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Sand-tire shred mixtures are useful as thermal backfills due to their lower unit weight and thermal conductivity than those of most soils. In this study, a series of thermal conductivity tests on sand-tire shred mixtures and pure sand were performed to investigate the effects of volumetric mixing ratio and tire shred particle size. A volumetric mixing ratio of 40% was found to yield the greatest decrease in thermal conductivity from that of pure sand, with a maximum percent difference of 72%. Using tire shreds with larger relative size ratio was found to result in higher thermal conductivity, and the maximum variation in the thermal conductivity percent difference with the relative size ratio can reach about 20% at a volumetric mixing ratio of 40%. An empirical model proposed to predict of the thermal conductivity of quartz sand-tire shred mixtures could capture trends in the experimental data.

Particle shape is known to affect the mechanical behavior of sands as it influences packing density and particle contacts. Even though the thermal conductivity of sands also depends on packing density and particle contacts, the effects of particle shape on thermal conductivity are not well understood. A series of thermal needle tests were conducted on five granular soil mixtures with different proportions of rounded and angular glass particles having the same mineral composition and gradation. The maximum and minimum void ratios and the packing density of these mixtures were found to depend on the overall regularity, defined as the average value of the particle's aspect ratio, convexity and sphericity. For a given overall regularity, the thermal conductivity increases with decreasing void ratio or increasing relative density. Interestingly, the overall regularity has a small effect on the thermal conductivity at a given void ratio but has a significant effect on the thermal conductivity at a given relative density. A particle shape-dependent empirical equation is proposed to quantify the effects of relative density and overall regularity on the thermal conductivity of the tested sand.

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