Fig. 3.14. (a) Distribution of total moisture content against depth of frozen samples, (b) density of same samples, of kaolinite (/) and montmorillonite (//) clays during compressional compaction by loads 0 MPa, (1), 0.25 MPa, (2), and 1.2 MPa, (3); t = -1.5°C).
obvious in water-saturated (G = 1) frozen clayey soils where it can be 40-80% of the total (stabilized) deformation.
The mechanism of unfrozen water movement in a frozen soil under compression is, apparently, mostly filtrational (under the application of an external pressure gradient), though the process of migration of film water to the periphery of the sample due to the gradient of the total thermodynamic water potential should be also taken into account. Seepage takes place when, at the boundary between a porous disc and frozen soil, the pressure in the unfrozen water is equal to atmospheric, whereas in the central part of the sample it is at a maximum. Concurrently, in the central part of the sample, due to higher pressure, the unfrozen water content grows as a result of ice melting and the water films get thicker compared to those in the peripheral parts. Therefore, seepage and migration induce the water to move from the centre of the sample to its periphery and beyond the limits of the frozen soil. The outflow of water from the soil system is accompanied by the redistribution of ice content and of the mineral skeleton through the height of the compressed sample, as is clearly shown by the curves of changes in water content and of density of the frozen soil (Fig. 3.14). Moreover, at first the initial consolidation involves the peripheral parts of the sample and later its central part as well. The cryogenic structure of frozen soil samples changed in the course of an experiment from massive (before compressive densifica-tion) to microlayered (the layers are oriented at right angles to the load acting) in the central part and micromesh in the peripheral part of the sample (after the compression load effect). A significant reduction of the total water content and first of all of ice as a result of seepage-migrational consolidation in the compressed frozen sample, can be clearly traced in Fig. 3.15.
Differentiation of the total compression compaction of frozen soils into seepage-migrational (primary consolidation) and attenuating creep deformation (secondary consolidation) is not easy because the two kinds of deformation are indivisible and continuous in time. Compaction of frozen soil as a result of secondary consolidation is usually associated with irreversible dislocations of ground particles, of their aggregates and of ice, which cause sharp changes in cryogenic microstructure. In the course of this process, the microaggregates of the ground approach each other and become larger, thus producing a denser packing. This process, however, does not exclude a possible breaking-up of individual larger aggregates. Concurrently with this dispersion and aggregation of the frozen soil skeleton in the course of attenuating creep, a reorientation of the basic planes of clayey particles and aggregates in the direction perpendicular to pressure, is observed. Densification of frozen soil, due to the decrease in porosity previously filled with ice and to more compact packing of the particles of the frozen soil, makes it much stronger owing to greater electromolecular bonds between the particles as they approach each other. It should be added that, when frozen soil is compressed, the structure of the schlieren and pore ice is also essentially reworked (into more fine-grained ice) with fusion of the sharp facets of ice crystals, and a larger number of viscoplastic flows of ice crystals and of their aggregates.
In general, we can state that the principal causes of compressional compaction of frozen soils under constant load and negative temperature are irreversible displacements of structural elements of the frozen materials which occur as a result of closing of empty pores and cracks and of viscoplastic flow of ice and because of volumetric (viscous) creep deformations of the mineral skeleton with concurrent squeezing out of unfrozen water from the frozen ground system.
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