Because the thermal conductivity of minerals is much higher than that of water and air, thermal conductivity of soil decreases with increasing porosity.
4.3.2 Degree of Water Saturation, Sr
The thermal conductivity of air in a soil or aggregate’s pores is negligible but the conductivity increases with increasing degree of water saturation.
Fine soils generally have a high porosity and a low quartz content and, consequently, the thermal conductivity of dry clay and silt is low. However, the fine pores of these soils more easily hold a higher amount of water and fine soils typically deliver thermal conductivities in the same range as other soils.
The thermal conductivity of ice is four times higher than that of water and consequently the thermal conductivity of soils with a high degree of water saturation increases dramatically at or below freezing. Here it should be kept in mind, that fine soils at temperatures below 0°C can still hold a large amount of unfrozen water and that thermal conductivity increases, therefore, progressively with decreasing temperature. Coarse soils typically have low degrees of water saturation and thermal conductivity does not increase significantly at freezing.
A typical relationship between thermal conductivity and water and ice content is shown in Fig. 4.2. The relationships shown assume only ice or only water, respectively.
