The surrounding (back ll) materials around the underground power cable systems are essential for dissipiating the heat away from it, during the exertion phases. The heat dissipiation restrains the thermal instability and risk of progressive drying of the back ll materials, thus, reduce thermal stress on power cable. Thermal instability is the reduction of thermal properties (conductivity or diffusivity) due to migration of moisture because of heat accumulation. Thus, the back ll materials should have adequate thermal properties and favorable water retention capacity, which will falicitate the heat transfer easily from the heat source to the surrounding area with minimal moisture migration. The bentonite have high water retention capacity, but low thermal conductivity. Sand/ y ash exhibit low water retention and have higher thermal conductivity than bentonite. The addition of bentonite promote the water holding capacity and thermo-physical properties of sand and y ash. Therefore, this study presents the thermal properties of back ll materials, bentonite-y ash (B-F) and bentonite-sand (B-S) at varying weigth-percent of sand and y ash with bentonite. various compositions of the mixtures were compacted to varying dry densities and water contents and thermal properties variation of back ll materials were measured using a dual thermal needle probe 'KD2 Pro 2008' at room temperature. The study deals with systematic evaluation of the volumetric speci c heat capacity, thermal conductivity and diffusivity of back ll materials against varying dry density and water content. The threshold water content (TWC) has been determined from the thermal diffusivity-water content variation curve and it has correlated with plastic limit (PL) and optimum mosite conetn (OMC). Thereafter, the e cacy two thermal conductivity prediction models also were statistically evaluated with respect to experimental results.

The knowledge of soil thermal resistivity is required for projects such as thermal backfills for underground power cables, high-level nuclear waste disposal, pipelines carrying hot fluid and so on. The role of the backfill is to dissipate heat easily with minimal loss of moisture and low shrinkage. Clayey soils such as bentonite have high water retention capacity but low strength, high shrinkage and high thermal resistivity. Cohesionless geomaterials such as sand and fly ash exhibit low shrinkage and water retention. These materials have lower thermal resistivity than bentonite. The mix of sandfly ash with bentonite contributes to higher strength. Therefore, it is possible to mix bentonite with sand or fly ash to obtain an optimal thermal backfill mix with requisite thermal and engineering properties. There are few studies that deal with the thermal resistivity of bentonite-based backfill mixes. The present study deals with the development of the thermal resistivity variation curve (TRVC) for such mixes, which is the variation of thermal resistivity with water content. The critical moisture content has been determined from the TRVC and has been correlated with geotechnical properties such as plastic limit and optimum moisture content.

The surrounding (backfill) materials around the underground power cable systems are essential for dissipiating the heat away from it, during the exertion phases. The heat dissipiation restrains the thermal instability and risk of progressive drying of the backfill materials, thus, reduce thermal stress on power cable. Thermal instability is the reduction of thermal properties (conductivity or diffusivity) due to migration of moisture because of heat accumulation. Thus, the backfill materials should have adequate thermal properties and favorable water retention capacity, which will falicitate the heat transfer easily from the heat source to the surrounding area with minimal moisture migration. The bentonite have high water retention capacity, but low thermal conductivity. Sand/fly ash exhibit low water retention and have higher thermal conductivity than bentonite. The addition of bentonite promote the water holding capacity and thermo-physical properties of sand and fly ash. Therefore, this study presents the thermal properties of backfill materials, bentonite-fly ash (B-F) and bentonite-sand (B-S) at varying weigth-percent of sand and fly ash with bentonite. various compositions of the mixtures were compacted to varying dry densities and water contents and thermal properties variation of backfill materials were measured using a dual thermal needle probe ‘KD2 Pro 2008’ at room temperature. The study deals with systematic evaluation of the volumetric specific heat capacity, thermal conductivity and diffusivity of backfill materials against varying dry density and water content. The threshold water content (TWC) has been determined from the thermal diffusivity-water content variation curve and it has correlated with plastic limit (PL) and optimum mosite conetn (OMC). Thereafter, the efficacy two thermal conductivity prediction models also were statistically evaluated with respect to experimental results.