Relationship between thermal conductivity and soil-water characteristic curve of pure bentonite-based grout

The relationship between the soil-water characteristic curve (SWCC) and thermal conductivity of pure bentonite and bentonite-sand grouts used as backfilling materials in ground heat exchangers was investigated in a laboratory experiment. The mix proportions utilized in this study were bentonite to 20% and 30% of the total weight, adding quartzite sand to 30% and 50% of the total weight (bentonite + water). Mixed grout specimens were prepared in rectangular parallelepiped shapes. The thermal conductivity, volumetric water content and matric suction of unsaturated specimens were measured as the saturated specimens were slowly dried at room temperature until there was little change in the measured values. The matric suction and thermal conductivity showed a bilinear relationship, with a breaking point identifying the air-entry value (AEV) and the SWCC describing the relationship between the matric suction and the volumetric water content (VWC). As the matric suction slowly increased to the AEV, the VWC of the specimens decreased, whereas the thermal conductivity increased; then, beyond the AEV, it rapidly decreased again. That is, as the specimens dried out, attaining the maximum at the AEV, their thermal conductivity decreased. The thermal conductivity and the VWC showed a parabolic relationship with the maximum thermal conductivity value at around the VWC corresponding to the AEV of each specimen. Revised empirical equations representing the relationship between these two parameters were suggested for prediction of the thermal characteristics of bentonite-based grout in geothermal heat pump applications.