Engineered granular materials for heat conduction and load transfer in energy geotechnology

Energy-related geosystems often impose extreme temperatures and loading conditions on the surrounding medium, so granular materials must be selected or engineered to satisfy heat transfer requirements and mechanical stability. In this work, the thermo-mechanical response of some natural and engineered granular materials was investigated by subjecting dense specimens to vertical load under zero lateral strain boundary conditions with concurrent thermal conductivity measurements. The materials studied were quartzitic sand with and without metal coatings, fly ash, diatomaceous earth, ceramic microspheres and hollow glass microspheres. Dry and densely packed hollow glass microspheres, ceramic microspheres and naturally occurring diatomaceous earth were found to be more compressible than sands, but exhibited very low thermal conductivity and very low stress-dependent gain in thermal conductivity. At the other extreme, dense sands combined the high thermal conductivity of quartz with the benefits of metal coatings to render the highest thermal conductivity values among the tested materials; while mechanically stable, dense sands were found to experience pronounced changes in thermal conductivity with stress. Analytical predictions show that saturation with high thermal conductivity liquids will enhance the effective thermal conductivity of granular materials more than the changes attained with metal coatings. Interparticle heat conduction processes and contact resistance explain the measured conductivity values obtained with the granular materials tested in this study.