The results of a series of gas permeability tests, with monitoring of gravimetric/volumetric moisture content and total suction, on a commercially available needle-punched geosynthetic clay liner (GCL) are presented. GCL specimens were partially hydrated with deionised water under 2 and 20 kPa confinement prior to testing. The tests were conducted at differential pressures ranging from 1 to 10 kPa. Gas permeability was found to decrease with an increase in gravimetric/volumetric moisture content and a decrease of suction. The effect of the preconditioning stress was found to be more pronounced at gravimetric moisture contents greater than 40% (25% apparent degree of saturation, 0·30 m3/m3 volumetric moisture content), and suctions less than 1·6 MPa
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.
Super-saturated salt solutions are used to control relative humidity (RH) and to infer the hydration (water uptake and loss) behaviour of three needle-punched geosynthetic clay liners (GCLs) with respect to time under conditions of both free swell and 20 kPa applied stress. It is shown that RH and applied stress play a key role in the hydration behaviour with time when GCL specimens were in equilibrium with water vapour. It was also observed that water uptake and loss was affected by the bentonite form (powdered or granular) and mineralogy of the bentonite. However, the effect of GCL structure (i.e. difference in geotextiles and bonding of needle-punched fibres to the carrier geotextile) on their hydration behaviour for GCLs with similar form of bentonite was not significant for RH ≤ 97.7%. The effect of GCL structure became more apparent at 100% RH (for all GCLs). The results presented in this study can be used to better assess the hydration of GCLs in field applications such as waste containment liners and cover systems at different RH and overburden stress conditions
The Vapour Equilibrium Technique (VET) has gained popularity as a reliable method for controlling relative humidity as well as vapour suction in various porous media. In this study, super-saturated salt solutions were used to control Relative Humidity (RH) and to infer the water retention curve for a needle punched geosynthetic clay liner with respect to time under free swell and 20 kPa applied stress. Results showed that GCL sample RH plays a key role in moisture absorption/desorption behaviour and equilibration time when RH is in equilibrium with suction. Water absorption isotherms for the bentonite component of GCL agreed well with literature data. It was also observed that the effect of applied stress on water retention was more pronounced at high relative humidity and corresponding low suction. The results presented in this paper can help improve the understanding the water retention behaviour of GCLs in field applications such as waste containment liners and cover systems.
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