Experimental And Numerical Study On Water/Leachate Transport In Landfill Of Municipal Solid Waste

Along with the rapid economic development and urbanization, the generation of municipal solid waste (MSW) increased drastically in China. As a consequence, land disposal of municipal solid waste becomes an even more common practice. Leachate water levels in the landfills in southern China are generally high, giving rise to serious slope stability and contamination problems. Water transport in landfills belongs to unsaturated-saturated flow. Therefore, it is of practical and theoretical significance to study the water/leachate transport in landfills.Focused on the issues of water in landfills, laboratory tests, field monitoring, and numerical analyses were carried out. The main contents and conclusions are as follows.(1) Soil-water characteristic curves (SWCCs) of MSW were measured using the samples obtained from borehole. The results show that the saturated water content and residual water content of MSW are relatively high, while the air-entry value is low. As the matric suction increase, the water content of MSW decreases greatly when the matric suction is low.(2) Saturated hydraulic conductivity of MSW was measured by constant head permeability test in laboratory. Hydraulic conductivity function of MSW was deduced based on the SWCC and was verified by laboratory tests. The results show that saturated hydraulic conductivity of waste decreases as an increase in burden pressure and fill ages. The slope of hydraulic conductivity function is steep when the matric suction is relatively low.(3) Perched water often exists in landfills in south of China. Unsaturated-saturated seepage analyses were performed on a landfill element to study the development of perched water on an interim soil cover. The analyses results demonstrate that rainfall intensity, hydraulic conductivity of interim cover, and initial water content of waste have a great influence on the perched water level. Soils with high or low hydraulic conductivity are not suitable materials to be used as interim covers.(4) Leachate levels in landfills in southern China are generally high. Numerical simulations were conducted to study the leachate levels corresponding to various situations. The leachate level and leachate production of Qizishan landfill were calculated. The calculated volumetric water content in unsaturated zone is about 40%, which is consistent with field measurement. The calculated leachate level isalso consistent with field monitoring. The calculated annual leachate production is close to the result of HELP model, and the calculated daily leachate production is more reasonable than that of HELP model.(5) The principle of double-ring infiltrometer was introduced. A serious of tests were carried out on the colluvium around Qizishan landfill to investigate the infiltrability and the influence depth. The results show that the infiltration rate in top soil layer is about 10^-4cm/s in magnitude and that the maximum infiltration depth is between 1m～1.2m. A method for calculating the subsurface water inflow was proposed, in which the complicated weather condition was taken into account. The reason for the ineffectiveness of the run-off interception ditch was revealed. Design methods to intercept the subsurface water are proposed.(6) Numerical simulations were carried out to analyze well-pumping in a landfill with an existing high leachate level. It was found that the hydraulic conductivity and SWCC of waste have great influence on pumping. The reasonable radius and spacing of pumping wells are analyzed. Pumping test was conducted in Qizishan landfill. The hydraulic conductivity of waste is determined to be 3 × 10^-4cm/s and the influencing radius is determined to be 20m.(7) The influence of high leachate level on landfill stability, leakage of leachate and on transport of contaminants was investigated by performing numerical simulation on the Qizishan landfill. The results show that the high leachate level reduces the safety factor of the current landfill and that it is necessary to take dewatering measures to increase stability. The safety factor of the expansion project will be less than 1 and translational failure will occur when the water level reaches a half of the landfill height. The high leachate level greatly accelerates the transport of contaminant through compacted clay liner. The permeability and the depth of the vertical curtain grouting greatly influence leachate leakage through it.Field and laboratory tests were carried out to investigate the water retention and permeability of municipal solid waste. Saturated-unsaturated flow analyses were conducted to investigate moisture transport in landfills and the results were verified by field monitoring. The issues related to water/leachate in landfills were analyzed. Advices and design methodologies such as interim covers, run-off interception ditch, leachate drainage system, pumping well and curtain grouting are proposed to improve the design and performance of present landfills. So this paper is of academic value and, at the same time, of practical value to landfill design and management.