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Fig. 8.4. Temperature dependence of thermal conductivity of materials of different grain size: 1 - rudaceous rock waste with sandy silty infilling material; 2 - fine sand; 3 - light fine sandy-clayey material; 4 - loess-like silty clay material; 5 - medium silty-clay; 6 - clay; 7 - well-degraded peat.

Fig. 8.4. Temperature dependence of thermal conductivity of materials of different grain size: 1 - rudaceous rock waste with sandy silty infilling material; 2 - fine sand; 3 - light fine sandy-clayey material; 4 - loess-like silty clay material; 5 - medium silty-clay; 6 - clay; 7 - well-degraded peat.

size of grains leads to a greater number of contact thermal resistances and is accompanied by the increase of hydrophilic nature and of ultraporosity which contribute to the higher content of liquid phase characterized by lower thermal conductivity compared to that of ice. This principle, as a rule, applies within the whole temperature range (from + 20 to — 20°C) including the range of intense phase transitions of moisture; it applies for soils with various moisture content.

The mineral composition of soils exerts a general influence over their thermal conductivity, although indirectly, via factors of rock structure. Thus, Fig. 8.5a shows an almost twofold difference in thermal conductivities of sandy silty materials of quartz and of vermiculite compositions in the temperature range —15 to +15°C. It is explained by the difference in thermal conductivities of quartz (~ 5 W m~1 K ~ *) and vermiculite (~ 2 W m-i £-1) Apart from mineral composition the value is influenced by organic matter content, because organic matter is characterized by a comparatively low thermal conductivity: for peat X = 0.46 W m~1 1. Therefore, the higher the peat content, the lower the thermal conductivity (see Fig. 8.5b).

One of the most important factors determining thermal properties of soils is their salinity, which influences their phase composition and structural transformations. Salinity leads to the increase of the liquid phase content thereby reducing thermal conductivity.

Since thermal conductivity of the soil mineral skeleton is, as a rule, higher than that of water and ice, compaction is accompanied by higher thermal conductivity. Wetting of a soil leads to substantial increase of X since air

X, W m "1 k~1

Fig. 8.5. Temperature dependence of thermal conductivity of soils of different composition: (a) - saturated soils of different mineral composition; 1-2 - heavy-sandy silty material (1 - quartz, 2 - vermiculite); 3-4 - clay (3 -montmorillonite, 4 - kaolinite); (b) - sand with various peat contents and peat: 1-3 - sand (1 -W = 18.6%, n = 0.94, pd = 1.6g cm"3, q = 0; 2 -W = 66.5%; n = 0.80, pd = 0.81 g cm"3, q = 0.25; 3 W= 78.3%, n = 0.80, pd = 0.67 g cm 3, q = 0.40); 4-peat, W= 116.5%,n = 0.80, od = 0.57gcm"3.

Fig. 8.5. Temperature dependence of thermal conductivity of soils of different composition: (a) - saturated soils of different mineral composition; 1-2 - heavy-sandy silty material (1 - quartz, 2 - vermiculite); 3-4 - clay (3 -montmorillonite, 4 - kaolinite); (b) - sand with various peat contents and peat: 1-3 - sand (1 -W = 18.6%, n = 0.94, pd = 1.6g cm"3, q = 0; 2 -W = 66.5%; n = 0.80, pd = 0.81 g cm"3, q = 0.25; 3 W= 78.3%, n = 0.80, pd = 0.67 g cm 3, q = 0.40); 4-peat, W= 116.5%,n = 0.80, od = 0.57gcm"3.

having a low thermal conductivity (0.023 W m 1 K is replaced by water with a higher thermal conductivity (0.57 W m_1 K-1) or ice (2.29 W m_1 K"1).

Unlike heat capacity, thermal conductivity of soils is not an additive quantity which makes X dependent on factors that determine soil structure, i.e. textural aspects. Of paramount importance is the manner of heat transfer in the soil - directly by particles, from particle to particle via contacts or from particle to particle through an intermediate medium.

Cryogenic structure also belongs to the factors responsible for soil texture and exercises substantial influence on soil thermal conductivity. As shown by experiments, at similar values of moisture content and density, clays having massive cryogenic structure are characterized by a higher thermal conductivity than those having schlieren structure. Account should be taken of the anisotropy of thermal properties for soils with schlieren cryogenic texture, which manifests itself in higher values of X (usually by 20-30%) when heat flows along the ice lenses rather than across them. Peat has a noticeable anisotropy of thermal properties.

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