A manufactured thermally conductive granular backfill product compacted in a trench around an underground electrical transmission cable, the manufactured granular backfill product including a mixture of a granular base material having an R value which is higher than 1.2 when compacted, with a sufficient amount of an additive of a granular iron compound having an R valve which is less than 1.2 when compacted, so as to provide a manufactured thermally conductive granular backfill product having an R value of 1.2 or lower when compacted.
