Gas advection-diffusion in geosynthetic clay liners with powder and granular bentonites

Gas diffusion and gas permeability tests were performed sequentially on powder and granular partially hydrated needle-punched geosynthetic clay liners (GCLs) over a range of gravimetric water content using a gas flow unified measurement system under 2 kPa and 20 kPa vertical stresses. Most of the changes in diffusion and advection occurred at intermediary levels of saturation or gravimetric water contents where diffusive and advective gas migration in the granular GCL tended to be higher than in the powder GCL. When the GCLs were relatively dry, their gas diffusion and gas permeability remained constant due to the large interconnected air voids present in the bentonites. For relatively wet conditions, the difference in their gas diffusion and gas permeability was minimal as the bentonites developed a relatively uniform gel structure. The results suggest that at a nominal overburden pressure of 20 kPa, GCLs such as the ones studied need to be hydrated to more than 160% gravimetric water content or >80% apparent degree of saturation before gas diffusion and permeability drop to 1.0 × 10⁻¹¹ m²/s and 2 × 10⁻¹³ m/s, respectively.