Effects of moisture content and dry bulk density on the thermal conductivity of compacted backfill soil

Soil backfilling and compaction are often involved in urban construction projects like the burying of power cables. The thermal conductance of backfill soil is therefore of great interest. To investigate the thermal conductivity variation of compacted backfill soil, 10 typical soils sampled in Zhejiang Province of China with moisture contents of 0%–25% were fully compacted according to the Proctor compaction test method and then subjected to thermal conductivity measurement using the thermal probe method at 20 °C. The particle size distribution and the chemical composition of the soil samples were characterized to analyze their effects on thermal conductivity. The results showed that the maximum thermal conductivity of fully compacted soils generally exceeds 1.9 W/(m·K) and is 20%–50% higher than that of uncompacted soils. With increasing moisture content, soil thermal conductivity and dry bulk density first increase and then remain unchanged or decrease slowly; the critical moisture content is greater than 20% in most cases. Overall, the critical moisture content of soils with large particle size is lower than that of those with small particle size. Quartz has the highest thermal conductivity in the soil solid phase, and the mass percentage of quartz for most soils in this study is more than 50%, while that for yellow soil is less than 30%, which leads to the thermal conductivity of the former being nearly twice as great as that of the latter in most circumstances. Based on regression analysis, with moisture content and dry bulk density as the independent parameters, the prediction formulae for the thermal conductivity of two categories of compacted backfill soils are proposed for practical applications.