Consolidation behaviour of bentonites in the presence of salt solutions

Abstract

Due to its high swelling capacity and lower value of hydraulic conductivity, compacted bentonite is used as a liner material at the waste disposal site. Salts presents in landfill leachates may cause the diffuse double layer of bentonite to shrink which in turn affects its swelling and compressibility behaviour. Consolidation is among important properties of the liner material which need to be studied for settlement analysis. This study was carried out to investigate the effect of NaCl and CaCl2 solutions of varying concentrations, on the consolidation behaviour of two compacted bentonites having different mineralogical composition. The bentonites were studied in terms of changes in the mechanical parameters including the coefficient of volume change (mv), coefficient of consolidation (cv), compression index (Cc) and time taken to complete 90% of consolidation (t90) in the presence of the salt solutions. The results showed that the Cc, mv and t90 of the bentonites decreased, whereas, cv increased with the increase in salt concentration. Bentonite of higher quality defined by having a higher cation exchange capacity, exchangeable sodium percentage and swelling capacity, exhibited higher values for Cc, mv and t90 in comparison to lower quality bentonite. Results also showed that irrespective of the salt solution present cv decreased, whereas, t90 increased with increase in the consolidation pressure. However, with increasing in the pressure, mv increased initially and after reaching a peak value it decreased. A higher value of theoretical void ratio was obtained from diffuse double layer theory for both the bentonites at lower overburden pressure and water as pore fluid. However, with increase in the consolidation pressure the theoretically obtained void ratio–pressure curve converged towards the experimentally obtained curve and crossed it at certain pressure and diverged again. However, with salt as a pore fluid a higher value of experimentally obtained void ratio was observed.