Textural characteristics of the frozen material

Cryogenic texture of the frozen material is assumed to be that of the ice framework, consisting of inclusions, intercalations differing in shape and size, orientation and spatial arrangement, by which the structure of the

Fig. 7.9. Curves of differential distribution of void volumes by radius (a) and dependence of unfrozen water content on temperature in siliceous (&), sandy (c) and clay-rich (d) soils and rocks: 1 - diatomitic clay; 2 - diatomite; 3 - tripoli; 4 - opoka; 5 - silica gel; 6-8 - sandstones (6 - weathered medium-grained; 7 - unweathered fine-grained, 8 - unweathered coarse-grained); 9 - 11 - compact clays; 12-13 - argillites; 14 - aleurite.

Fig. 7.9. Curves of differential distribution of void volumes by radius (a) and dependence of unfrozen water content on temperature in siliceous (&), sandy (c) and clay-rich (d) soils and rocks: 1 - diatomitic clay; 2 - diatomite; 3 - tripoli; 4 - opoka; 5 - silica gel; 6-8 - sandstones (6 - weathered medium-grained; 7 - unweathered fine-grained, 8 - unweathered coarse-grained); 9 - 11 - compact clays; 12-13 - argillites; 14 - aleurite.

mineral skeleton is subdivided into individual structural varieties. The concept of the frozen soil texture distinct from cryogenic texture implies study of textural features of both ice inclusions and the organo-mineral portion of soils.

The fine granular texture of soils forms in the course of their lithification. Specific features of cryogenic texture generation are dependent on chemical-mineral variability and variability of facies as determined by the different modes of freezing (syngenetic, characterized by cryogenic transformation of sediment simultaneous with its accumulation, and epigenetic, exemplified by freezing of lithified materials). The cryogenic texture of soil is much dependent on the soil's texture and structure prior to freezing. The availability of textural attributes in the initially unfrozen soils (lamination, Assuring, oriented inhomogeneity) leads to the formation of inherited cryogenic textures. In homogeneous materials (both in composition and structure) the superimposed cryogenic textures are formed in the process of freezing.

Heterogeneous cryogenic textures encountered in natural conditions are created by different mechanisms of cryogenic texture generation among which the main are migration-segregation, pressure-migration, injection and orthotropic compression. The different types of ice, segregation, injection, ablimation, etc., are dependent on the process of ice formation in cryogenic structures. The main mechanism of superimposed cryogenic texture formation is that of migration-segregation. This is associated with development of different processes including heat and mass exchange, physical-mechanical as well as physico-chemical ones (coagulation, aggregation, shrinkage, swelling, etc.) that substantially transform the primary structure of soils. These processes result in development of stresses, breakage of structural links and segregation of ice. Classification of migration-segregation cryogenic textures based on the study of their mechanism and conditions of formation was elaborated by E.D. Yershov (9) (see Table 4.1).

Cryogenic texture of rock is largely determined by its Assuring, saturation with water and freezing. There are, as a rule, inherited cryogenic textures. Size, shape, orientation and spatial arrangement of ice inclusions correspond to the geometry of fissures. In magmatic igneous rocks (granites, diorites, andesites, etc.) there are fissure and fissure-vein cryogenic textures. In fissures ice is in the form of films, crusts and nodules of crystals, with ice-cementing of loose infilling material which completely or partially fills up the cavities. Almost completely filled fissure cavities are typical of fissure-vein cryogenic structures looking like substantial veins.

In consolidated (cemented) sedimentary rocks the following types of cryogenic structures are distinguished: stratal-fissure, stratal-fissure-porous and stratal-fissure-karst (Table 7.1). The size of ice interlayers in the inherited structure is dictated by the opening of fissures and varies from fractions of a millimetre to tens of centimetres. With respect to ice thickness the following cryogenic structures are distinguished: thin, medium and thick fissure and fissure-wedge.

