The Thermal Behavior of Cable Backfill Materials

The thermal behavior of a number of soils including those frequently used for high-voltage cable backfill has been studied in the laboratory and in the field. Initially, thermal resistivity tests using a probe and thermal moisture migration tests were carried out in the laboratory. From these tests it was found that well-graded granular materials behaved the best. In particular "limestone screenings," a by-product from rock quarries, was found to have highly desirable thermal properties. A full-scale field test was made by means of a simulated cable installation. The materials tested included a uniform sand, a manufactured sand and "limestone screenings." The placement moisture and density of the materials was varied as well as the heat input. Observations of moisture and temperature gradient were made over a two and one- half year period. The results of this study confirmed the results of the earlier laboratory tests. The screenings and the manufactured sand, both of which were well graded, exhibited a low resistivity which remained stable during both dry and wet periods. The uniform sand had a higher initial thermal resistivity and became unstable due to moisture migration during dry periods of the year. Subsequently, a few simple tests have been made to investigate the flow of vapor and liquid in soil under a thermal gradient. A tentative explanation of the varying behavior of the materials, based on their respective porosity characteristics, is given.