Formation of structure in freezing and thawing soils

The wide spectrum of physico-chemical and physical-mechanical processes which accompany freezing and thawing of soils causes considerable structural transformations of their organic-mineral skeleton manifested by change in the size, shape, relations and orientation of structural elements (primary particles, mineral and organic-mineral aggregates). The size of structural elements can increase or decrease during freezing. The decrease is a result of dispersion effects and the increase is caused by coagulation and aggregation.

When the ground freezes quickly at low negative temperatures (about — 40 to — 60 °C) and there is no water migration, then concurrent generation of crystallization centres and the growth of ice crystals produces a process of disintegration of particles and aggregates. Chiefly, the large particles and aggregates of the mineral skeleton (sands and coarse dusty fractions) are intensively disintegrated, being more inhomogeneous and with a larger number of defects than the structural elements of smaller size (Fig. 4.6a). When the temperature of freezing is higher (above — 30°C), enlargement of aggregates dominates over their destruction. For example, for polymineral sandy-clay material after freezing to — 30°C, as with freezing to — 60°C, the content of coarse silty and sandy aggregates grows but as a result of aggregation of smaller structural elements (Fig. 4.6b). Therefore, the intensity and duration of phase transitions of water are important for the direction of change in grain-size composition, i.e. lower intensity may cause migration of intra-aggregate water to centres of ice formation and can improve conditions for plastic rearrangement of mineral elements, thus contributing to their mutual approach and enlargement by coagulation and aggregation. Quick phase transitions cause only fragmentation along mineral boundaries and finer grain size.

In the case of water migration to the crystallization front, the leading role in structural transformation of freezing ground belongs to the processes of mass transport into the freezing zone, to differentiation and deformation of the soil mass during formation and growth of segregated ice layers and to dehydration and shrinkage of the unfrozen part of the ground. Reconstitution of structure in this process involves considerable compaction and strengthening of the mineral skeleton, which provokes greater strength of the soil in general, and formation of ice-cement adhesion, in particular. In b

Fig. 4.6. Changes in grain size composition of clayey soils (from differential curves of small particle analysis): a, b - all-round freezing of heavy silty clay at t equal to — 60°C and — 30°C, respectively; c - freezing polymineral clays with dominant Na+ content; d - same, Ca2+ and Mg2+; e,f- thawing polymineral clays consolidated under loads of 0.05 and 1 MPa, respectively. 1 - initial state; 2 - after freezing; 3 - after thawing. Weight content of particles by fractions, ordinate axis, and the size of particles, abscissa.

Fig. 4.6. Changes in grain size composition of clayey soils (from differential curves of small particle analysis): a, b - all-round freezing of heavy silty clay at t equal to — 60°C and — 30°C, respectively; c - freezing polymineral clays with dominant Na+ content; d - same, Ca2+ and Mg2+; e,f- thawing polymineral clays consolidated under loads of 0.05 and 1 MPa, respectively. 1 - initial state; 2 - after freezing; 3 - after thawing. Weight content of particles by fractions, ordinate axis, and the size of particles, abscissa.

the unfrozen dehydrated zone of freezing soils concurrent dehydration of structural elements takes place, as they approach each other and form larger aggregates and blocks. Porosity is reduced and the mineral skeleton is densified; concurrently, the reorientation of particles and aggregates along the direction of migration flow and the formation of slit-like pores also take place (Fig. 4.7). The size of aggregates and blocks is controlled by the degree and intensity of dehydration and by the character and degree of development of deformations and shrinkage stresses. The shape of structural separations depends mainly on the mineral composition and crystallo-chemical features of the soil-forming clay minerals.

Unfrozen Water Fraction
Fig. 4.7. Microstructure of the mineral skeleton of freezing kaolinite clay in the unfrozen dehydrated zone at the boundary of freezing (a) and in the heaved frozen zone (b).

