, the nature and intensity of which are essentially different depending on whether migration of water occurs in the material or

Physico-chemical and mechanical processes in freezing soils

When ground freezes without water migration, then the free or semibound water in the pores increases its volume by about 9% as it turns into ice. If the pores are filled with water at G = 1, then the freezing ground expands volumetrically and local densifications or particle aggregates appear, caused by considerable crystallization pressure as water turns into ice. For example, if the volumetric expansion is obstructed, the freezing water may reach a pressure of 2200 Mpa. Between the large soil aggregates, experiencing the greatest densification (compression) are the largest pores where water freezes first (at negative temperatures close to 0°C). Therefore liquid water is squeezed from the smaller pores inside the aggregates into the larger interaggregate pores and crystallizes there, destroying the interaggregate structure connections. As the negative temperatures continue to fall not only are the ground micro- and mesoaggregates compressed, but bound water in the intra-aggregate pores begins to freeze and turns into ice. If it has no free escape, it breaks up the structural intra-aggregate ties and disperses (fragments) the mineral aggregates of the soil. This destructive cryohydra-tion process can take place simultaneously with the breakdown of the sandy and partly of the coarse silt fractions of freezing soils as a result of thermal weathering, which involves destruction due to different thermal expansion of the various minerals and elements composing the mineral component.

In soils the breakdown process can take place together with development of aggregation and stronger structural ties both between elementary particles of the ground and between small aggregates as a result of dehydration of aggregates (as part of the water freezes out) and of their approach to each other due to squeezing. For example, during freezing of moist soils, the concentration of ions increases in the unfrozen water. In this way, the threshold of coagulation can be reached and soil particles will coagulate (aggregate) with decrease of the general surface activity and dispersion. Therefore, a slightly more concentrated soil solution, which is normal during freezing, is sufficient to achieve deep mutual coagulation of soil particles and to form microaggregates of dust-size fraction.

The physico-chemical processes in soils that freeze with migration of water are much more varied, because the unfrozen layer, lying below the freezing front, is intensively dehydrated, so that the films of bound water are thinner and the particles and aggregates more closely packed. Dehydration causes considerable contraction and lowers the porosity of the unfrozen part of soils and forms larger aggregates and blocks of soil due to coagulation and aggregation. The soil particles, aggregates and blocks are more densely packed and are oriented in the direction of the water flow in accordance with the minimal hydrodynamic resistance. The pores inside the aggregates become smaller, whereas the volume and number of inter-aggregate pores grow and these acquire the shape of elongated slits.

Uneven shrinkage (both vertically and horizontally within individual aggregates and blocks of ground) in the unfrozen part of freezing soils causes the appearance of zones with various 'defects' of strength where contraction stresses are concentrated. Under the effect of the developing gradients of local stresses, the water migrates into these zones. If the zones of stress concentration receive insufficient amounts of water, then microfissuring may develop in the unfrozen dehydrated part of the freezing ground.

In the frozen part of the soils, as in the case of freezing without water transportation, intensive phase transition of water into ice takes place with an increase of water volume by 9% and the splitting up (disintegration) of large aggregates and blocks. Since ice appears first of all in large (interaggregate) pores, the size of aggregates is reduced and their density increases due to radial compression by the growing ice crystals. The size of skeletal interaggregate pores increases by several times compared to pores in the unfrozen dehydrated part of the soil.

Additional supply of water (due to migration) from the unfrozen part to the freezing part of the soil causes, on the one hand, a wedging pressure in films of unfrozen water, i.e. swelling of the whole water and ice saturated ground and, on the other hand, a progressive moving apart (disintegration) of macro- and meso-aggregates due to the increase of volume of the migrated water as it turns into ice. Consequently, in the frozen part of the freezing soil, the swelling-heaving is greatly intensified due to water migration, and macro- and mesoaggregates are fragmented and reorientated. Microblocks and mesoaggregates are rotated and the previous orientation (observed in the unfrozen part of the soil) is changed but inside them the orientation of particles remains the same.

