Study On The Water Transport In The Landfill Earthen Final Cover And Its Controlling Method

The principal goal of landfill final cover is to prevent or control infiltration of precipitation, thereby reducing leachate production, and to control the emission of landfill gas into the atmosphere. The alternative earthen final covers (AEFCs) are constructed with relatively non-plastic soils with greater durability and lower cost and it requires relatively lower post-closure maintenance than conventional covers, so AEFCs are good choices for the landfill final closure. At present, many landfills in China are close to its service life, so they need a final cover. Therefore, it is of practical significance to study the performance of AEFCs.The research works in this paper are based on the general project "Study on the water transport between landfill earthen final cover and atmosphere and its controlling method in humid areas" funded by National Science Foundation of China. Numerical simulations, model tests and analytical methods were carried out to study the issues. The main research works and conclusions are as follow.(1) The influence of rainfall patterns on the infiltration into landfill earthen final cover was investigated based on the infiltration-runoff boundary conditions proposed by Gitirana (2005). The results show that as the peak rainfall intensity occurs earlier, the cumulative infiltration will be larger for the non-uniform pattern under the same daily precipitation; the cumulative infiltration of advanced pattern (A1) will be larger than the uniform pattern under certain conditions; the cumulative infiltration of rainstorms will be less than the rainfall pattern with 24 hours duration; not only the uniform pattern but also the advanced pattern (A1) should be taken into account in the study of long term performance of AEFCs in humid areas.(2) An analytical solution for water movement in an infinite slope was presented on the basis of the governing equation for unsaturated flow. To obtain the analytical solution, it is assumed that both the soil-water characteristic curve and permeability function can be described with exponential functions. The analytical results were compared with the results obtained from the numerical simulation, which shows that the analytical solution is reasonable. The influence of soil properties and antecedent rainfall and subsequent rainfall intensity on the water movement and storage were investigated. The results show that the less the saturated permeability, the shallower the wetting front; the lager the desaturation coefficient, the shallower the wetting front; the larger the water storage capacity (θs-θr), the shallower the wetting front; for the capillary barrier, the volumetric water content and matric suction in the fine-grained layer changes greatly, but the volumetric water content profile in the lower layer almost remains at the initial value.(3) The model test system comprised of model tank, rainfall simulator, water collection and measurement system and monitoring system was developed. The water transport between capillary barrier and atmosphere and the performance of capillary barrier with unsaturated drainage layers under high rainfall intensity were investigated through the model test. The results show that runoff occupies 69%～77% of the total precipitation. The UDL can drain the water from AEFCs through lateral diversion. The lower the saturated permeability of surface layer, the lateral diversion will be more dominant than the deep percolation.(4) Numerical models combined the landfill earthen final covers with wasted disposal were developed. The influence of rainfall intensity, the thickness of the UDL, the thickness and saturated permeability of surface layer, the inclination of the cover on the diversion length were investigated through numerical simulations. The results show that the UDL can increase the diversion length of the common capillary barrier and reduce the occurrence of perched water. The thickness of the UDL can influence the diversion length in the initial stage of rainfall, but it can not increase the final diversion length of AEFCs subjected to continuous high rainfall. The steeper the cover slope, the more water will be drained from the cover, especially in the UDL. Some suggestions on the utilization of lateral diversion capacity of inclined capillary barriers were proposed to optimize the cover design.(5) The numerical model that can describe the interactions between landfill earthen final cover-vegetation-atmosphere was developed. The model based on the non-isothermal flow equation considering the thermal moisture coupling takes the meteorological condition, the evaporation from the ground surface and the transpiration of the vegetation into account. The performance of AEFCs in humid, sub-humid, semi-arid areas in China was investigated. Some suggestions for the AEFCs design in China are given based on the numerical simulations.