Thermal conductivity is a key parameter for the design of various geothermal structures such as geothermal energy piles, ground source heat pumps and soil borehole thermal energy storage. In this paper, the thermal conductivity of sand-kaolin clay mixtures prepared at varied moisture content, dry density and clay content was measured using a newly designed thermo-time domain reflectometry probe. The data revealed that the thermal conductivity of the sand-kaolin clay mixtures increased with an increase in moisture content and dry density. The thermal conductivity of the moist sand-kaolin mixtures decreased with an increase in clay content in two distinct regimes including a slow decrease in thermal conductivity above a critical clay content, which was found to be 10%. Five soil thermal conductivity models were selected to predict the thermal conductivity of the sand-kaolin clay mixtures, and the predictions were compared with the experimental results. Two models were modified to account for the effect of clay content on the thermal conductivity of the sand-clay mixtures. Recommendations regarding the design of energy geostructures were made based on the measured thermal conductivity data.