Mechanical properties of frozen ground

Mechanical (deformation and strength) properties of frozen soils are usually expressed by quantitative coefficients that determine a functional relationship between the value and type of mechanical action and response of the material. Deformation characteristics of frozen soils comprise moduli of total and elastic deformation, Poisson's ratio, indices of rheologic curves of flow and curves of creep, coefficients of viscosity and compressibility; strength characteristics are short-term and long-term values of shear strength (coefficients of friction and cohesion), compression, tension and equivalent cohesion.

The relationship between stress and deformation of frozen soils during

Fig. 8.8. Dependence of diffusion coefficient Kn on moisture content of unfrozen and frozen soils: 1 - silty clay material (see extreme right of graph); 2-4 - clay (2 - polymineral, 3 - kaolinite, 4 - montmorillonite). Freezing zone of the ground is shown by the dotted line.

creep at constant temperature is non-linear and can be described by a power function. With higher negative temperatures and with a progression from coarse-grained to finer-grained soils the same value of relative deformation is observed at smaller stress. Thus, when temperature is increased from — 20 to — 5°C similar values can be observed at stresses 3 to 4 times less. The relationship between stress cr and deformation s of the frozen material under uniaxial compression can be expressed by the modulus of total deformation as follows:

cr = E((T, t) e, where E is the modulus of total deformation (Pa) on compression dependent on time x and stress cr. Deformation s is expressed in fractions of a unit. Modulus E represents the tangent of the angle to the deformation axis drawn through ordinate point cr¡ corresponding to value t¡ of the isochronous curve.

Modulus of total deformation has a definite physical meaning directly reflecting resistance of the frozen soil to deformation. It diminishes with greater stress and longer time of load (Table 8.3). Higher dispersion (finer grain size) and higher temperature of the frozen soil also lead to reduction of total deformation modulus. Among factors that exert an influence on total deformation modulus are physical-mineral composition, moisture content, degree of saturation with ice etc.

The relationship between deformations and stresses in the elastic region is expressed by the modulus of normal (elongation) elasticity - Young's

Table 8.3. Modulus of total deformation offrozen soils (t = —2°C) in uniaxial compression (after Ye. P. Shusherina)

Rock (moisture

Rock (moisture

Table 8.3. Modulus of total deformation offrozen soils (t = —2°C) in uniaxial compression (after Ye. P. Shusherina)

content %)

Duration of loading

Load range MPa

E, Mpa

Sand (27)

1 min

+1 0

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