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The thermal conductivity of dry soils is related closely to air pressure and the contact areas between solid particles. In this study, the thermal conductivity of two-phase soil systems was determined under reduced and increased air pressures. The thermal separation of soil particles, i.e., the characteristic dimension of the pore space ( d ), was then estimated based on the relationship between soil thermal conductivity and air pressure. Results showed that under both reduced and increased air pressures,  d  estimations were significantly larger than the geometrical mean separation of solid particles ( D ), which suggested that conductive heat transfer through solid particles dominated heat transfer in dry soils. The increased air pressure approach gave  d  values lower than that of the reduced air pressure method. With increasing air pressure, more collisions between gas molecules and solid surface occurred in micro-pores and intra-aggregate pores due to the reduction of mean free path of air molecules. Compared to the reduced air pressure approach, the increased air pressure approach expressed more micro-pore structure attributes in heat transfer. We concluded that measuring thermal conductivity under increased air pressure procedures gave better-quality  d  values, and improved soil micro-pore structure estimation.

Thermal separation of soil particles from thermal conductivity measurement under various air pressures