The Effect of Long-Term Permeation of Inorganic Salts on the Geotechnical Characteristics of Bentonite-Amended Clay Liners

Abstract

Bentonite-amended clay liners in engineered landfills are designed to prevent the migration of hazardous pollutants from leachate into groundwater. This study aimed to assess the changes in the geotechnical properties of bentonite-mixed clays when exposed to long-term contact with inorganic salts found in leachate. The plasticity, swelling, compressibility, and hydraulic conductivity of clayey soil mixed with 10% and 20% bentonite were tested under the permeation of water and 1M solutions of CaCl₂, MgCl₂, and NaCl. These results were then compared with samples soaked in 1M solutions of CaCl₂ and NaCl for 180 days to observe the effects of prolonged leachate exposure. In contrast to the results obtained when water was the pore fluid, the results showed that the liquid limit, plastic limit, free swell index, and free swell of bentonite-amended soils decreased in the presence of CaCl₂, MgCl₂, and NaCl solutions. Samples immersed in salt solutions showed significantly lower liquid and plastic limits than fresh samples prepared with the same salt solutions, indicating the detrimental effects of these salts on the liner's plasticity. Coefficient of consolidation values for the 10% and 20% bentonite samples immersed in CaCl₂, as well as for the 10% bentonite sample immersed in NaCl, showed an increasing trend with higher effective stress. After 180 days of immersion in salt solutions, the hydraulic conductivity of all samples exposed to CaCl₂ and NaCl dropped with effective stress and reached values less than 1 x 10⁻⁹ m/s. The addition of more bentonite did not significantly improve hydraulic conductivity when exposed to different salt solutions in long term. In conclusion, samples with higher bentonite content were found to be more susceptible to attack by inorganic salts over the long term.