Study On Moisture-gas Coupled Flow In Loess/Gravel Final Cover And Control Of Landfill Gas Emission

The principal goal of landfill final covers is to decrease infiltration of precipitation and to control the emission of landfill gas into the atmosphere. Alternative earthen final covers (AEFCs) are constructed with relatively non-plastic soils with greater durability and lower cost and they require relatively lower post-closure maintenance than conventional covers, so the AEFCs are good choices for the landfills of non-humid areas. Most of the previous studies concentrated on the rainwater infiltration, and the study of landfill gas flow in the AEFCs was not investigated sufficiently. In the northwest of China, the climate is mainly non-humid, and loess is widely distributed. The use of loess as AEFCs material is promising in this area. So, the study on the moisture-gas coupled flow and control of landfill gas emission of loess/gravel final cover in this area should be warranted.A full-scale gas flow simulation testing facility of a loess/gravel final cover was constructed at the Xi’an landfill of municipal solid wastes. Field gas permeation tests were performed to measure the gas permeability of the compacted loess before and after the planting of vegetation on the cover. The Air/w module of GEO-SLOPE commercial software was used to set up a numerical model of moisture-gas coupled flow of the AEFCs. The numerical model was calibrated by the experimental results from a soil column experiment. The calibrated numerical model was used to study the moisture-gas coupled flow of a loess/gravel final cover. The Vadose/w module of the GEO-SLOPE commercial software was used to set up water-heat coupled flow model and to analyse the daily saturation degree profiles of a loess/gravel final cover and a loess monolithic cover under the climatic conditions of the wettest year and driest year in Xi’an. Then, the daily saturation degree profiles were input to the moisture-gas coupled flow model to study the performance of controlling landfill gas emission of the two covers under the two extreme climatic conditions. Based on analytical solutions, simplified methods were presented to evaluate the performance of limiting percolation and controlling landfill gas emission of the AEFCs. At last, measures were presented to enhance the perforamce of controlling landfill gas emissions of the loess/gravel final cover. The main research conclusions are as follow.(1) The results of field gas permeation tests show that before planting vegetation, the field measurements of gas permeability ranged from 3.67×10-12 m2 to 5.73×10-14 m2 when the volumetric water content changed from 36% to 46%. The gas permeability of the compacted loess slightly reduced with a large increase in saturation degree when the saturation degree is less than 0.85. However, when the saturation was greater than 85%, the gas permeability significantly decreased with a further increment in saturation degree. After planting vegetation, the field measurement of the gas permeability was one order of magnitude lower than before planting vegetation. This is because that the growth of vegetation roots tends to fill the large pores in the compacted loess, which would cause a decrease in gas permeability of the loess. The field measurement of gas permeability was about one order of magnitude greater than the laboratory measurement. This is because the loess material used in the field tests contained large soil clods, which resulted in an increase in gas permeability due to an increase in pore size and a decrease in tortuosity.(2) The experimental results from a loess soil column were used to calibrate the numerical model of moisture-gas coupled flow. The experimental results reveal that gas preferential flow channels exist in the soil column. The numerical model was able to simulate the water infiltration process and capture the gas pressure change trend of the soil column. Then, the calibrated numerical model was used to study the moisture-gas coupled flow of a loess/gravel cover. The research findings are as follows:The influence of the gas pressure on the rainwater infiltration can be neglected before the surface layer’s arriving at saturated state. When the applied gas pressure at the bottom of the cover was less than the standard of 0.75 kPa, the gas pressure had small effect on the water storage capacity of the cover, and the water storage capacity of the cover was about 4% smaller than the result without considering gas pressure. A capillary break at the interface between the loess layer and the gravel layer increases the water storage capacity of upper loess layer form the field capacity (i.e.,0.32) to close to the saturated water content (i.e.,0.37), and significantly decreases its gas permeability from 1.6 × 10-9 m2 to 9×10-12 m2. And then the landfill gas emission rate of the cover can be maintained to 0 when the bottom gas pressure being 0.75 kPa and without considering decrease in water content of the cover due to evapotranspiration. The capillary break can drastically enhance the performance of the cover with respect to reducing landfill gas emission.(3) The moisture-gas coupled flow model was used to study the performance of controlling landfill gas emission of a loess/gravel final cover and a monolithic loess cover under the climatic condition of the wettest year and driest year in Xi’an. The landfill gas was quantized by methane in this paper. The research results show that, corresponding to the wettest year and with the average measured gas pressure of 0.11 kPa being applied at the bottom of the two covers, the annual methane emission of the loess/gravel final cover and the monolithic loess cover was 9 kg·m-2 and 20 kg·m-2, respectively. The two results were both less than the emission standard of Australia (i.e.,22 kg·m-2). The performace of controlling landfill gas emission of the loess/gravel final cover was better than the monolithic loss cover. Corresponding to the driest year, the annual methane emission of the loess/gravel final cover was 59 kg·m-1. The 59 kg·m-2 can be decreased to 21 kg·m-2 when the initial water content of the loess layer was increased from 0.3 to satureated water content (i.e.,0.41).(4) An analytical solution was developed in this study for evaluating the infiltration and deep percolation of rainwater into a monolithic cover on the basis of the two-dimensional governing equation for unsaturated flows. To obtain the analytical solution, it was assumed that both the soil-water characteristic curve and the permeability function can be described by using exponential functions considering entry air value and the bottom boundary of the cover was set as unit gradient boundary. Based on the analytical solution, a simplified method was presented to evaluate the total percolation of the monolithic cover. A case study was performed to demonstrate the method based on the water balance monitoring data of a model test on a silty soil cover reported in the literature. The case study indicated that the prediction of percolation from the method is 34% greater than the measurement. For practical application of the analytical solution, further work is required with respect to the calibration of the model parameters with detailed meteorological data and an accumulation of water balance monitoring data for monolithic covers. The methane emission of the monthly average saturation degree profile was close to the monthly accumulated methane emission of the cover. So the monthly average saturation degree profile can be used to evaluate the methane emission of the cover. A method was presented to evaluate the methane emission of the AEFCs. The measured data of the loess/gravel final cover testing facility in Xi’an was used to verify the feasibility of the method. The accumulated methane emission in 2015 of the loess/gravel final cover was calculated by the method as 18 kg·m-2, which is 9% greater than 16.5 kg·m-2 corresponding to the measured data.(5) The landfill gas emission of the AEFCs is mainly related to the saturation degree profile of the cover and the gas pressure at its bottom. Controlling the bottom gas pressure by gas extraction and keeping the soil moisture by irrigation can be conducted to decrease the gas permeability of the cover and enhance the performance of reducing landfill gas emission.