Fundamental problems in thermal conductivity measurements of soils

Measurements of thermal conductivity, open porosity, water content and water saturation degree have been made on two different kinds of samples, one incoherent (earth) extracted at different depths from the surface in a building excavation in the nearby of Rome, the second, a compact sedimentary rock (blue malmstone) just under the surface from the ills surrounding Alba (Piedmont, north of Italy). For the thermal conductivity the probe method (line heat source) has been used, taking care of the practical problems arising during tests (minimizing thermal contact resistance, drilling hole for the probe housing). Results show the typical behaviour of thermal conductivity vs. temperature of a porous medium containing water. Thermal conductivity and saturation degree clearly increases with depth under the soil surface due to increased compactness and less water evaporation in air. Also open porosity reduces with depth due to increased compactness. Results show also the presence of four typical phenomena happening during the soil heating: storage of heat due to thermal capacity of the material, heat conduction driven by thermal conductivity, water evaporation due to heating and vapour diffusion and circulation.