As the negative temperatures continue to fall (far from the freezing front, in the zone of low-intensity of phase transitions of water) the unfrozen water freezes out in the thin intra-aggregate pores. This process causes dispersion of meso- and micro-aggregates (disintegration and peptization) and their orientation, observed in the unfrozen part of the soil almost completely disappears (see Section 4.3).

The differentiation of soil into a massive-frozen part (skeletal-mineral) and the visible fixed layers of migration-segregation ice is nothing less than the essential and unique physico-chemical and mechanical process of freezing of soils with water migration (compared with freezing without water migration). The ice layers appear in the zones of stress concentration, whose configuration in general corresponds to the future type of cryogenic structure. The bound water, supplied as a result of grad t and grad P activity, at first produces the wedging effect, which overcomes the local resistance of the soil to destruction and then freezes and increases its volume.

If the ice layers contain soil inclusions, then their displacement also occurs if grad t is active, but to the parts with higher negative temperatures, that is, the ice 'purifies' itself from admixtures and inclusions. This process is associated with uneven thickness of films of unfrozen water (there being a temperature gradient) on the opposite facets of soil inclusions. Migration of water to thinner films where it freezes causes squeezing out of soil inclusions towards higher temperatures. In general, the presence of solid, liquid and gaseous inclusions in ice largely depends on the rate of freezing of soils and of pore and migration ice. A lower rate of freezing reduces, and a higher rate increases, the amount of inclusions in ice.

The freezing of soils where there is a water supply to the frozen zone usually causes expansion of the zone with heaving of the surface of the ground. The unfrozen part of the soil is consolidated by shrinking due to dehydration and sometimes also by its compression under the effect of the heaving of the overlying frozen part (if this cannot be deformed upwards).

Physico-chemical and mechanical processes in thawing soils

The thawing of coarse clastic and sandy soils, both with low and high ice content, is usually associated with fairly simple physico-chemical processes, such as consolidation, dehydration, settlement and other processes connected with compression and re-orientation of fragments and sand particles, and with runoff down a tilted impermeable layer or infiltration of gravitational water into underlying horizons. The processes of thawing of fine-grained soils (silty sands, silty-sandy clays and clays) are different and more complicated. Two types of thawing are distinguished in the first approximation (as in freezing): with and without water migration from the thawing zone into the frozen part of the ground.

The thawing of fine-grained soils without water supply to the frozen zone of the sample normally occurs when either the thawing front moves quickly or the soils have low water (ice) content, which practically always results in deformation by consolidation. The water that is formed in soils under rapid thawing of pore or schlieren ice, either goes to hydration of soil particles (without water migration) or escapes from the soil under the force of gravity. It is obvious that during quick thawing of soils with large ice inclusions (for example, thawing of soils with large stratified-meshed cryogenic structure), cavities and empty cracks are formed in the thawed part of the soil, i.e. a specific post-cryogenic structure is formed. The melting of pore, contact and film ice as the negative temperature of frozen soils rises produces greater amounts of unfrozen water with its higher mobility, thus creating conditions for local migration of water within ground elements dehydrated during freezing (aggregates, blocks, particles) and for their hydration and swelling. The process of osmotic swelling along the partings in the soil dehydrated during freezing has a notable effect in the change of structural ties between the soil elements. This swelling is most clearly demonstrated at 0°C when the thawing of pore ice and ice inclusions terminates and the structural elements can move apart. In this process, the intra-aggregate ties become more relaxed than the inter-aggregate ones, and a transition takes place from close coagulation contacts to longer distance coagulation (aggrega-tional) contacts. During thawing of soils with a high ice content, their moisture content may even exceed that at the liquid limit, which accounts for a wide distribution of thixotropic soils in the Far North.

