| The research on the water transport in municipal solid waste(MSW)landfill is important for issues such as leachate production,leachate recharge,landfill gas generation and collection,and landfill slope stability.Regarding the analysis of the water transport in landfill,MSW is generally regarded as a uniform porous medium with a certain permeability,and the method of saturated-unsaturated flow analysis is used.However,the high heterogeneity is the most prominent characteristic of MSW,and the composition of MSW is very complex.It contains both fine-grained matrix such as slag and humus and large-sized components such as plastic and bamboo which has macropores characteristics.Therefore,the water transport in MSW has both the characteristics of matrix flow and preferential flow,and the preferential flow will occur under certain conditions.There is no unified understanding of whether unsaturated flow or preferential flow is used to simulate the water movement in MSW at present.Researching on the boundary between unsaturated flow and preferential flow in MSW is the premise of selecting a suitable mathematical model for analysis of water transport in landfill.Based on the National Natural Science Foundation project“Preferiential flow character of municipal solid waste and its influence on water movement in landfills(51478256)”,the hydraulic characteristics and preferential flow features of MSW were studied through laboratory experiment.Combined analysis of skew coefficient with numerical simulation,the applicability of unsaturated flow and preferential flow were verified.The main work and corresponding results of this article are as follows:(1)The pore structure characteristics of MSW were analyzed based on the particle gradation test.The results show that the unevenness coefficients C_u of the MSW particles in the shallow,middle,and deep layers are 173,121,and 21,respectively,which are significantly larger than 5,indicating that the particle size of MSW varies widely.The curvature coefficients C_c are 0.54,0.17 and 0.73respectively,which are obviously less than 1,indicating that the lack of intermediate particle group and insufficient filling of fine particles in macropores.(2)The unsaturated hydraulic characteristics of MSW were studied by using the complete soil-water characteristic curve and transient profile test.The results show that the saturated water content and the residual volume water content of MSW are relatively high,which corresponds to the characteristics of MSW containing macropores and inherent water,respectively.The field capacity of MSW is 42%,and the steep drop of the SWCC appears in low matrix suction range.The saturated permeability coefficient of MSW is large,and the permeability coefficient decreases significantly with the decrease of water content.The unsaturated hydraulic conductivity of matrix suction 9.5,9,and 8.9k Pa(at the height of 15,10 and 5cm of waste column)of the middle layer waste obtained by the transient profile method are(3)The characteristics of preferential flow in MSW were analyzed by dye tracer test and image processing technology.The results show that the larger the dyeing area ratio,the higher the proportion of preferential flow involved in movement.The deeper the dyeing depth,the lower the matrix flow ratio,and the more obvious the preferential flow in MSW.The greater the infiltration intensity,the higher the preferential flow in MSW.The higher the initial moisture content in MSW,the more obvious preferential flow.The preferential flow is more likely to occur in shallow MSW.The rules for vertical slice are the same for that of horizontal.(4)The breakthrough curves obtained from the Cl~-breakthrough tests under different infiltration rates were analyzed for skew coefficients.The corresponding infiltration rate limit value when the preferential flow occurred was quantitatively depicted.The results show that the smaller the infiltration rate,the smaller the value of the skewness coefficient,which only tends to a small value of 1.12.At low infiltration rate(8,5mm/h),the skewness coefficient does not change much,so the infiltration rate of 8mm/h which corresponding to the steady skewness coefficient S=1.12 is regarded as the limit infiltration rate for the occurrence of preferential flow.It has important reference significance for choosing unsaturated flow model or preferential flow model to study water transport of MSW.(5)SEEP/W which applied to unsaturated flow was used to simulate the measured values of the transient profile test to verify the flow characteristics at 5mm/h infiltration rate.The results show that the change process of the matrix suction and volumetric water content of each section of the simulated waste column with time is closed to the results of the transient profile test.It is verified that the water migration at 5mm/h infiltration rate can be simulated by unsaturated flow.(6)Water leaching tests at different infiltration rates were performed,and the flow characteristics under the infiltration rate of 12mm/h were verified by HYDRUS-1D simulation.The results show that the water content of macropores involved in water transport in water leaching test increases with the infiltration rate.The infiltration rate of 8mm/h can be used as the limit infiltration rate of middle-layer waste.The change of water content at 12mm/h infiltration rate simulated by HYDRUS-1D which suitable for preferential flow is approximately the same as the test result.It is verified that the flow at 12mm/h infiltration rate conforms to the theory of preferential flow. |
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