Basic types of cryogenic structures

To explain reliably the particular features of generation and growth of different types of superimposed and inherited cryogenic structures (Table 4.1) we should know the mechanism and pattern of formation of the vertical and horizontal ice layers in soil during freezing (thawing).

Superimposed cryogenic structures normally appear in soils which are relatively homogeneous (prior to freezing), as a result of the freezing process and not as a result of the primary (initial) structure of the unfrozen soil. Three basic types are usually distinguished among the superimposed cryogenic structures of freezing soils according to the presence, shape and location of ice layers in them. These structures are: the massive, where ice schlieren are not visually apparent; the layered, with elongated-oriented ice layers; the mesh form, when the ice schlieren in the section form a net or a grid.

The massive cryogenic structure is formed when the physical-mechanical conditions of segregated ice formation are not met or when the thermal-physical conditions for formation of cryogenic structure are in general not satisfied. The latter case is usually when there is either a high rate of freezing of water-saturated or unsaturated soils, or at any rate of freezing of low moisture content fine-grained soils or of coarse-grained soils - when water migration is practically absent, while the ground water in pores is fixed by freezing. In this case the contact, film, pore and basal types of ice cement formation are the most common.

The formation of massive cryogenic structure can also take place with water migrating in the freezing ground but only if the physical-mechanical conditions for segregated ice formation are not satisfied and the stresses in the ground do not overcome its local strength. The local strength can be responsible for controlling the general increase of ice content and for the absence of segregated ice formation in over-compacted or cemented soils and in soils whose initial moisture content is less than that of the shrinkage limit. In these cases, however, there is the possibility of formation of separate

Table 4.1. Classification of cryogenic structures by conditions of their formation in freezing soils

Lithological features of soils

Thermal physical conditions of formation of cryostructures

Qph + I

Physical-mechanical conditions for the appearance of cryostructures

Type of cryostructure

Condition of formation of ice schlieren || to freezing front

'^côh + ^dora)

Condition of formation of ice schlieren _L to freezing front Pn = Pshr ~

p prup

>» li« coh

Soils with varied composition, structure, bedding and properties

Not satisfied

Not satisfied due to unsatisfied thermophysical conditions

Massive (contact, film)

Satisfied

Not satisfied

Massive (pore, basal)

Fine grained soils homogeneous in composition, structure, bedding and properties

Satisfied and provides migration of water into the frozen part of freezing soils

Sporadically satisfied

Porphyryitic

The condition is only beginning to be satisifed providing for formation of only incompletely developed schlieren

Not satisfied

Discontinuous semi-layered

Sporadically satisfied

Semi-layered phorphyritic

The condition is only beginning to be satisfied providing for formation of only incompletely developed schlieren

Semi-meshy

Satisfied

Layered with discontinuous vertical schlieren

Satisfied

Meshy

Not satisfied

Layered

ice schlieren and ice nests (porphyritic cryostructure) due to the instability (episodic nature) of the necessary physical-mechanical conditions and due to large pores, micro-cracks and other inhomogeneities in the soil.

The massive cryogenic structure sometimes appears when stresses in a soil could overcome the local cohesion of the soil but a sufficiently large external load (e.g., Pdom) prevents this.

The layered cryogenic structure is formed during freezing of soils, if the thermal-physical and physical-mechanical conditions are satisfied for formation of segregated ice layers, parallel with the freezing front, but the conditions are insufficient for formation of vertical ice schlieren. Generation and growth of the layered cryogenic structure usually take place in the interval of negative temperatures from —0.2 to — 3°C. This type of structure occurs predominantly in fine soils (sandy-silty, silty and clay soils) and sometimes in dusty sands. The layered cryogenic structure, compared to other types of schlieren cryogenic structure, is more frequent in nature and is most evident at low freezing rates. Its formation begins with the appearance of individual thin lenses and ice microschlieren. Later their size increases, they merge and compose a single elongated ice schlieren parallel with the freezing front which can be traced visually. If the freezing rate ofr decreases and the water flow towards the horizontal ice layer /w increases, then thicker ice streaks are formed.

