Development of migrationalsegregated ice interlayers

The particular features of thermal-physical, physical-mechanical and physico-chemical processes in freezing ground create conditions for the formation of migrational-segregated ice layers with various orientation, frequency and thickness; in other words, they cause the appearance and development of cryogenic structures. These processes in freezing ground essentially transform the structure, density and strength of soils. For example, as a result of dehydration of the unfrozen part of the ground (migration of water to the frozen part), the processes of coagulation, aggregation and reorientation of soil particles develop intensively. These processes induce the formation of soil aggregates of different size and shape, thus creating various 'defective' zones which may later become zones of 'concentration' of significant stresses following from further development of shrinkage and swelling-heaving stresses in the freezing ground. They are generated in the still unfrozen desiccated part of the freezing soil. The bound water in the soil in the zones of critical stresses is under tension and, therefore, has a lower thermodynamic potential. This results in movement of moisture towards these zones (horizontal, vertical and tilted). As a result, the water migration in the unfrozen and freezing zones of the ground is determined not only by grad t, but by grad Pshr and grad Psw-ho as well, i.e.

The increase of water content in the zones of concentration of stress, which are located at the boundaries of the aggregates formed, continues only up to complete filling of the pores with water in the zone. Subsequent increase of water content is possible apparently only if ground cohesion is overcome by local microcracking, when a quick (jumplike) transition of water into ice takes place (the algebraic value of the potential increases sharply) in the former zones of 'concentration' of stresses. Owing to the increase of water volume by 9%, another force becomes active; the crystallization pressure of ice. The generation of a continuous ice layer is gradual (as is the formation of a gaping shrinkage crack in the unfrozen dehydrating soil). At first, in the freezing part of the ground near the freezing front, the discontinuous streaks of ice (seen only under a microscope) appear in jumps in the zones of concentration of stresses. At later stages, they become thicker schlieren, merge and form continuous linear layers of segregated ice, seen clearly in experiments not near the freezing front, but in the already frozen part of the ground (Fig. 2.12).

Therefore, the process of shrinkage and swelling-heaving in freezing soils on the one hand causes changes in volumetric stresses, and, on the other hand, to a certain degree, determines the configuration, or the type of the future cryogenic structure.

It is obvious that knowledge of the thermal exchange and migration of water in freezing soil is essential for the study of the mechanism and kinetics of segregated ice formation. However, the role in the formation of cryogenic structure of physico-chemical and physical-mechanical processes should be accounted for in full measure. In this respect, the thermal-physical conditions (heat and mass exchange) are necessary but insufficient for the appearance of segregated ice layers. Needed for this purpose are those physical-mechanical conditions under which the local strength of soil is overcome (without gaping cracks) and the microlayers of ice appear.

At present, the thermal-physical conditions of formation of cryogenic structures in freezing grounds are studied in greater detail than the physical-mechanical ones. The previous chapters reviewed the heat and water transportation and ice accumulation in freezing soils. This chapter is devoted to the general analysis of thermal-physical conditions, which cause the growth of ice layers and which are controlled by the heat flow relations in the unfrozen and frozen parts of the ground. Since the formation and growth of segregated ice layers in freezing soils occurs not only near the freezing boundary, but also within the range of negative temperatures, the general expression of the thermal conditions for the formation of schlieren segregated ice will be:

where Q[r and Quni indicate the quantity of heat passing, respectively, through the cooling surface in the freezing zone of the ground and from the unfrozen zone into the frozen zone; Qph and QJ,™ indicate the heat of phase transitions, that which escapes during formation of ice-cement from water fixed by the freezing process in situ, gpSh, and that from formation of segregational ice layers or ice-cement from migrating water in the frozen part of the soil, Q^.

It is obvious, that at Ag>0 the ground will freeze and its cryogenic structure will be formed. If AQ = (2pSh, freezing leads to a massive cryostruc-ture. If AQ > gj,sh, water migration and, possibly, formation of segregated ice layers or ice-cement will take place (when QJ,™ > 0). The thermal conditions for the growth of segregated ice in the frozen zone of thawing ground can be similarly expressed, if a little more complicated.

