Thermal diffusivity of bentonite–sand based engineered backfill material

Due to the advantages of high swelling and water retention capacity, bentonite is commonly used as one of the thermal backfill materials for geothermal structures application. However, the low thermal diffusivity of pure bentonite indicates the poor conduction of heat from the geothermal systems to surrounding ground which leads to the thermal instability of geothermal systems. Sand has high amount of thermal diffusivity with low water retention capacity than the bentonite. This study presents the results of laboratory investigation on the enhancement of thermal diffusivity of bentonite, with the addition of sand. For the experimental investigations, the sand content, in the order of 20–80%, was mixed with bentonite. Moreover, the bentonite, sand and bentonite–sand mixtures were tested in laboratory using the transient heat flow probe method, at varying dry densities (1.0–1.4 Mg/m3) and moisture content (0–53%). The effect of volumetric water content (combined effect of dry density and moisture content) and sand content on the thermal diffusivity of tested samples were studied. The experimental results were compared with previously developed empirical models, which are based on the volumetric water content and texture of soil. Further, the influencing parameters were discussed based on Spearman correlation coefficient among the various geotechnical parameters and thermal diffusivity of engineered backfill materials. From, the results, it has been observed that the dry density, soil textures and degree of saturation are the most influencing parameters to couple the heat-water transfer in the vicinity of heat source embedded with engineered backfill material.