A model also provides a tool, which has the ability to predict gas concentration levels, and could ultimately provide a foundation for engineering tools capable of predicting landfill behaviour for the purpose of design or remediation. The numerical model currently under development incorporates the presence of a contaminant gas within the gas phase. The pore gas phase is assumed to be a multicomponent mixture of dry air, water vapour and a contaminant gas, i.e. methane or carbon dioxide. The model is comprised of four non-linear partial differential equations for the system variables of capillary potential, temperature, pore air pressure and the molar concentration of contaminant gas. The effect of the thermal gradients on the migration of the contaminant gas and the pore water is included due to the elevated temperatures that occur in landfills due to decomposition.
Unsaturated soil is a three-phase system comprised of liquid, gas and the solid soil particles.
In this formulation, the liquid phase is considered to be water containing the dissolved contaminant gas and air, whilst, the gas phase is considered to be a binary mixture of dry air and a contaminant gas. Both the liquid and gas phases are assumed to flow through a rigid porous matrix, consequently, the proposed model reduces to a two-phase system. The aim of this work was to simulate the behaviour of gas migration through highly saturated compacted clays and to ascertain the applicability of a two-phase flow model based on Darcy's law in this regime. In order to validate the two-phase flow approach, the numerical model was compared against an experimental program to determine the breakthrough pressure which was carried out with the Department of Civil and Geological Engineering, University of Manitoba . The test program consisted of a series of one-dimensional bench scale gas conductivity tests on clay specimens of approximately 50 mm diameter by 24 mm height with varying moisture content, dry density and degree of saturation.