In the freezing part of clay-rich soils (i.e., in the region of significant phase transitions), it is not the initial unfrozen soil structure that is being reconstructed, but the structure already transformed by dehydration and shrinkage of the unfrozen part. The freezing out of water in large pores and the growth of ice crystals when the temperature falls causes heaving of the mineral skeleton and differentiation of the soil mass (when segregated ice micro- and macro-layers are formed). The structure of the mineral skeleton becomes loose, though the strength of soil is greatly increased by cementation by ice. The process of phase transition is often accompanied by partial fragmentation and reorientation of soil blocks and aggregates as a result of deformation by heaving but inside the aggregates the orientation of elementary (initial) particles usually remains intact. As a result of growth of ice inclusions, mostly in large pores and at structural boundaries, the pores retain their slit shape and enlarge (Fig. 4.8). In the process of further freezing (fall of temperature), the phase transitions of water occur in the smaller intra-aggregate and interparticle pores, thus causing disintegration of soil particles and their disorientation within aggregates and blocks of soil. As a result, the frozen soil has a disordered structure similar to that before freezing (in never-frozen ground).

Quantitative microstructural changes during freezing are, on the whole, the result of composition and initial structure. The mineral composition determines the shape of structural partings which appear during freezing. Since montmorillonitic soils are more easily deformed under the effects of physico-chemical processes concurrent with freezing, these show transformation of the initial, usually skeletal-matrix, microstructure into a turbulent

Soil Aggregate Microstructure
Fig. 4.8. Vertical orientation (parallel to migration flow of water) of mineral particles and aggregates in the unfrozen dehydrated zone of the freezing polymineral clay.

one even under conditions of modest dehydration and deformation of unfrozen parts of the soil far removed from the freezing front.

Crystallochemical features of the structure of kaolinite particles, making up the rigid structural elements, control the possibility of their fragmentation in the zone of ice formation and the reorientation of their basal surfaces along the boundary between the mineral and ice layers during growth of segregated ice. The initial (before freezing) grain size and the mineral composition determine the extent of structure-forming processes in freezing soil by influencing the intensity of water exchange, ice formation and dehydration of soil. Structure formation is more obvious in finer-grained (clay) soils and becomes less as the initial grain size increases. The chemical composition has an equally important effect on the formation of structure and on the direction of changes of grain size of freezing soils. In clay soils containing Na ions, under conditions of deep dehydration (caused either by low temperatures of freezing, or by intensive water exchange during slow freezing), the dominant processes are coagulation and aggregation of structural elements. Freezing of clays containing multivalent cations (Ca2+ and Mg2+), however, is accompanied by dispersion along structural separations (Fig. 4.6c,d).

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Fig. 4.9. Basic types of ice-cement in frozen soils: a - basal, b - pore; c - film; d - contact; 1 - soil particles, aggregates; 2 - ice cement; 3 - parts of pores free of ice and water.

Fig. 4.9. Basic types of ice-cement in frozen soils: a - basal, b - pore; c - film; d - contact; 1 - soil particles, aggregates; 2 - ice cement; 3 - parts of pores free of ice and water.

The difference in formation of structure is caused by a higher content of clay-colloidal material in the Na-containing soil in the initial state (prior to freezing) resulting in a more active manifestation of physico-chemical processes, by development of a mostly local water exchange in the soil and consequently significant dehydration at discontinuities in the mineral assemblage.

If the water exchange in compacted soils is obstructed at low water contents, then disintegration of structural elements is observed. On the contrary, in relatively loose, water-saturated soils coagulation and aggregation dominate.

The formation of ice as a structural element in freezing soils cardinally changes the initial (unfrozen) structure. The resulting cryogenic microstructure is a result of both the initial composition and structure of unfrozen soil and the freezing conditions. The typical feature of microstructure of clastic and sandy soils is the presence of ice cement which binds the previously loose soil into a solid mass. Several types of ice cement can be formed depending on the initial water content in sandy soils; for example, the cuff (contact) type, the film (crustal) type and the porous and basal types of ice-cement (Fig. 4.9). The cryogenic microstructure of clay soils (clay-rich and fine-silty sands, clays) is similar to the macrostructure of frozen soils (massive, laminated, mesh and cellular). As the fineness increases, the thickness of microlayers of ice and their frequency increase (Fig. 4.10).

