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The thermal properties of soil have drawn great research attention in recent years. Thermal conductivity is the most important such property, particularly for the design of shallow geothermal systems such as energy piles and borehole thermal energy storage. A newly developed thermo–time domain reflectometry probe was used to measure the thermal conductivity of quartz sands with the peak value method. Three rounded quartz sands with different particle sizes were used in experiments to investigate the effects of particle size and fines content on thermal conductivity. Results revealed that thermal conductivity increased with increasing particle size for uniform sands. The thermal conductivity was lower in fine sands than in coarse sands under dry conditions but higher in fine sands than in coarse sands at low moisture contents. The thermal conductivity of coarse sands was increased to peak value by increasing the fine sands content to the critical fines content corresponding to maximum thermal conductivity. The critical fines content increased with a decrease in the mean grain size ratio of fine sands to coarse sands; the content also was related to the grain shape.

Geothermal energy particularly shallow geothermal energy has become increasingly popular in the geotechnical engineering community. Design of optimized and efficient geothermal systems requires better understanding of soil thermal behavior which relies on accurate characterization of soil thermal conductivity, along with moisture and density. In this study a thermo-TDR probe was designed and evaluated against a standard heat probe for thermal property measurement. In addition, the probe also enables the simultaneous measurement of moisture and density, along with thermal conductivity. This unique feature makes it possible to obtain high quality data for geothermal related studies. Ottawa sand and Kaolin clay soils at different moisture conditions were tested using the developed thermo-TDR probe. The obtained data show the thermo-TDR probe can measure moisture content and density, as well as thermal properties with reasonable accuracy.

In the design of shallow geothermal systems such as energy piles and ground source heat pumps, it is essential to have accurate characterization of soil thermal properties. In general practice, soil thermal properties are estimated from design charts or semi-empirical models developed in the middle of the last century. These methods do not consider site specific conditions and often result in inaccurate estimation. In this study, a thermo-TDR probe sensor was developed to accurately measure soil moisture content, density as well as thermal properties. The developed thermo-TDR probe was calibrated and tested for brown sand at different moisture content and density. The performance of the thermo-TDR probe was verified with a standard heat probe as well as model predictions. Results show the thermo-TDR probe can be very useful for thermal study of soils.