Thermal conductivity and shrinkage characteristics of bentonite-fly ash and bentonite-sand backfill material

Thermal backfill is an integrated part of underground electrical cable infrastructures systems, ground heat source pumps and radioactive waste repositories, as it minimizes resistance to heat transfer away from these systems. The heat transfer capacity and current carrying capability of underground electrical cables are significantly affected by thermal conductivity of backfill material and the surrounding soil media. Therefore, this research paper compares the thermal conductivity and shrinkage results of compacted (low to high densities) fly ash- and sand-bentonite mixtures with bentonite contents of 30%, 50%, 60%, 80% and 100%. The thermal conductivity of mixtures increased from 1.05 Wm⁻¹K⁻¹ to 1.20 Wm⁻¹K⁻¹ with the addition of fly ash content from 20 to 70% by weight in bentonite. The thermal conductivity bentonite-sand mixture was also found to be increased from 1.21 Wm⁻¹K⁻¹ to 1.83 Wm⁻¹K⁻¹ with increasing sand content. Additional to this, the bentonite-sand and bentonite-fly ash-based backfill materials surrounding heat-sensitive structures experience shrinkage and desiccation cracking due to thermal drying. Therefore, the desiccation volumetric shrinkage tests of bentonite-sand and bentonite-fly ash mixtures were conducted and found that the presence of sand or fly ash reduces shrinkage strain. Based on the experimental results, this study suggests a sustainable utilization of fly ash up to 50%-70% as an effective thermal backfill material in electrical cable infrastructure systems. Thus, the application of fly ash as a construction material reduces environmental impact and cost, aligning with the goals of sustainable development.