Experimental Investigations on the State-Dependent Thermal Conductivity of Sand-Rubber Mixtures

Soil-rubber mixtures are potential construction materials for use in civil engineering applications such as the thermal insulation layer for buried pipelines in the permafrost zone and underground energy storage systems. Currently, however, the thermal conductivity of soil-rubber mixtures in various stress, saturation, and void ratio conditions is not fully understood. In this study, a stress-controlled apparatus based on the thermal needle probe technique was developed. It was employed to study the influence of the aforementioned factors on the thermal conductivity of sand-rubber mixtures. Thirty-four tests were completed with a wide range of net stresses (0-600 kPa), initial degrees of saturation (0%-100%), void ratios (0.45-0.95), and rubber contents (0%, 10%, and 20%). Three replicates were used in each condition. The results showed that the thermal conductivity was strongly dependent not only on void ratio, rubber content, and degree of saturation but also on net stress. For instance, the thermal conductivity of a dry sand-rubber mixture increased by 30% when the stress increased from 0 to 600 kPa. This increase in thermal conductivity can mainly be attributed to an increase in interparticle contact area, accounting for about 25% of the 30% increase; the reduction in void ratio played a minor role. A semiempirical equation was proposed to calculate the thermal conductivity of sand-rubber mixtures at different stress levels.