Thermophysical Properties of Bentonite-sand/fly ash Based Backfill Materials for Underground Power Cable

Abstract

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.