Frozen Thermal Conductivity Of Gneiss

taken into account that the thermal conductivity of water, ice and air is linearly dependent on temperature. Thus, with a temperature drop of 1K the coefficients X of water and air diminish by 2 x 10"3 and 9 x 10"5', respectively, while that of ice increases by 5 x 10-3Wm~1K~1.

Heat capacity values of rocks and minerals are rather steady and comparatively well studied experimentally. Specific heat capacities of soil components (mineral skeleton, ice, water-gas and peat) vary within a narrow range; Csk = 0.71-0.88kJ kg"1 K1; C, = 2.09kJ kg"1 K1; Cw = 4.19kJ kg-1 K_1; Cg= 1.02kJ kg"1 K_1; Cpeat = 0.8-2.1 kJ kg"1 K1. Heat capacity of soils is an additive quantity and is the sum of the specific heat capacities of each component multiplied by the amount of their mass.

Thermal conductivity of frozen rocks and soils

Comparative analysis of experimental data shows that the thermal conductivity of intrusive rocks increases from 2to5Wm~1K~1ina series dunites - gabbro - syenite - diorites - granites, i.e. from basic to acid rocks. This is explained by the difference in Si02 content - the higher the content, the greater is the thermal conductivity. Thermal conductivity of extrusive rocks is also dependent on their chemical-mineral composition and degree of crystallization and varies, as shown by experimental data, within the range 2.0-3.6 W m"1 K" With higher Si02 content their thermal conductivity increases in the series: porphyries - andesites - trachytes - basalts. As shown by analysis of thermal conductivity of metamorphic rocks, X varies within a wide-range - from 0.8 to 7.4 W m~1 K~ it increases from slates to gneiss to quartzites which is explained by the gradual disappearance of schistosity in this series. Thermal conductivity of sedimentary cemented rocks differs substantially between three subgroups: 1) cemented rock waste; 2) cemented silty and clayey materials; 3) chemical and biochemical. The first one is represented by rudaceous and coarse rocks - conglomerates, gritstones and sandstones with X from 1.5 to 4.5 W m~1 K"1. The range is determined by the particular thermal conductivity of the fragments and cement.

Thermal conductivity of unfrozen silty and clay-rich cemented materials represented by aleurolites and argillites is, on the average, lower than that of rudaceous and fragmental rocks and varies in the range from 0.8 to 2.2 W m-i t^ explained by their finer-grained structure for which a greater number of contact thermal resistances is typical.

The subgroups of chemical and biochemical rocks, for example, siliceous rocks of marine origin (tripoli, diatomite) are, in general, characterized by lower thermal conductivity as compared with all the above, 0.8-1.7 W m"1

K-1, which is explained by the high porosity in combination with low thermal conductivity of the skeleton of these rocks. Such monomineral rocks as dolomite and anhydrite are characterized by the highest thermal conductivity, respectively, 7.2-11.9 and 3.7-5.8 W m"1 K"1, while limestones, 5.7 W m"1 K"1, and marls 2.6W m"1 K"1, have a lower thermal conductivity.

Rudaceous rocks, being multi-component and multi-phase systems, have a wide range of thermal conductivities. The upper limit of thermal conductivity for rudaceous rocks is that of hard rock fragments (up to 3-9 W m"1 K"1), while the lower limit (0.3-0.5 W m"1 K"1) is set by the thermal properties of fine-grained material. With higher content of big fragments X increases as the thermal conductivity of large fragments is greater than that of fine-grained rock waste. Rudaceous rocks with high moisture content have higher thermal conductivity in the frozen state compared with the thawed; this is associated with transition of ground water into ice and the four-fold increase of its thermal conductivity.

Variation of the thermal conductivity of rudaceous rocks is to a significant degree dictated by phase composition of moisture, dependent on temperature and type of infilling material. Thus, within the temperature interval from —10 to — 1 °C, X of rudaceous materials with sandy infilling is practically constant. This is explained by the fact that the main phase transformations of moisture in this material take place in the range of 0 to

— 1 °C. Below — 1 °C the ice to unfrozen water ratio virtually does not change. In contrast, the thermal conductivity of materials with sandy silty and silty clay infilling diminishes with higher temperature, the rate of reduction depending on the liquid phase/ice ratio. A marked increase of X, equalling 25-30% on the average occurs between 0 and — 5°C. Within the

— 5 to —10°C range thermal conductivity is assumed to be constant since phase composition of the moisture does not vary. Within the positive temperature range (0-25 °C) X of rudaceous rocks increases linearly, but insignificantly, associated with the linear dependence on water temperature.

It should be noted that thermal-physical characteristics of rudaceous rocks with a different type of infilling become more uniform with higher content of coarse particles. This is explained by the fact that material characterized by different coefficients of thermal conductivity (sand, sandy silty, and silty clay-rich materials) are gradually replaced by rock waste having uniform (unchanging) values of thermal conductivity.

With other conditions being equal, thermal conductivity of soils diminishes with decrease of grain size in the following sequence: rudaceous -sandy - sandy-silty - loess - silty-clayey - clay - peat (Fig. 8.4). A smaller

X, W m"1 k 1

1 1 • • a

0 0

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