Fig. 8.10. Rheologic curves for flow deformation of soils of different grain size and mineral composition (uniaxial compression with constant velocities) (according to Y.V. Kuleshov): 1-3 - clays (1 - bentonite; 2 - poly mineral, 3 - kaolinite); 4 - silty clayey material; 5 - sandy silty material; 6 - sand; 7 - ice.
and rate of deformation, is caused by the slower rise of the yield point <rcr with lower temperature compared with other bounding stresses. At the same time a diminished Bingham part is explained by higher fragility of the frozen soil which prevents visco-plastic deformations.
It has been confirmed by many studies that frozen soils of different granulometry and mineral composition have different resistance to the development of flow deformation (Fig. 8.10) and, accordingly, their effective viscosity coefficients differ. In a general case, other conditions being equal, the viscosity coefficient increases with coarser grain size and higher rigidity of mineral skeleton. First of all, it is induced by increase of their short-term strength. At the same time, according to the data furnished by N.K. Pekar-skaya, obtained after creep tests, at small stresses and fairly low negative temperatures to — 30 °C, long-term strength of clays usually exceeds that of sands. This is associated with the effect of primary structural cohesion of mineral particles, typical of fine-grained materials. At low rates of deformation the values of effective viscosity for clays and sands can become similar.
Data from investigations show that moisture content (ice content) affects viscosity of the frozen materials greatly and this influence has an extreme nature. In general, when pores are not filled completely (G < 1 ), soil viscosity increases with higher ice content, reaching a maximum value at G = 0.8 to 0.9. After that, viscosity diminishes and with still higher ice content tends to reach the value of the viscosity of ice.
Frozen fine-grained soils under the pressure of overburden or overlying engineering structures become consolidated as a result of complex physical-
mechanical and physico-chemical processes. As shown in the experiments of S.S. Vyalov and N.A. Tsytovich and later A.G. Brodskaya, the ground has a substantial compressibility under load. According to the investigations, consolidation of the soil is conditioned by deformability and displacement of each component: gaseous, liquid (unfrozen water) and plastic-viscous (ice and solid mineral particles). In general, the compression curve (curve of consolidation) of the frozen soil has a pattern as shown in Fig. 8.11.
Three main parts can be distinguished in the compression curve of frozen soil aav a1a2 and a2a3 . The part aa1 is the elastic and structurally recoverable deformation. The pressure value corresponding to point oi1 is near to the structural strength of the frozen soil and when this is exceeded compaction begins and the porosity decreases. The part ata2 of the compression curve is the structurally irreversible main deformation and is up 70 to 90% of the total deformation. The subsequent part of the curve represents a strengthening at greater loads.
The total (stabilized) subsidence of frozen ground due to consolidation 0Cq is determined in compression tests as the value of the relative compressibility coefficient of the frozen ground, using the equation a£0 = Sx, /(hP), where S^ is the stabilized subsidence of the soil layer with the controlled negative temperature; h is thickness of the soil layer in the oedometer; P is the applied pressure.
Taking into consideration mineral and granulometric composition frozen soils can be arranged in the following series according to compressibility: montmorillonite clay > polymineral clay > kaolinite > sandy silty material > sand (Fig. 8.12, Table 8.5). Thus, at a pressure of 0.3 MPa and a temperature of — 1.5 °C the compressibility coefficients of montmorillonite clay are 1.5 times that of kaolin, 1.2 times that of polymineral clay and almost twice that of sand. Coarser materials are less subject to deformation by compression. The high content of unfrozen water in montmorillonite clays contributes to more complete and longer development of creep processes in them. Of particular importance are irreversible displacements of mineral particles and aggregates associated with both inter- and intraaggregate porosity. The microaggregates themselves undergo substantial transformations, losing their thin coating of fine particles, and becoming compact and dense.
Salinity of frozen soils drastically increases their compressibility owing to the development of seepage-migration stages of deformation. This is explained by the higher content of unfrozen water due to salinity and the reduced ice component which makes the properties of the frozen soils nearer to those of unfrozen ones.
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