Hydraulic Defects And Their Healing Of Bottom Liner At MSW Landfill Site

With the development of society, urbanization is speeding up throughout China, total municipal solid waste (MSW) generation has increased greatly. MSW treatment and disposal has become the focus of attention. Now, landfill is the primary method of municipal solid waste disposal in the world. In order to determinate a suitable landfill system in defining the acceptable seepage rate of leachate into the surrounding environment (soil and groundwater), we should focused on the permeability. The primary problem is the seepage of liner systems at the bottom or covering in the landfill.Geomembrane (GM) and geosynthetic clay liner (GCL) are commonly used as a part of barrier systems in modern landfills. In this study, we focus on the anti-seepage of two kinds of material and the influence factor. Primary discussion for influence factor of hydraulic defect and the self-healing of material were progressed according to the result of experiment.The impact of various parameters on the leakage flow through defect in the geomembrane or GCL were studied in the following conditions:(i) the influence of the confining stress on the material;(ii) the hydraulic head applied on top of the material;(iii) the size of defect, and (iv) the solution used in the hydraulic conductivity test. According the characteristic of the GM and GCL, leakage through defect in geomembrane and GCL were examined with laboratory tests using HM-4160A flexible wall permeameter produced by Humboldt Mfg. Co. The parameters of the hydraulic performance were concerned in this study. Then we discussed the permeation principles of the defect geomembrane or GCL base on the analysis of force applied to material and the permeable condition. Microstructures of the specimens were observed by scanning electron microscope (SEM). The photographs from scanning electron microscope revealed the reason of the geomembranes’ hydraulic conductivity changes base on micro structure. And photographs showed the different of GCL’s pore structure with different water content.The main research contents and results are as follows:(1) Test results show that the flexible wall permeameter is a useful instrument to determine the hydraulic performance of geomembrane since it can be accurately measure the amount of flow in a period of time. Under0.1MPa confining stress, when the hydraulic heads change from0.1MPa to0.6MPa, the hydraulic conductivity of the geomembrane has three kinds of variation trend: (1) remains essentially constant;(ii) increases with the increase in hydraulic head and (iii) decreases with the increase in hydraulic head. The heterogeneity of the physical properties of geomembrane and penetration channel pattern is the main influence factors of the variation tread.(2) According to the Shapiro-Wilk W test, under the significant levels0.05(P<0.05), the results of the physical and mechanical parameters of the specimen used in this study obey the normal distribution with bigger difference in dispersion. Thus, in order to evaluate quality of the composite geomembrane, the sample sizes should be large enough to judge by statistical parameters. As for the parameters of the hydraulic performance of composite geomembrane use in this test, the distribution of hydraulic conductivity and the flow rate are both follow a power law. So, we can compare the two kind of composite geomembrane by the exponential model. The comprehensive assessment on the composite geomembrane should include the parameters of the hydraulic performance, physical and mechanical. We can test the composite geomembrane using the following steps:standardize the index; non-dimension; choose the satisfactory weighting parameters. And then we can get quantitative evaluation by the comprehensive evaluation. The weighting parameters is principal element for the the results of assessment. So, weight coefficient should be determined according to the actual situation.(3) Leakage through flaws in geomembrane is examined with simulation test in the lab. The confining stresses (range20-100kPa) and different hydraulic heads (range20-100kPa) are concerned in this study. There is no good linear relation between the flow rate to the size of defect and hydraulic head. It is found that at a lower flaw length (<6mm) the flow remains essentially constant, less than5.0×10"5cm3/s. And the tests indicate that the flow rate of GM with7mm length flaw under20kPa increases with the increase of the hydraulic head. As for the flaw geomembrane, the7mm length flaw may be a critical length when porous stone was put on each side of geofilm.(4) The difference between the hydraulic performances of complete saturated GCLs is evaluated by permeability test. The GCL with the low hydraulic conductivity is about10"9cm/s and another GCL is10-6cm/s. The volume change for specimens under backpressure is tested in the saturation time. For the volume of the inflow pipe, there is a rise in the first day, because there is a compression for the specimen under backpressure20kPa. There is a slight decrease on liquid level of the inflow of GCL in the17days. Similarly, the volume of the outflow pipe, there is a drop in the first day, and then there is a slight increase in liquid level of the outflow of GCL in the next17days. After18days, the liquid level of the inflow and outflow were the same, specimens were complete saturated. So it is very useful to saturate GCL under backpressure20kPa and it takes some times for the saturation process. The time depended on the character of specimen, the backpressure and so on. No matter what the hydraulic conductivity of GCL, seepage consolidation happened in the hydraulic conductivity test. So confining stress contributed to low bulk GCL void ratios and hence lower hydraulic conductivity values, even the backpressure is20kPa. The decrease in hydraulic conductivity rate is about half order of magnitude. The GCL with penetrated flaw has self-healing capacity to some extent. If the hydraulic conductivity is the reference value, the results suggest that GCL, with penetrated flaw, may pose significant problems and its self healing is incomplete repair when the backpressure is20kPa. The hydraulic conductivity of defect GCL is twice than before. We can get the information from ion chromatography test that the ion exchange of sodium against calcium normally occurs in the test. When CaCl2solution is permeation liquid in the hydraulic conductivity test, it will result in a higher hydraulic conductivity of sodium bentonite GCL compare with distilled water.(5) The composite liner consists of geomembrane and GCL can take advantage of two kind of material in the landfill. The hydraulic conductivity of the composite liner is limited. When the geomembrane has defect, penetration prevention depended on the size of the defect in composite liner. As for the defect geomembrane with hole, if there are three1-mm diameter holes, the geomembrane has little contribution to hydraulic performances of composite liner. At that moment, the hydraulic performances of composite liner are also influenced by the confining stress. With the increase of the confining stress, it is advantageous to reduce the flow area under the stress. And the hydraulic conductivity of the composite liner will be low. Only from the point of view of anti-seepage, if there is no geotextile fabric on the geomembrane, the area of the size will be very small between the GCL and geomembrane, the hydraulic conductivity of composite liner is lower than the composite liner consist of composite geomembrane and GCL. When the hydraulic heads range from20kPa to80kPa, there is no significant change in the hydraulic conductivity of the composite liner. Its value depended on the area of the defect, contact relation between the GCL and geomembrane, and the confining pressure.