A large variety of layered cryogenic structure (layered, lens-like, lens-like-woven, etc.) is caused by the specific composition, structural and geological-genetic features of soils and the various conditions of freezing and intensity of thermal and mass exchange and the structure-forming processes. A number of varieties of layered structure is distinguished by the shape of schlieren, their length and orientation, by the relation between thickness of ice schlieren and mineral soil layers etc. (Chapter 7).

The mesh cryogenic structure is formed when the thermal-physical conditions are consistently satisfied and concurrent physical-mechanical conditions for formation of horizontal and vertical ice layers are also satisfied (conditions 4.3^1.4). Since horizontal shearing stresses are always greater than the normal (vertically oriented) stresses, the formation of purely laminated cryogenic structure is possible, whereas the formation of only vertical ice schlieren (without horizontal ones) is in principle impossible because of the migrational-segregational mechanism of their generation. In some soils, as the rate of freezing grows a gradual transition takes place from the layered structure to the mesh-like and massive cryogenic structures. Therefore, under certain freezing regimes, conditions are created for the formation more equally of vertical and horizontal zones of concentration of critical stresses.

Specific features of development and further growth of the mesh cryogenic structure, in contrast to the layered one, are determined by water migration within the block both horizontally and vertically. Moreover, the water moves to schlieren perpendicular to the freezing front only under effect of grad Pn, whereas it moves to schlieren parallel with the freezing front under the effect of the temperature gradient (or grad Wunt) and the stress gradient grad Psh.

The mesh cryogenic structure varies both in shape (prisms, parallelepipeds, cubes, etc.) and in size of soil blocks as a result of different relations between normal and shearing stresses in freezing soils of different composition, structural-textural features and strength. The frequency of distribution or the distance between horizontal /| and between vertical l± ice layers are determined by the relations: /| = /(1/grad Psh) and l± = /(1/grad Pn).

It should be specially mentioned, however, that an increase in freezing rate (because of a low dehydration rate of the unfrozen part and small swelling of the freezing zone of the ground) leads to the end result of a decrease of values of shearing and normal stresses and a considerable reduction of the zone of generation and growth of vertical and horizontal ice layers. This process encourages (due to the short time of ice schlieren formation) the appearance of partially developed cryogenic structures (semi-mesh, semi-layered, angular-discontinuous, etc.).

With the increase of the rate of freezing, the conditions may arise when vertical schlieren can no longer be formed and an often fine but frequently discontinuous-layered cryogenic structure is formed (or discontinuous-lenslike, scaly, etc.).

The type of cryogenic structure is not determined only by conditions of freezing but depends substantially on the lithology of the freezing layer. The cellular cryostructure, which is a variety of block structure, proves this. It differs from the mesh and layered structures in the mode of formation and occurs mostly in bentonitic clays. For example, near the freezing front bentonite shows distinct cells of soil of irregular polygonal shape with a framework of ice. The cellular cryogenic structure of bentonite is entirely dependent on and predetermined by its specific microstructure and texture, which creates cellular or honeycomb structures in the unfrozen dehydrating part of the soil.

An analysis of a variety of superimposed cryogenic structures demonstrates that there is in principle the possibility of formation of independently developing inclined or even vertical ice layers, as observed by many re-

Fig. 4.3. Inherited cryogenic structures: a-b- Zones of structural defects, general view of soil with dislocated structure before and after freezing: 1, 2 open cracks before freezing, 1*, 2* - after freezing, 3,4, 5 - closed (healed) cracks before freezing, 3*, 4*, 5* - after freezing; c - zones of influence of extraneous inclusions; d - zones of concentration of stresses (interbedding of kaolinite clay and quartz sand at 45° angle to direction of thermal and migration flows.

