Influence of Soil Gradation, Moisture Content, and Density on Thermal Resistivity of Soils for Buried Cable Applications

The thermal resistivity of sand backfills significantly affects buried power transmission cables’ performance and longevity. This study examines thermal resistivity of poorly graded (SP) and well-graded (SW) sands under varying relative density, moisture content, and atmospheric exposure. Laboratory tests used the Transient Line Source method on sand samples at 20%, 50%, and 80% relative density with 0%–8% moisture content. Samples with 8%, 4%, and 2% initial moisture were exposed to atmospheric conditions for up to 24 hours to assess moisture loss effects. Results show thermal resistivity decreases significantly with higher moisture content and density, with SW sand exhibiting 10–15% lower resistivity than SP sand due to better particle packing. At 8% moisture, resistivity ranged from 0.489–0.640 m.K/W for SP sand and 0.389–0.543 m.K/W for SW sand. Atmospheric exposure increased resistivity by 123–158% over 24 hours, especially at high initial moisture. A multilinear regression model incorporating sand type, moisture, density, and exposure time achieved high accuracy (R² = 0.97, RMSE = 0.089 m.K/W). These findings offer valuable insights for optimizing sand backfill selection and design, enhancing thermal management and cable ampacity in underground installations.