Thermal conductivity evolution of sand-clay mixtures under one-dimensional compression

The aim of this study was to reveal the evolution law of natural sand-clay mixtures under varied stress conditions. A serial of one-dimensional compression tests, dry density, and thermal conductivity measurements were conducted on both dry and saturated samples to investigate the variations of dry density ρd as well as thermal conductivity k with increasing stress level and clay content f, and the influences of pore fluid on packing behaviors and heat flow efficiency of the mixtures. Based on the binary packing theory, a new thermal conductivity model of sand-clay mixtures considering effects of stress state and clay content was developed and verified. In addition, the microstructure characteristics and the thermal conduction mechanisms of binary granular mixtures were discussed. The results indicated that dry density ρd of sand-clay mixtures exhibits a trend of ‘increasing first and then decreasing’ with an increase in clay content f, where maximum ρd value is detected as the f is in the range of 30% to 40%. Increasing vertical stress is beneficial for improving the compaction behavior of the mixtures, and the presence of pore fluid imposes a negligible effect on ρd. Thermal conductivity k presents a similar evolution with that of ρd, while pore fluid not only significantly reduce the thermal resistance among soil particles, but also increases the sensitivity of thermal conductivity to both clay content f and stress level. The maximum k value is found around f of 40%. The critical clay content f* and the minimum void ratio emin are closely related to the state of stress, pore fluid, and particle morphology. The proposed calculation model of thermal conductivity for the sand-clay mixtures comprehensively incorporates the hybrid effects of state of stress and fine content, which is also consistent with the binary packing theory and has good applicability. Particle deformation and breakage, and spatial structural evolution of the granular matter mixtures are recommended to further explore the thermal conduction mechanism.