From experimental data on the thickness of the forming ice layers, it is obvious that the layers, parallel h\ and perpendicular hL, to the freezing front, can be represented, in a general way, by the following:

where f, I± represent intensity of the water migration flow to the ice layers which are parallel and perpendicular to the freezing front; the intensity is a function of grad P and grad t in the frozen part of the soil; AX| and Ax± are the intervals of the areas of growth of the ice layers which can be obtained (as shown below) only with allowance for the physical-mechanical conditions of segregated ice formation; ufr is the rate of freezing.

The generation of ice microlayers probably begins near the freezing front and reaches its maximum near the boundary where the direction of deformation changes (Fig. 3.3) and where intensive ice separation is observed (Fig. 2.12). This boundary is practically the boundary of equality of shrinkage and swelling-heaving stresses. It is not permanent with time, depends on the composition and structure of the soil and on conditions of freezing, and is usually located where temperatures are from —0.2 to — 4°C.

Ice interlayers parallel to freezing front

These appear as a result of development of horizontal zones of concentration of stresses caused by dislocating (or shearing ) stresses Psh = Psht + Psw-ho which appear during variously oriented deformations in the dehydrating and swelling layers of the ground. The horizontal shearing stresses naturally reach their maximal values on the boundary (£def) of change in the direction (sign) of the soil deformations (Fig. 4.1). These stresses are directly observed in the whole freezing zone. In the frozen part and in the unfrozen dehydrated part of the soil this effect is indirectly active through the overlying and underlying layers of soil. The result of this

Fig. 4.1. Development in freezing soil of stresses of shrinkage Pshr, swelling Psw, heave Ph0, horizontal shear Psh and normal volumetric stresses Pn and thus the wedging pressure of film water P™d, local cohesion Pcoh and cohesion at the boundary 'soil - ice layer P®~h'. Boundaries ¿;fr, idef are those of freezing, visible segregated ice formation and change in deformation direction in the sample, respectively; I, II are frozen and thawed parts of the ground, respectively.

Fig. 4.1. Development in freezing soil of stresses of shrinkage Pshr, swelling Psw, heave Ph0, horizontal shear Psh and normal volumetric stresses Pn and thus the wedging pressure of film water P™d, local cohesion Pcoh and cohesion at the boundary 'soil - ice layer P®~h'. Boundaries ¿;fr, idef are those of freezing, visible segregated ice formation and change in deformation direction in the sample, respectively; I, II are frozen and thawed parts of the ground, respectively.

interaction is horizontal shearing (dislocation) stresses primarily along zones of defects parallel with the freezing front. Boundaries of aggregates and blocks of the ground become such zones. In this way, the horizontal zones of concentration of stresses in soil form and, when the shearing stresses and the wedging pressure of thin water films, P"'ed develop further, they can be transformed into zones of local destruction of the soil. This occurs as a result of overcoming of the local cohesion of the ground (resistance to dislocation P^sh), when

■fsh + -Pwed = (^shr + ^sw ho) + P\ved > ^coh + ^dom (^-3)

where Pdom is the distribution of the external (including anthropogenic) pressure according to the depth of freezing ground. The satisfying of this condition means that water in the horizontal zones of concentration of stresses is no longer under stress, transforming by jumps into ice, generating horizontal ice microschlieren.

The area of the ground Ax||, where the condition of horizontal segregated ice formation is satisfied (Fig. 4.2), is a potential zone of concurrent generation and growth of ice layers parallel with the freezing front. It is determined, according to the plot in Fig. 4.1, by intersection of curves: psh + ^wed =/(*) and Pcoh + Pdom +/(*)•In the upper part of the freezing

Fig. 4.2. Formation of superimposed cryogenic structures when the boundary of freezing soil is irregular: 1 - surface of the ground, 2 - ice streaks.

