Use of a thermo-TDR probe to measure sand thermal conductivity dryout curves (TCDCs) and model prediction

- Thermo-TDR probe is capable of measuring sand TCDCs satisfactorily in a holistic manner.
- The probe provides a more accurate estimation of sand thermal conductivity under varying moisture content condition.
- Thermo-TDR probe is a promising tool to investigate sand thermal and geotechnical properties for geothermal applications.

Thermal conductivity is a governing soil thermal property for geothermal applications. Measurements of soil thermal conductivity and moisture content are usually obtained separately by different sensors, which may lead to errors in measured thermal conductivity dryout curves (TCDCs). Use of a newly designed thermo-time domain reflectometry (TDR) probe to measure sand TCDCs and model prediction were carried out in this study. Laboratory experiments were performed on four quartz sands with different grain sizes. The calibration relationships for dielectric constant (Ka) and electrical conductivity (ECb) were first established by a multiple specimen method. The thermo-TDR probe was then used to measure sand thermal conductivity, and predict its moisture content and dry density in dryout process produced by a modified hanging column device (MHCD) for establishing sand TCDCs. Sand thermal conductivity was also predicted by three alternative soil thermal conductivity predictive models, and then compared to measured values. Statistical analyses were conducted to further evaluate the model performance and validate the reliability of the proposed method. It is concluded that the thermo-TDR probe is capable of measuring sand TCDCs satisfactorily in a holistic manner, and providing a more accurate estimation of sand thermal conductivity under varying moisture content condition. Measurement error might be increased when sand is under near dry condition due to the heat convection effect.