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An Apparatus to Measure Gas Diffusion and Gas Permeability of Unsaturated Cap Barriers Subjected to Distortion

Landfill cap barriers are commonly designed to limit infiltration into the landfill, sustain deformation, and prevent the migration of landfill gases to the atmosphere. The gas flow through the unsaturated cap barrier could be due to advection, diffusion, or both based on the degree of saturation of the cap barriers. In the present study, a custom-designed test setup is developed to measure the gas permeability and diffusion coefficient of unsaturated cap barriers sequentially under various settlement rate and distortion levels. The apparatus comprises cylindrical lower and upper chambers, perforated overlapping triangular plates, worm gear, screw jack, motor, controller, and sensors. The lower chamber equipped with an oxygen sensor will be used to measure the variation in the oxygen concentration inside the chamber, leading to the measurement of gas diffusion of cap barriers. The measured quantity of nitrogen gas stored in the gas reservoir is allowed to flow through the cap barrier, leading to the measurement of gas permeability by advection. The gas permeability and gas diffusion at various distortion levels can be determined with the help of perforated overlapping triangular plates and a calibrated screw jack-motor system. The usage of this test setup in assessing the gas flow characteristics of the unsaturated cap barrier under the chosen distortion level is briefly illustrated using two types of cap barriers (compacted clay liner and geosynthetic clay liner) at different moisture contents.