Critical Review of Thermal Conductivity Models for Unsaturated Soils

Although it is well established that heat conduction in unsaturated soil depends on liquid saturation, there are several models available to consider the changes in thermal conductivity during drying and wetting. The key factors affecting thermal conductivity of unsaturated soil are evaluated through a critical examination of these different models and their development. Depending on the principles and assumptions employed, these models are categorized into three groups: mixing models involving series/parallel elements; empirical models where thermal conductivity values at dry and saturated states are used; and mathematical models based on phase volume fractions. Experimental data for different soils are used to assess the quality of prediction for these models. It is found that all the existing models do not realistically account for pore structure or interface properties, and thus are not capable of properly predicting thermal conductivity as a function of liquid saturation. A conceptual model based on soil-water retention mechanisms, is proposed to overcome the pitfalls of the existing models and can be used to establish quantitative thermal conductivity models for variably saturated soils in the future.