Study On Mechanism Of Capillary Barrier Evaportranspiration Covers

Final cover systems are used in landfills to reduce rainfall penetration,hence to reduce leachate output and the operating costs of landfills.Conventional cover is a resistive barrier cover which takes advantage of lowpermeability compacted clay or geomembrane to prevent water penetration. Thecompacted clay is prone to crack because of seasonal shrinkage or non-uniformsettlement of landfill surface, geomembrane is added to the conventional cover,forms composite barrier cover, the costs of final cover increase greatly andslippage easily occurs along the geomembrane interface. Therefore,evapotranspiration cover is becoming increasingly popular overseas.Evapotranspiration(ET) cover by using soil layers store water during the periodsof high precipitation, and evapotranspiration during the periods of lowprecipitation to control percolation. ET cover dose not have especiallyrequirement about the types of soil materials and generally uses local soils.Compared with conventional cover, ET cover is not easily cracked and havegood durability and resistance to differential settlement. Capillary barrier coveris one of the ET covers, it is composed of a vegetation layer, a thicker layer offine-grained soil which is suitable for vegetation growth, such as silty clay, and alayer of coarse-grained soil, such as coarse sand. Capillary barrier reducemoisture infiltrate into coarse-grained soil, and increase the water storagecapacity of fine-grained soil relying on the difference of the water storagecapacity between the two kinds of soils. At present, many landfills in China areapproaching their service life and need final cover, but moisture migration in ETcover and mechanism of capillary barrier cover have not been known clearly.Therefore, it is of practical significance to study the performance of ET covers.The research works in this paper are based on the project “Numerical model Study on the working mechanism and the performance evaluation of ETcover in landfills” funded by National Natural Science Foundation of China.Using model tests and numerical simulation to study on moisture migration inET cover and mechanism of capillary barrier cover, discussing the applicabilityof capillary barrier cover in Hangzhou region. The main research work andconclusion are as follow.（1）Respectively from unsaturated seepage theory, cover and atmosphere waterbalance theory, capillary barrier theory and preferential flow theory toillustrate the mechanism of capillary barrier evapotranspiration cover. Asimple test was done to verify capillary barrier action.（2）To study the mechanism of capillary barrier evapotranspiration cover, soilcolumn model was built and vegetation was planted at a open area inHangzhou, at the same time rainfall, surface runoff, percolation and soilmoisture content were measured. The results show that evapotranspirationof the vegetation is very important for the performance of ET cover, by theeffect of capillary barrier action and the moisture migration,evapotranspiration can also be used in the whole thickness of ET cover;Capillary barrier evapotranspiration cover becomes invalid temporarilywhen the fine-grained soil on the bottom reaches saturation and capillarybarrier interface is breakdown, then it still works normally with the recoveryof capillary barrier interface’s function; Percolation accounted for0.91%oftotal rainfall during the test, it shows that test’s ET cover can effectivelyreduce rainfall infiltration; The soil in the cover may produces a largeamount of preferential flow during the heavy rainfall periods in summer,greater than100mm/d of the rainfall may generate preferential flow thatbreakdown capillary barrier layer, and the cover become invalidtemporarily.（3）In order to verify the reliability of the numerical models on moisture migration of capillary barrier evapotranspiration cover in landfills,measured metrological data in Hangzhou was used to make numericalsimulations of long-term and short-term. Compare the consequences withmodel tests to discuss the cover’s working performance. The results showthat VADOSE/W model can simulate the moisture migration of capillarybarrier evapotranspiration cover truly and accurately by long-termsimulation, the consequences are closed to the model tests; Thedual-porosity model (HYDRUS-1D model) can well depict the process ofpreferential flow migrate in capillary barrier evapotranspiration cover underthe period of short-term heavy rainfall, the consequences closer to themodel tests compared with unsaturated seepage model (SEEP/W model).（4）VADOSE/W model was used to make design parameters sensitivityanalysis of the capillary barrier evapotranspiration cover, about soil types,soil thickness, vegetation parameters and meteorological parameter’s effecton the moisture migration and working performance of covers. The resultsshow that under weather conditions in Hangzhou in2012, choose goodvegetation, root depth50cm, the soil layer’s thickness of1.4m which hasstrong storage capacity’s capillary barrier evapotranspiration cover canreduce rainfall infiltration effectively, meet design requirements and can beapplied to landfills in Hangzhou region.