Effects of wetting–drying cycles and NaCl solution on the hydro-mechanical behaviour and microstructure of compacted red clay-bentonite mixtures

Abstract

Compacted clays are widely used as barriers in municipal solid waste (MSW) landfills due to their low permeability, and the hydro-mechanical behaviour of such materials can be greatly affected by the environmental conditions. In this study, a series of wetting-drying (W-D) cycle tests and hydraulic conductivity tests were conducted on compacted red clay-bentonite mixtures. Various concentrations of NaCl solution were introduced during wetting to investigate the chemical effects. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests were performed to analyze the evolution of soil microstructure. Results indicate that the compacted mixtures undergo cumulative shrinkage during W-D cycles, reaching an elastic state after three cycles. The hydraulic conductivity decreased as bentonite content increases, becoming lower than 1 × 10 -9 m/s when the bentonite content exceeds 10%, which satisfies the requirement for the waste barriers. For a given bentonite content, the relationship between the logarithm of hydraulic conductivity and the void ratio can be well described by a linear regression equation. Additionally, the hydraulic conductivity initially increases and then decreases during the W-D cycles, peaking during the second wetting process. The presence of NaCl solution accelerates the microstructural evolution and cumulative shrinkage, particularly for pure red clay. Therefore, adding an appropriate amount of bentonite mitigates the effect of NaCl solution on the volume change. Furthermore, the addition of bentonite exhibits a dual effect: the lubrication effect dominates for the mixtures with low bentonite content, while the filling effect prevails as volume change decreases due to the restriction of aggregates rearrangement when the bentonite content is high enough to fill the macropores. • The compacted clay mixtures undergo cumulative shrinkage during W-D cycles. • The hydraulic conductivity of mixtures decreased as bentonite content increases. • The logarithm of hydraulic conductivity is linearly related to the void ratio. • Bentonite mitigates the effect of NaCl solution on the volume change of red clay. • The lubrication and filling effects of bentonite govern the volume change.

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