Modeling Water Flow In Landfill Basing On The Preferential Flow And Anisotropy

Leachate level is a key problem in landfill stability, settlement and capacity, pollution control and gas collection. The high leachate level is common in the domestic, however, the usual dewatering methods can’t effectively control the water level, and the theoretical analysis is often difficult to reflect the engineering practice. The water flow of landfill is highly non-uniform for the multiple dimension porosity, which is caused by the heterogeneous character of the waste material itself. There is a significant difference in the hydraulic properties between landfill and homogeneous body, which mainly reflected in two aspects:1) the landfill water flow is dominated by the preferential flow, and the large pore channel is the main path of the water flow; 2) the pore in the landfill is compressed to show a significant anisotropy, caused by the layered disposal and compaction procedure. Based on this, the thesis focuses on the preferential flow and anisotropy, to reveal the mechanism of leachate migration, and obtain the temporal and spatial variation of the drawdown at landfill under the condition of dewatering, mainly through the field test and numerical, theoretical analysis. The main research works and conclusions are followed: (1) This thesis investigated the waste compression changes of pore shape, pore size distribution and pore alignment, following the compression characteristics from the domestic and foreign research. Based on that, we derived a saturated permeability model according to the Poiseuille equation, which establish the relationship between the permeability and pore distribution, mainly about the generation and development of preferential and anisotropy at landfill. The permeability model can accurately describe the change of horizontal, vertical saturated permeability coefficient and anisotropy value under increasing pressure, when comparing with the indoor and outdoor test datas from literature.(2) This thesis investigate the influence of hydraulic characteristics of landfill on leachate migration by the numerical simulation of SEEP/W, basing on the pumping effects of the vertical well at 3 landfill sites in the domestic. We laid special stress on analyzing the variation of pumping flow and the drawdown caused by the permeability changes along the depth, the existence of anisotropy and preferential flow. The simulation results show that the landfill flow characteristics of anisotropy and preferential flow is directly related to the difference in the performance of vertical well dewatering.(3) In this research, the leachate distribution and landfill hydraulic properties in-situ landfill material have been examined quantitatively and qualitatively at Changan landfill in the ShiChuan. Through the well recovery and pumping test, we get that the water flow in landfill has obvious heterogeneity, exhibiting the difference of pumping performance between multiple vertical wells at the same landfill site. In addition, the investigation about changes of resistivity in the process of leachate pumping and salt tracer by electrical resistivity technical (ERT) reveald the landfill hydraulic properties, that there are significant anisotropy and preferential flow characteristics in the field. In general, the in-situ test further proved that the anisotropy and preferential flow of landfill have a significant impact on the leachate migration.(4)Finally, according to the Neuman well flow model, a vertical well flow model for modeling landfill dewatering has been developed in this thesis, which accounting the landfill hydraulic properties of anisotropy and preferential flow. The results show that the landfill drawdown curve is obviously influenced by the anisotropy and preferential flow, different from the homogeneous soil, it gradually incline to the horizontal direction with the increase of time. The calculation drawdown by using this model, are in good agreement with the numerical simulation results and the field measured data.