Experimental study of the environmental and geotechnical properties of landfills under long-term leachate effects: macro–microscopic tests on in situ clays

Clay structures are widely used as impermeable barriers to landfill pollution, although waste can degrade for more than 30 years, no studies concerning the long-term service performance of clay layers under leachate have been reported. In this paper, the in situ clay leaching in landfill for more than 20 years was studied by drilling and sampling. The results show that clay particle contents in soils decreased under long-term leachate effects, causing the liquid limit and plasticity index of the soil to reduce substantially, and the plastic limit to decrease to a lesser extent. The internal friction angle of clay increased, its cohesion decreased, and its compressibility increased, which caused the hydraulic conductivity of clay to increase by 2–4 times at constant porosity following leachate interaction. The long-term effects of leachate led to a reduction in the number of clay minerals, such as montmorillonite, in the soil, the coalescence of soil particles, an increase in the volume of large pores by 132%, a decrease in the volume of small pores by 14.4%, and increased pore connectivity. When landfill waste is undergoing middle and later stages of degradation, the leachate pollutant diffusion depth in the bottom clay layer is approximately 1 m. Since the groundwater is not polluted, our results confirm that the long-term safety aspect of landfill meets the bottom clay impermeability requirements specified in GB16889-2008 (hydraulic conductivity < 10⁻⁷ cm/s, thickness > 2 m).