Fig. 4.3. Inherited cryogenic structures: a-b- Zones of structural defects, general view of soil with dislocated structure before and after freezing: 1, 2 open cracks before freezing, 1*, 2* - after freezing, 3,4, 5 - closed (healed) cracks before freezing, 3*, 4*, 5* - after freezing; c - zones of influence of extraneous inclusions; d - zones of concentration of stresses (interbedding of kaolinite clay and quartz sand at 45° angle to direction of thermal and migration flows.

searchers, in Quaternary deposits. These ice layers are a result of the numerous cracks of various shapes in the unfrozen dehydrating soil (cracks due to bending, warping, flaking etc.). Moreover, in natural conditions, there is often an irregular boundary of freezing (which is tilted relative to the surface, undulating, angular, etc.). The fact that ice schlieren are generated and grow mostly parallel with or perpendicular to the front of freezing/thawing implies the existence in superimposed cryogenic structures, of a complicated system of ice schlieren (undulating, radial, ring, rhombic, etc.) caused by the irregular surface of the freezing or thawing boundary (Fig. 4.2).

Inherited cryogenic structures in freezing soils are quite common. They are caused by different kinds of lithological inhomogeneities and defects of loose deposits. The major genetic types of inherited cryogenic structures are associated with strength-defective and strained contact zones and with extraneous inclusions (Fig. 4.3). Their formation, as that of superimposed cryogenic structures, cannot take place without the thermal-physical and physical-mechanical conditions for segregated ice formation.

The inherited cryogenic structures in the zones of structural defects of soils (the strength-defective type) are confined to zones of mechanical crumpling, dislocation, sliding planes, closed cracks and other defects of structure and texture. The ice schlieren follow the shape of structural defects in the weakened zones of the freezing soil. The critical stresses for local destruction of the soil first of all are concentrated in these zones during deformation by shrinkage-swelling and the formation of structure in the freezing ground.

The inherited cryogenic structures in the zones of stress concentration at the contacts between different soils (contact-stress structures), in contrast to the strength-defect cryostructures, appear not in the weakened zones of the soil but in the zones of contact of soils which differ in thermal and mass exchange and in shrinkage-swelling characteristics. At the contacts, the differences in the amounts of deformation due to shrinkage and swelling give rise to critical stresses (stress concentration zones).

Consequently, the cohesion of the soil is overcome primarily in these places and ice schlieren are generated. For example, during freezing of stratified soils, as a rule elongated segregated ice streaks appear at the interfaces of layers (Fig. 4.4). They inherit completely the initial lamination (horizontal, vertical, tilted, or oblique) of the loose fine soil deposits.

In the freezing of soils, inherited cryogenic structures may be caused by extraneous inclusions and have the following characteristics. Firstly, the water resistant inclusions block migration flow to the freezing front, and downstream from them the ground is dehydrated and massive cryostructure is formed (or the ice schlieren become thinner), whereas behind them the ice content grows and the schlieren cryostructure appears. Secondly, these inclusions, which differ greatly from the soil in their deformation properties, concentrate stresses at their contacts with soil thus regularly causing the appearance of ice schlieren. Moreover, behind the extraneous inclusion (at a certain distance from it) the geometry of ice schlieren follows the contour of the inclusion, i.e. it repeats the character of the moving freezing boundary, which in its turn is predetermined by the presence of the inclusion with a thermal conductivity different from that of the ground.

In the frozen part of thawing soil which has a temperature gradient, the accumulation of migration-segregation ice and the formation of cryogenic structure are also possible (Fig. 4.5). Experiments show that the development of cryogenic structures in the frozen part of thawing soils has the same features as during freezing. During this process both the superimposed and

Fig. 4.4. Inherited cryogenic structures in marine lenticular clays (photo by G.I.

Dubikov). Largest ice lens at bottom is c. 3 cm.

Fig. 4.4. Inherited cryogenic structures in marine lenticular clays (photo by G.I.

Dubikov). Largest ice lens at bottom is c. 3 cm.

the inherited types of cryogenic structure can develop, in which the generation, growth and configuration of ice schlieren are predetermined by the initial composition and structure of the soil and by the conditions of its thawing. The formation of these structures of course requires the presence of thermo-physical and physical-mechanical conditions of schlieren ice formation during the process of thawing.

