The microstructure of frozen soils implies the totality of microstructural and microtextural features inherited from the unfrozen materials as well as those acquired during freezing and in the frozen state. The concept of microstructure includes the size, shape, pattern of surface and the quantitative proportions of elements that compose the frozen soil and the nature of their relationships. Microstructure is defined as the totality of attributes that characterize the relative arrangement and distribution of elements of the frozen soils in space.
The elements of the frozen soil microstructure include primary mineral grains, particles, their aggregates that compose the mineral skeleton, ice crystals, unfrozen water, air inclusions and foreign admixtures of a size, as a rule, of less than 1 mm.
Ice crystals in the frozen soil comprise ice-cement as well as various ice inclusions. The following kinds of ice-cement may be recognized in the filling of the pores with ice: cuff-type (contact) found in the adjacent cuffs and contacts between grains and aggregates of the skeleton; the film-like form that envelops the surface of grains and aggregates of the skeleton with part of the pores unfilled; pore type, that fills the pores completely; and the basal type, that accounts for the main mass of the material and divides the grains and aggregates of the mineral skeleton. There is also needle ice-cement of ablimation origin which is isolated in the form of ice spicules in pores and on the surface of grains and aggregates. Along with all these, there
are also massive, porphyry, ring-like, laminated, and latticed cryogenic microstructures. Their principal types are, in general, similar to the well-studied cryogenic macrotextures. However, unlike the latter, microstructures are to a greater extent curved, discontinuous and inconsistent.
The frozen soil microstructure is largely heterogeneous. Generally, the formation of cryogenic microstructure is controlled by the conditions of accumulation and freezing. In general, two principal genetic types of frozen soils are distinguished: epigenetic and syngenetic.
The microstructure of epigenetically frozen soils reflects their microstructural and microtextural peculiarities acquired before freezing. These are represented in the degree of aggregation, compactness of mineral skeleton and the inherited nature of ice segregation. Thus, for the silty clay icemarine deposits of the Salekhard suite which have undergone a stage of lithification before freezing, a matrix-like microstructure of the mineral skeleton is typical, which is similar to the nonfrozen deposits. Distinctive features of the frozen soil are demonstrated by the morphology of ice-cement and ice inclusions, as well as by the microstructure of the adjacent zone (Fig. 7.13a).
The microstructure of syngenetically frozen soils is characterized by a
specific nature with higher ice content, predominance of basal ice-cement and the occurrence of a loose organo-mineral skeleton (Fig. 7.13b). Such a microstructure, according to V.V. Rogov, is typical of the syngenetically frozen deposits of the ice complex, which are characterized by the poorly aggregated loose organo-mineral skeleton divided by the basal ice-cement into aggregates sized 0.2-0.3 x 0.5-2 mm. Apart from ice-cement there are ice lenses and bands 0.1-0.2 mm thick with crystal axes oriented almost vertically.
The microstructure and microtexture of frozen soils are closely associated with grain size (Fig. 7.14). Rudaceous and sandy materials which are non-cohesive, have a massive cryogenic texture. Their mineral skeleton does not experience any change at freezing. The contact between structural elements is of a point type. Irrespective of the freezing regime, water is retained in pores or is partially pushed out. The type of ice-cement varies from needles and cuffs to basal.
Microstructure of frozen clay-rich soils differs from that of rudaceous soils in having a composite nature caused by physico-chemical processes which substantially transform the morphology and size of the mineral skeleton, aggregates and void spaces, leading to the formation of porphyritic inclusions of ice and segregation microschlieren. In the mineral intercalations with schlieren cryogenic texture, the ice is present in the form of individual crystals arranged between mineral particles and their aggregates as well as in the form of chains parallel to the longitudinal extension, which serve as a transition between ice-cement and microschlieren (16). Ice crystals in the microschlieren have a columnar, rarely tabular, shape with regular
straight edges, with their main optic axes being, as a rule, oriented perpendicular to the longitudinal extension of the schlieren.
Microstructure of frozen clayey soils is to a great degree dependent on their mineral composition. This is evident, firstly, when clay minerals of the montmorillonite, kaolinite and hydromica groups are present in the soil skeleton. Differences in their crystallo-chemical structure are reflected in the morphology of particles, aggregates and inclusions of ice-cement and in the quantitative ratio of ice-cement to unfrozen water, as well as in the intensity of heat and mass exchange with physico-chemical and physico-mechanical processes giving rise to a specific cryogenic microstructure (Fig. 7.15). A loose chaotic structure of tabular aggregates with isometric inclusions of ice-cement uniformly distributed in the inter-aggregate voids is typical of
kaolinite clays. Favourable conditions for moisture migration in the kaolinite clays promotes ice segregation with ice microschlieren varying in thickness from a hundredth of a millimetre upwards. A distinctive feature of the frozen montmorillonite clay microstructure is a cellular microtexture. The microstructure of the mineral blocks and the size of ice schlieren is determined by freezing conditions. In general, the blocks typically have a compact structure of aggregates and an insignificant amount of ice-cement, which is explained by the local contraction of the mineral skeleton resulting from moisture migration in all directions from the centre to the bordering ice framework. A characteristic feature of hydromica clay microstructure is slit-like pores of irregular shape conditioned by the morphology of particles and aggregates having a leaf-like form of skeleton with slightly curved surfaces and tortuous boundaries.
The microstructure of frozen fine-grained soils reflects the inherited nature of the microtexture existing before freezing. Among the inherited features are their structureless nature, laminated with availability of aggregates, etc. Thus, the undisturbed sample of a silty-clay from a seasonally thawing layer showed: compact soil aggregates; ice inclusions of porphyroid kind with part of the pores not filled with ice; and 'ice-covered sites' were practically absent. Microstructure of a disturbed silty-clay sample was characterized by a uniform distribution of the sandy-silty fraction, by the circular, isometric, less compact aggregates, by the absence of purely clay formations and by big pores as well as by the presence of both pore ice-cement and ice microschlieren.
Restructuring of the initial microstructure is much conditioned by freezing regime. As a rule, with higher rate of freezing, the more uniform is the distribution and correlation of structural elements; their average size is reduced; ice microschlieren become less thick and more frequent. Freezing of the soil in conditions of moisture inflow leads to a poorly aggregated mineral skeleton with small loose aggregates, a predominance of basal ice-cement and the presence of frequent ice microschlieren. In the absence of moisture seepage a compact skeleton forms having big aggregates with porous ice-cement and rare ice microschlieren. With a certain regime of cyclic freezing in the clay soil circular microstructures are formed, resulting from differentiation of the materials that comprise the soils (16).
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