As regards spatial arrangement and orientation there are the following cryogenic structures : ordered-latticed (or regular latticed) and orderless-latticed (non-regular-latticed). The latter are, as a rule, confined to zones of tectonic crushing and intense weathering, while ordered-latticed cryostruc-tures are remote from such zones. Differentiations in composition and structure of ice in the inherited cryogenic structures of rocks are primarily associated with different mechanisms of ice formation. The migration-

Table 7.1. Classification of cryogenic structures of the perennially frozen rocks (after A. A. Kagan, N.F. Krivonogova)

Main types of soil for which cryostructure is Texture name Description typical

Fissure Ice occurs along fissures in the form of films, crusts along walls or cement and schlieren in the fissure infill

Fissure veined Ice fills the whole cross-section forming veinlets

Stratal-fracture Ice veins along fissures of bedding form the appearance of cryostructure; ice content of fractures of other systems has a subordinate nature Stratal-fissure Ice veins along fissures of bedding porous and ice-cement in pores; ice content of other systems has a subordinate nature Stratal-fissure Ice veins along fissures of bedding karst and cavities of leaching and dissolution form the appearance of cryostructure ice; content of other systems of fissures has a subordinate nature

Magmatic - granites, diabases, andesites, basalts

Metamorphic - hornfels, quartzites, crystalline schists, gypsum Sedimentary - dolomites, limestones, marls, aleurolites, argillites

Sedimentary -sandstones, clayey schists, marls

Sedimentary -limestones, dolomites (karst), gypsum, rock salt segregation type of ice in rock is encountered only in fine-grained water-saturated fissure infillings. However these are the main mechanisms of ice formation as distinguished by the authors A.A. Gagan and N.R. Krivonogova:

1) infiltration, dependent on the infiltration of surface and ground water; ice is polygranular with random orientation of crystals (allot-riomorphogranular);

2) injection, associated with hydrodynamic intrusion of water into fissures and its freezing in them; injection ice formation usually leads to cryogenic expansion of fissures and development of highly fissured rocks; injection ice in big fractures is mostly transparent, while the textures themselves are called inherited-expanded;

3) ice cement, typical of fissured masses frozen below the level of ground water; it is also typical of saturated rocks in the seasonally thawing layer;

4) ablimation (sublimation) ice which is formed due to freezing of vapour in bigger fractures reaching to the ground surface or other sources of recharge. Ablimation ice has a granular texture.

Cryogenic structures in the lithified materials of the loose cover differ in their composition and structure and conditions of freezing. At the present time many authors (B.I. Vtyurin, Y.M. Katasonov, E.D. Yershov, L.N. Maksimova et al.) have elaborated classifications of all known types and kinds of cryogenic structure encountered in the natural environment. Several principal types of cryogenic texture were distinguished in accordance with orientation and spatial arrangement; namely, massive, laminated, latticed and cellular. Different types of cryogenic texture were distinguished according to the size of ice layers and the spacing between them (Table 7.2).

One of the main processes determining the heterogeneity of types and kinds of migration-segregation structures is migration of moisture. The intensity of the migration flow of moisture and its duration determine the thickness of segregated layers while its intensity promotes development of shrinkage tensile stresses and deformations, their frequency and orientation. The intensity of moisture flows increases with higher dispersion of soils. Therefore, thick schlieren cryogenic texture is most often encountered in clayey soils. Changes in grain size of soils have a greater effect over the appearance of the cryogenic structure, i.e. the frequency of layers and their thickness, and a lesser effect over the type of cryogenic structure. The types of cryogenic structure are to a greater extent determined by mineral composition of materials. Thus, with higher content of minerals of the montmoril-lonite group in clays the type of cryogenic structure changes from horizontally laminated to cellular. With higher content of the montmoril-lonite group of minerals the moisture flow intensity and thickness of ice layers diminish, but, on the contrary, they increase with higher content of minerals of the kaolinite group.

At high rates of freezing a massive cryogenic texture is mainly formed; with less fine material, a lower rate is required for the formation of massive cryogenic texture. With a lower rate of freezing in fine-grained soils massive texture grades into streaky cryogenic structures. The lower the rate of freezing the longer the time required for the growth of ice layers and their thickness. At shallow depths of ground water under conditions of an open system, the formation of horizontally laminated thick schlieren cryogenic texture is most likely. In the process of cryogenic texture formation a

Table 7.2. Classification of schlieren cryogenic structures (according to A. Vtyurina, B.I. Vtyurin)

Type (according to schlieren arrangement and nature of ice cement)

Subtype

(according to ice schlieren orientation)

Kind (by the interval between schlieren: rare laminated and big netted - over 100 mm; medium laminated and medium netted -10-100 mm; frequently laminated and finely netted -1-10 mm; microlaminated and micronetted - under 1 mm)

Variety (by the thickness of ice schlieren: thick schlieren over 10 mm; medium schlieren 5-10 mm; microschlieren less than 1 mm)

Laminated

Horizontally, obliquely, vertically laminated

Rare-, medium-, frequent, microlaminated

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