On the basis of this analysis, a genetic classification of cryogenic structures (based on appearance and development) was suggested grouping logically practically all known types and varieties of cryostructures (8). Table 4.1 illustrates this classification. Each class of structure (with and without schlieren) is divided into the types described earlier, and every type is subdivided into kinds of cryostructure according to the frequency of generation and thickness of ice schlieren. The number of kinds of structure (by frequency of occurrence of ice schlieren) within each type can vary and the different distances / between schlieren are determined in natural conditions both by the lithology of the freezing or thawing deposits and by external thermodynamic conditions, with which the stress gradients are connected.

Fig. 4.5. Generation and growth of segregational ice schlieren in the frozen part of soils thawing from above (superimposed cryostructures): a - layered structure (slow thawing); b - block structure (rapid thawing) in kaolinite clay with initial massive cryostructure; c - thawing of clay-rich soil with initial schlieren cryostructure.

Fig. 4.5. Generation and growth of segregational ice schlieren in the frozen part of soils thawing from above (superimposed cryostructures): a - layered structure (slow thawing); b - block structure (rapid thawing) in kaolinite clay with initial massive cryostructure; c - thawing of clay-rich soil with initial schlieren cryostructure.

Division of the kinds of cryostructures into large-, middle- and small-schlieren (using numerical values) is not practical for all cases but only within any type of structure as applied to an actual type of ground. Otherwise this subdivision loses its purpose, because it will not relate to their formation. Therefore, in this classification of cryogenic structures, only the extreme variants are taken into account, in other words, the pattern of formation (direction of change) of horizontal and vertical ice schlieren depending on grad Psh and grad Pn. Then for each of these variants, four extreme types of structure are recognized according to the thickness of ice schlieren, and between them there can be a great number of varieties. In fact, the transition of one type into another is smooth and gradual. The simple division of types into thick-, middle- and fine-schlieren structures (without due account of the type of structure, of the geology and genesis of the material and of the freezing-thawing conditions) is tentative, because it does not show, and even hides the patterns in the thickness of ice layers.

The cryogenic structures in freezing soils described above are of migration-segregation genesis. In natural conditions, however, the cryogenic structures may also appear as a result of other mechanisms. For example, in very wet weakly lithified soils the mechanism of cryogenic structure formation is associated with the freezing out of mostly free water and selective orthotropic ice formation. This mechanism is a result of the predominant matrix structure of silts and weakly lithified fine-grained porous soils, in which the larger part of water is free (unbound), while the soil particles cannot actively interact. Freezing of water produces an ice framework (block-framework cryogenic structure), in which the vertical and inclined ice layers appear earlier and much more rapidly than the horizontal crossmem-bers. The ice crystals push out the mineral particles in the direction of their growth thus concentrating them inside the blocks, i.e. the spaces between the walls of an ice framework are filled with the wet mineral part of the soil. As the ice walls of the framework grow (becoming thicker and longer), the mineral part of the freezing soil is dehydrated and densified by compression. This mechanism of formation of structure is called orthotropic-compres-sional (12). Moreover, with finer-grained material all other conditions of freezing being equal, the soil more often separates into ice and mineral layers and forms a cellular cryogenic structure. Block cryogenic structure with fewer ice layers is formed in coarser soils. The same type of pattern is observed as the rate of freezing decreases.

In natural conditions the injection mechanism of formation of ice layers is also active. The formation of injection and mixed (migration-pressure) cryogenic structures is possible only in the case when hydrostatic pressure of the water being injected exceeds the strength of the structural connections in soils. At high pressures, a hydroburst can occur even in frozen soil, with instantaneous injection of water and formation of larger layers of injected ice. Under hydrodynamic pressures above the critical long-term strength of the soil, the long-term injection of thin water films occurs with pressure-migrational segregated ice formation.

Cryogenic structures formed as a result of freezing change with time under the effects of negative temperature gradients, mechanical stresses, concentration of the pore solution, and of other, external fields. During this process, both in syngenetic and epigenetic layers of perennially frozen soils and in the layer of seasonal freezing-thawing, the already existing ice layers may grow and new ice schlieren can be generated and, in some cases, also disappear.

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