| Final cover systems are used in landfills to reduce rainfall penetration, henceto reduce leachate output and the operating costs of landfills. Conventional coveris a resistive barrier cover which takes advantage of low permeability compactedclay to prevent water penetration. But the compacted clay is prone to crackbecause of seasonal shrinkage or non-uniform settlement of landfill surface,which lead to significant decline of its seepage control performance. In order tosolve this problem, geomembrane is added to the conventional cover, formscomposite barrier cover. Although this method can improve its seepage controlperformance, the costs of final cover increase greatly and slippage easily occursalong the geomembrane interface. Therefore, evapotranspiration cover hasreceived considerable attention overseas. Evapotranspiration cover, also known asET cover, control percolation by water storage during periods of highprecipitation and evapotranspiration during periods of low precipitation. ET covercan be divided into two types: monolithic cover and capillary barrier cover.Monolithic cover is composed of a single thicker layer of fine-grained soil, suchas silt or silty clay, which is suitable for vegetation growth. The fine-grained soilcan store large amounts of water by capillary action. Capillary barrier coverconsists of a layer of fine-grained soil over a layer of coarse-grained soil, such asgravel or coarse sand. Capillary barrier cover reduces moisture infiltration intocoarse-grained soil and increases the water storage capacity of fine-grained soilrelying on the difference of unsaturated permeability between fine-grained soiland coarse-grained soil. Compared with conventional cover, ET cover does nothave especial requirement about soil materials and generally uses local soils, sothe construction cost is low. What’s more, ET cover is not easily cracked which isinduced by seasonal shrinkage or non-uniform settlement of landfill surfacebecause of the usage of soil with low degree of compaction, therefore ET covercan maintain its stable performance for a long time. At present, many landfills inChina are close to its service life, so they need a final cover. Moisture migrationin ET cover and the performance of ET cover in Yangtze River delta region have not been known clearly. Therefore, it is of practical significance to study theperformance of ET cover.The research works in this paper are based on the project “Numerical modelStudy on the working mechanism and the performance evaluation of ET cover inlandfills’’ funded by National Natural Science Foundation of China. Model tests,numerical simulations are carried out to study on Moisture migration in ET coverand the performance of ET cover in Yangtze River delta region. The mainresearch works and conclusions are as follow.（1）Based on the working process, the working mechanism of ET cover is statedrespectively from unsaturated seepage theory，soil-vegetation-atmosphereinteraction theory and capillary barrier theory. Capillary barrier coverreduces moisture infiltration into coarse-grained soil and increase the waterstorage capacity of fine-grained soil relying on the capillary barrier actionbetween fine-grained soil and coarse-grained soil. A simple test was done toverify capillary barrier action. The test results show that it can obviouslyimprove the water storage capacity of fine-grained soil by adding a layer ofcoarse-grained soil to the bottom of fine-grained soil.（2）To study the working performance of capillary barrier cover, soil columnmodel was built and vegetation was planted at a open area in Hangzhou, atthe same time rainfall, surface runoff of, percolation and soil moisturecontent were measured. The model test results show that evapotranspirationis very important for the working performance of ET cover andevapotranspiration plays a role in total thickness of ET cover. ET coverbecomes invalid temporarily if the fine-grained soil on the bottom reachessaturation and capillary barrier interface is breakdown, afterwards it stillworks normally with the recovery of capillary barrier interface’s function.（3）In order to verify the reliability of simulation results on moisture migrationof ET cover in landfills with using Vadose/W model, soil property of modeltest and measured meteorological data in Hangzhou in2012were used asinput parameter in Vadose/W model to simulate water transport and workingperformance of capillary barrier cover. Compared with model test results,simulation results are close to model test results on the performance of ET cover. Generally speaking, Vadose/W can simulate the moisture migration ofET cover truly and accurately.（4）Vadose/W model was used to analyze and compare water transport andworking performance of monolithic cover and capillary barrier cover underweather conditions in Shanghai in2002, then parametric analysis on designparameters of ET cover was done. The simulation results show the workingperformance of monolithic cover and capillary barrier cover both meetdesign requirements, but the performance of capillary barrier is superior tothat of monolithic cover, so ET cover can be applied to landfills in YangtzeRiver delta region in China. Parametric analysis results show that theperformance of ET cover can be improved to some extent by increasing thethickness of cover, selecting appropriate compactness of soil and increasingleaf area of vegetation. |
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