Fig. 4.2. Formation of superimposed cryogenic structures when the boundary of freezing soil is irregular: 1 - surface of the ground, 2 - ice streaks.

zone and of the frozen zone, generation of new horizontal ice schlieren of the second and subsequent generations may occur. This process takes place in any case however, mostly as a result of sharply increasing wedging activity of fine films of unfrozen water (P™d) because the shearing stresses diminish (Fig. 4.1).

The enlargement of already formed ice microlayers may occur also within the frozen zone of the freezing ground at sufficiently low negative temperatures. This is related not only to the sharply increasing wedging by thin water films, but also to the need to overcome at the ice schlieren boundary not the local ground resistance to dislocation (P^sh), but the much smaller cohesion (Psc~h') of the ice layer to the frozen soil (Pl~h'<Pcohl When the condition Psh + P™ d > Pcoh + Pdom is not satisfied, then the appearance and growth of horizontal ice layers cease - for example, in the following cases, when:

1) the ground freezes at a very high rate, when water migration can barely take place and volumetric stresses do not develop in the ground;

2) a highly cemented rock is freezing the strength of which cannot be overcome by volumetric stresses;

3) freezing occurs under very high external pressure (for example, under high overburden pressure).

In the latter case, water migration occurs only until the pores of the ground are completely filled with ice, because after that the film water, due to lack of free space, will not move even if there are gradients grad ¡uw or grad f'wed =/(grad t).

Vertical ice schlieren

These ice streaks in the freezing ground are associated with tensile (normal) stresses as a manifestation of irregular shrinkage varying with depth, in the unfrozen part of the freezing ground (to the level of the change in the sign of deformation mdef). The swelling-heaving part of the freezing soil which lies above, obstructs the shrinkage deformations of the dehydrating part. Consequently, in the case of freezing, the volumetric strain (normal) stresses Pn must be equal to the difference between the stresses of shrinkage and of swelling-heaving. In this aspect the boundary of the change of direction of deformations ^def must be the conditionally 'free' ground surface, where normal stresses are equal to 0 (Pn = Pshr — Psw-h0 = 0). Below this boundary lies the maximum of normal straining stresses (Fig. 4.1).

The stresses Pn, distending the ground horizontally, must first of all affect the vertical and tilted zones of defects which are the lateral boundaries of soil macroaggregates and blocks, formed during the dehydration of unfrozen and freezing parts of the soil. In this way the vertical zones of concentration of stresses appear. The water in these zones is in a state of tensile stress, which permits the water to be supplied there under the effect of grad Pn and provides the possibility of overcoming the local strength of the ground to the point of rupture, — (with further development of strain-inducing normal stresses). The vertical microlayers of ice appear as a result of this process. The area of the possible generation of their Ax± is determined by the intersection of curves Pn =f(x) and =/(*), while a condition for generation of vertical segregated ice layers is:

This condition (Fig. 4.4) does not exclude the possibility of generation of vertical microcracks and fissures in the unfrozen part of the freezing soil (during a particular kind of freezing process).

The wedging activity of thin films of migrating water does not participate in the generation and growth of the vertical ice schlieren (contrary to the horizontal ones). This is attributed to the movement of water to the vertical zones of concentration of stresses only under the effect of grad Pn and not under the effect of grad f, which would cause the appearance of grad P™ a-This may relate to the frequent observation in nature of thinner vertical ice layers than horizontal ones.

A comparison between the generation of the horizontal and of the vertical ice layers shows that, in the first place, the probability of generation and growth of layers parallel with the freezing front is greater, because the total sum of Psh stresses and of the wedging pressure of the film water P™ed are involved. The formation of ice streaks perpendicular to the freezing front, on the contrary, is determined by the difference of stresses Pn. Secondly, the area of generation and vertical growth of horizontal layers in the freezing soil is larger and, therefore, the time of their formation is longer. Consequently, they must be thicker and more frequent than the vertical ice streaks. Thirdly, the vertical ice layers always appear at lower negative temperatures and grow in a wedge form pointing downwards, thus creating more favourable conditions for their rapid progression to the freezing boundary. Therefore, in experiments, as a rule, the growth of vertical ice layers and cracks is observed visually to overtake the growth of the horizontal layers and cracks in the unfrozen zone.

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