Derivation of a correlation formula for the effective thermal conductivity of geological porous materials by the three-phase geometric-mean model

A general formula to express the effective thermal conductivity of dry or moist geological materials has been derived by means of a three-phase model with the data obtained for eighteen kinds of porous rocks of relatively simple mineral compositions under various moisture conditions. The formula is mathematically based on the weighted geometric mean, and it is modified to take the effect of porosity into account. The porosity of the rocks was covered from 5% to 51%. The conductivity was measured by a transient hot-wire comparative method at 20 degrees C. The experimental data have been expressed with a formula with an accuracy of +/-22%. Furthermore, this formula was found to be applicable to altered rocks of more complicated mineral compositions, artificially compacted bentonite as buffer material for high-level radioactive nuclear waste, and some kinds of concrete.