A Numerical Model to Explain Experimental Results of Effective Thermal Conductivity Measurements on Unsaturated Soils

Effective thermal conductivity measurements on unsaturated soils by means of the probe method (thermal conductivity probe, TCP) often present a nonlinear trend of T versus ln (τ ). Three different slopes are present in the plots, while a homogeneous material should present only one. Being soils composite structures made of different phases (solid earth, liquid water and air), a possible explanation is the presence of phenomena other than pure conduction, such as water evaporation and vapor migration through the soil structure.Anumerical model based on finite differences has been developed to simulate these phenomena. The model takes into account several factors including heat conduction, heat storage due to thermal capacity, water evaporation and water diffusion through a porous medium. Results show that two of the three slopes can be successfully simulated by the model, confirming the interpretation of the phenomena. However, the third slope from the experimental data is lower than the model’s slope, likely indicating the presence of other phenomena not yet taken into account, such as capillarity.