Closed-Form Equation for Thermal Conductivity of Unsaturated Soils at Room Temperature

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Thermal conductivity is a fundamental physical property governing heat transfer in soil. It depends on soil types, pore structure, temperature, and moisture content, and can vary over an order of magnitude. Some theories have been established to address the effect of temperature on thermal conductivity. Other theoretical works focus on the effect of moisture content on thermal conductivity of different sandy and loamy soils. This work addresses the effect of all soil types and moisture content on thermal conductivity for ambient temperatures from 20 to 25°C. Thermal conductivity varies little within the hydration and capillary regimes, varies moderately within the funicular regime, and varies greatly within the pendular regime. A closed-form equation is proposed and validated by explicitly considering the soil-water retention regimes. Results taken from the literature for 25 soils and from the current study for two clay soils show that the closed-form equation can accurately predict thermal conductivity of all types of unsaturated soil.