Slow thawing of soils and water migration from the thawed part to the frozen part activates practically all the physico-chemical processes which occur in thawing soils without water migration. Several other physico-chemical processes, however, are also active. With penetration of migrating water into the frozen part of thawing soils, the ice content is increased and often migration-segregation layers of ice are initiated and grow. In the thawed part of thawing soils the soil aggregates are dehydrated, and shrinkage brings them closer: they become larger and denser.

In general, rapid or slow thawing of soils increases their fine-grained nature as a result of dispersion and peptization of soil aggregates and blocks and fragmentation of initial sand particles. The joint effect of temperature and hydration mechanisms of destruction is a determining factor in this transformation of structural elements.

During cyclic freezing and thawing, soils experience physico-chemical processes typical of freezing and of thawing ground. The specific feature of repeated freezing and thawing is accumulation in soils of particles of coarse silt fractions due to destruction of sand particles.

Cyclic thawing and freezing often results in differentiation of freezing soils by grain size, which is caused by displacement (heaving) of the larger fractions of the soil (fragments, large particles) as the temperature falls. The results of field observations and laboratory experiments indicate that after several hundreds of cycles of freezing and thawing, the soils become graded by size, i.e. the larger particles (more than 1-2 mm) move towards the source of cooling and accumulate in a surface layer, whereas the smaller particles remain in place and accumulate in the lower part of the layer. In this process, the facets of the moving particles are abraded and smoothed out.

Deformations and stresses of shrinkage and heaving in freezing soils

An analysis of deformations and consolidation (shrinkage) stresses in the unfrozen part of unilaterally freezing fine-grained soils has shown that these effects are most active near the freezing front in the region of maximum dehydration. This observation is confirmed by data on the shifts of deformation recorders in a sample with depth and time (Fig. 3.2a). For example, the deformation values increased with longer time periods of dehydration as the freezing front approached the recorder. Moreover, the total value of shrinkage deformation in the vertical direction is by an order of magnitude greater than that in the horizontal direction. This can be attributed to a considerable 'prevention' of horizontal shrinkage of the unfrozen part of the soil due to its adhesion to the frozen part which resists shrinkage. Therefore, as the water-saturated soil freezes with intensive migration of water and ice accumulation, a narrow 'neck' is formed in the frozen part of the zone of dehydration which extends into the unfrozen part of the sample (Fig. 3.2b).

Fig. 3.2. Freezing of sample of kaolinite clay, (a) Change of temperature with time (dashed lines) and movement of deformation recorders with time in the vertical direction. (b) Vertical profile showing displacement of recorders (1) in the horizontal direction at the end of the experiment (rB « —5°C,h = height of sample, £si = segregation ice front, £fr = freezing front)).

Fig. 3.2. Freezing of sample of kaolinite clay, (a) Change of temperature with time (dashed lines) and movement of deformation recorders with time in the vertical direction. (b) Vertical profile showing displacement of recorders (1) in the horizontal direction at the end of the experiment (rB « —5°C,h = height of sample, £si = segregation ice front, £fr = freezing front)).

The 'prevented' shrinkage in freezing soils probably occurs not only as a result of interaction between unfrozen and frozen zones in the sample, but also because of the irregular distribution of moisture by depth, similar to the phenomenon in the unfrozen dehydrated soil. The irregular shrinkage deformations in the unfrozen part of the ground follow from volumetric-gradient all round stresses.

Experiments show that in freezing soils the shrinkage stresses grow with time to a certain maximum value determined at every moment of time by the relation of forces of structural adhesion of the soil with the forces trying to destroy it. The stress recorders indicate the beginning of the growth of stresses only when the frozen zone is already formed in the soil and water starts its migration to the front of ice segregation (Fig. 3.3 a). These stresses normally reach their maximum in the freezing zone (Fig. 3.3b) between the freezing front and the visual boundary of ice segregation, because here dehydration of soil is still active as the water slowly freezes.

Finer soil particles cause higher shrinkage stresses and higher stress gradients. Experiments show that the greatest values Pshr develop in clays a

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