A New Method for Simultaneous Measurements of Gas Dispersion Coefficient and Gas Coefficient of Permeability of Unsaturated Soils

This study developed a new method to simultaneously measure gas dispersion coefficient and gas coefficient of permeability (GCP) of an unsaturated soils with due theoretical consideration of steady-state gas transport mechanisms. Correspondingly, a soil column apparatus was developed. Based on the proposed method, gas mixture containing air (with a volumetric concentration >90%) and tracer gas were applied to the column base. The gas dispersion coefficient was estimated based on a measured steady-state tracer gas concentration gradient and a dispersive flux of the tracer gas, the latter of which was determined based on mass conservation of the tracer gas. On the other hand, the GCP was determined based on the total flux of gas mixture and gas pressure gradient following Darcy's law. The new method has the following advantages over existing methods: (1) has an ability to determine gas dispersion coefficient and GCP at any soil depth rather than an average value of a test region; (2) measures the two transport properties at varying soil moisture conditions rather than being limited by uniform moisture distribution; and (3) requires only a simple hand-calculation method to estimate the gas dispersion coefficient without the need to apply nonlinear regression on a gas breakthrough curve. The proposed method to determine GCP and gas dispersion coefficient of a silty sand was verified by independent element tests.