Laboratory investigation of thermally induced desiccation of GCLs in double composite liner systems: Fid-Bau Portal

Laboratory investigation of thermally induced desiccation of GCLs in double composite liner systems

Azad, Farzad M. / Rowe, R. Kerry / El-Zein, Abbas / Airey, David W.

The experimental results presented in this paper give some insight into the behavior of geosynthetic clay liners (GCLs) under conditions representative of those occurring in double composite liner systems (DCLSs). Under certain conditions, thermally driven water redistribution can cause desiccation of GCLs. As a result, GCLs must be used with care at the base of DCLSs where desiccation of GCL may occur. Based on this investigation, the following conclusions can be drawn: 1. The best means of minimizing the risk of GCL desiccation is to keep the thermal gradient as low as possible. 2. The lower the initial subsoil water content, the greater the risk of desiccation of a GCL Thus the placement of GCLs over relatively dry subsoils should be avoided for landfill applications. No GCL desiccation was observed in these experiments when the subsoil below the GCL had an initial water content of 10% or higher and the primary geomembrane temperature was about 40 °C or less. However, desiccation cracking did occur a) for GCLs in a primary composite liner on a foundation layer with 10-11% initial water content under a geomembrane with a temperature of 45 °C and b) for GCLs in a secondary composite liner on subsoil with initial water content of about 5% under a geomembrane with a temperature of 40 °C. 3. Low initial GCL water content can increase the risk of desiccation when the GCL is not in direct contact with the subsoil. This shows that there is a need to hydrate the GCL in the primary system before a significant thermal gradient is applied. In the present experiments better performance was observed for GCLs with a higher mass of bentonite at a given water content. However this may only be true as long as it is possible to hydrate the GCL (e.g., by introducing water to the leak detection system) and may not be true if the only mode of moisture uptake is from the relative humidity in the leak detection system. 4. For the conditions examined with temperatures on the primary geomembrane up to 45 °C, there was a low risk of desiccation of the GCL in the secondary composite liner when the underlying silty-sand subsoil had initial water content of 10-11 % or higher. 5. Although the bentonite can form open cracks upon drying, the cracks do not necessarily lead to a permanent increase in hydraulic conductivity of the GCLs. In flexible wall permeameter tests on desiccated samples permeated with deionized water, the bentonite swelled and the cracks closed upon wetting. Thus the high initial hydraulic conductivity decreased with time to values similar to the initial values for the conditions examined. While this is encouraging, the permeation with distilled water is a best-case scenario. In real landfill situations, the hydraulic conductivity may not recover to the same extent because of bentonite chemical interactions with leachate permeant. 6. The key factor in hydraulic performance of desiccated GCL is its exhumed water content. The lower the water content, the higher the initial hydraulic conductivity.