Changes in the thermal conductivity of bentonite-based grouts with varying volumetric water content

Abstract

In this study, the thermal characteristics of pure bentonite and bentonite–sand grouts for backfilling geothermal heat exchangers were investigated. Mix proportions were 20% and 30% of the total weight of bentonite, to which 30% and 50% of the total weight of quartzite sand were added. Specimens were prepared in rectangular parallelepiped shapes, and the thermal conductivity of partially saturated specimens with varying volumetric water content was measured. As the saturated specimen slowly dried out at room temperature, the change in volumetric water content with the elapsed time showed a typical soil–water characteristics curve. Thermal conductivity of the specimens increased and, after reaching the peak value, decreased again. A parabolic trend curve could be drawn to deduce the relationship between the thermal conductivity and the volumetric water content for each specimen. Empirical equations representing the correlation between these two parameters were also suggested for predicting the thermal characteristics of bentonite-based grouts in geothermal heat pump applications.