The experimental study on thermal conductivity of backfill material of ground source heat pump based on iron tailings

•The thermal conductivity of iron tailings is higher than that of loess at same condition.•Mixtures of tailings and loess present higher thermal conductivity than single use of loess and iron tailings.•When loess and tailings are mixed with the mass ratio of 3:7, the thermal conductivities are higher than that of the others.•The thermal conductivity of backfill materials shows great relationship with saturation.•A empirical equation of thermal conductivity of backfill material was proposed.