The specific features of formation of cryogenic microstructure in soils with different chemical composition of the pore solution and exchange complex is first of all evident in the change of aggregation of the mineral skeleton and in the phase composition of the water. The microstructure of salinized sandy soils shows a higher content of unfrozen water and a new structural element, the crystals of salts, which precipitate from the pore solution and cement the mineral skeleton, thus forming a new type of contact, i.e. the crystallization type. Structure-forming processes are much

Fig. 4.10. Basic types of cryogenic microstructure in soils: a - massive; b - lens-like; c - laminated; d - mesh; 1 ice; 2 - soil particles.

Cryogenics Microstructure
Fig. 4.11. Microstructure of frozen silty-clays, saturated with (a) Na+ and (b) Ca2 + . 1 - ice; 2 soil particles.

more complicated in clay soils where, moreover, ion exchange occurs; these processes, with allowance for the nature of the salts, cause either aggregation or dispersion of the mineral skeleton.

When clay soils with multivalent cations in the exchange complex freeze, the intensity of segregated micro-ice formation increases and laminated cryogenic microstructures appear. In soils saturated with univalent cations, discontinuous and thin ice micro-schlieren with a confused orientation are produced (Fig. 4.11).

The formation of cryogenic microstructure is influenced not only by the initial chemical-mineral composition, dispersivity, moisture content and density of soils but to a great extent by freezing conditions as well (rate of freezing, temperature gradients, the presence or absence of water supply, etc.). For example, in fine-grained soils as the freezing rate is increased the thickness of ice microlayers and the distance between them decrease, as do the average size of inclusions of ice cement and the size of its crystals. Inside the mineral blocks and aggregates, the inhomogeneity in microstructure increases from the centre towards the periphery of aggregates. During slow freezing, the aggregates acquire a more homogeneous microstructure, though a more distinct differentiation into purely mineral areas is observed (without ice inclusions), as are areas with higher ice content. The latter effect is, probably, caused by local redistribution of moisture/ice content (Fig. 4.12).

In the course of thawing, the structure of frozen soils is also transformed. On the basis of, as yet, a small amount of experimental data, we may assume that in most cases, and particularly during rapid thawing of soils, the general tendency is to disintegration of larger elements. Thawing is accompanied by weaker structural ties, lesser strength and greater permeability to water. This is shown in the lesser (to a tenth or less) coefficient of aggregation of the

Icy Ground

Fig. 4.12. Differentiation of ice and mineral particles in freezing silty clay due to local redistribution of water: 1 - ice and icy areas; 2 - soil particles.

soil which is most conspicuous in dense soil with low ice content. For example, an analysis of changes in aggregation of thawing soils, consolidated before freezing by loads of 0.05 and 1 MPa, shows that during thawing of soils with relatively low moisture and high density, the microaggregate composition changes to a greater degree than in samples with lower density and greater moisture/ice content (see Fig. 4.6e,f). Transformation of structural elements of soils in this case mostly means disintegration during thawing along structural partings of large coarse (sand and dust) fractions and an increase in the smaller fractions. This process finally reduces the mean statistical size of structural separations of denser soils by 13.8 /¿m in polymineral clay and by 3.3 ¿um in heavy sandy clays. In the low density samples with higher moisture/ice content, the process of thawing caused a decrease of the mean size of aggregates by only 1 /¿m in clay and by 0.3 /.¿m in sandy loam. This difference in the disintegration of structural elements in loose and dense thawing soils is attributed to more intensive wedging activity of fine water films in overdensified samples.

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