Bentonite as sealing material in geosynthetic clay liners - Influence of the electrolytic concentration, the ion exchange and ion exchange with simultaneous partial desiccation on permeability

摘要

Geosynthetic clay liners (GCLs) are sealing layers, which usually contain natural sodium bentonite as a effective sealing element, which is encapsulated between geotextile components. They are used in a variety of sealing applications, predominantly in hydraulic engineering and groundwater protection. They are most commonly employed to replace compacted clay liners (CCLs). In contact with water, bentonites swell and thereby close their pore spaces, which accounts for their low permeability. This extreme swelling requires water with a low content of electrolytes. Contact with chemicals in leachates and other solutions can lower the swelling of a GLC and thereby lead to a higher permeability. Commonly GCLs are covered with soil to create counter pressure against the swelling process in order to receive a denser structure with low hydraulic permeability. Besides this, the soil cover protects the GCL against damaging. In this geo-chemical environment an ion exchange of sodium-bentonite to calcium-bentonite occurs due to two reasons. Firstly calcium is often predominating, secondly bivalent Ca~(2+) is more easily exchanged against monovalent sodium (Na~+) than vice versa. This exchange typically takes place over a period of several month to few years, if in contact with cover-soil seepage. The ion exchange effects an increase in hydraulic conductivity of approximately 1/2 to 1 decimal exponent. This is however not alarming, as long as planning engineers take this effect into consideration. Landfill capping systems as well as sealings along highways through water protection areas are characterized by unsaturated conditions, as often found in applications with low confining stress (<15kN/m~2, less than 0.75 m soil-cover). In such cases, GCLs tend to show desiccation cracks, which cause a significant increase in permeability. In contrast to compacted clay liners, where self sealing is unlikely to occur due to low confining stress, a self sealing of calcium-bentonite GCLs takes place by swelling and plastification of bentonite, if a soil-cover of more than 0.75, better 1.0m (equals a confining stress of 15-20 kN/m~2) is provided.