Fundamental study of thermal conduction in dry soils

The thermal conductivity of the different soil components-mineral, liquids and air-varies across two orders of magnitude. Two studies are implemented to explore the role of contacts in heat conduction in dry granular materials. The first set of experiments is designed to elucidate heat transfer at contacts, and it is complemented with a numerically based inversion analysis for different local and boundary conditions to extract proper material parameters. Then, the thermal conductivity of dry soils is measured at different packing densities to address the relevance of coordination number and particle shape effects. Together, both studies confirm the prevailing effect of contact quality and number of contacts per unite volume on heat conduction in granular materials. Interparticle contacts and the presence of liquids in pores play a critical role in heat transfer, and determine the ordered sequence of typical thermal conductivity values: k _air < k _dry-soil < k _water < k _{saturated-soil} < k _mineral.