Oxygen diffusion through partially hydrated geosynthetic clay liners

One of the prime purposes of a cover system in sulphidic mine tailings is to limit oxygen intrusion into the waste deposits so that generation of acid mine drainage is minimised. In areas where humid climates tend to prevail, a resistive barrier is usually used, either as part of a conventional multi-component cover or as a capillary break cover. Geosynthetic clay liners (GCLs) can be used in either case because of their low saturated hydraulic conductivity, provided they can be kept close to saturation to minimise oxygen diffusion. This paper presents the results of a laboratory investigation into oxygen diffusion through partially hydrated needle-punched and stitch-bonded geosynthetic clay liners. It was observed that the diffusion coefficient decreased with an increase in the degree of saturation. The variations in diffusion coefficient followed the same trend for the GCLs investigated, with variation of up to four orders of magnitude for the conditions investigated, indicating that low oxygen diffusivity can be achieved if GCLs have a high degree of saturation. However, it was found that the way that GCLs are held together as a composite material has a significant effect on the variation of gas diffusivity. Finally, unique empirical relationships between the effective diffusion of needle-punched and stitch-bonded GCLs and degree of saturation appear to exist and can be used to obtain an estimate of the diffusion coefficient value for use in calculations of diffusive flux through cover systems containing stitch-bonded or needle-punched GCLs.