A model on assessing effects of gas diffusion in multifield coupled process for unsaturated soils

Abstract

Gas diffusion is an important process in thermo-hydro-mechanical coupled problems of unsaturated soils, e.g., nuclear waste repositories or heated pipeline engineering, which usually strongly affects process of liquid water evaporation and water vapor condensation, migration of air and water, heat transfer and skeleton deformation. This paper develops a simple model to assess the effects of gas diffusion on multifield coupled process of unsaturated soils, providing predicting guidance and uniform judgment rather than establishing a numerical coupled model by traditional methods. A dimensionless characteristic number is first proposed and expressed by sediment properties and environmental conditions. Rationality of this characteristic number is then assessed and validated by numerical results, and studies indicated that the gas diffusion is crucial when the characteristic number exceeds a critical one, which is reflected in early valleys of gas / liquid pressure increment induced by vapor condensation and the final maximum value. It can be found from the expression that this characteristic number is highly relevant to diffusion coefficient, permeability, temperature, initial saturation and gas pressure. Parametric studies indicate that gas diffusion appears to be critical for unsaturated clayey soils, provided the diffusion coefficient is larger than 10-7 m2/s and permeability smaller than 10-14 m2. The effect can be neglected for soils with diffusion coefficient is lower than 10-9 m2/s or permeability greater than 10-14 m2.