Evolution of thermal conductivity of sand-clay mixtures under one-dimensional compression

The aim of this study is to reveal the evolution law of natural sand-clay mixtures under varied stress conditions. A series of one-dimensional compression tests and dry density and thermal conductivity measurements are conducted on both dry and saturated samples to investigate the variations of dry density ρ_d as well as thermal conductivity k with the 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 for sand-clay mixtures considering the effects of stress state and clay content is developed and verified. In addition, the microstructural characteristics and the thermal conduction mechanisms of the binary granular mixtures are discussed. The results indicate that the thermal conductivity k of sand-clay mixtures exhibits a trend of 'increasing first and then decreasing' with an increase in the clay content f, and the pore fluid significantly reduces the thermal resistance among soil particles and increases the sensitivity of thermal conductivity to both the clay content f and stress level. The maximum k value is found around f of 40%. The dry density ρ_d presents a similar evolution with that of k, where the maximum ρ_d 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, while the presence of the pore fluid imposes a negligible effect on ρ_d. The critical clay content f* and the minimum void ratio e_min are closely related to the state of stress, pore fluid and particle morphology. The proposed model for the thermal conductivity of 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. The particle deformation and breakage and the spatial structural evolution of the granular matter mixtures are recommended to further explore the thermal conduction mechanism.