Study On Leachate And Landfill Gas Generation,Transportation And Induced Slope Instability In Municipal Solid Waste Landfill

With rapid increase of waste cleaning-transportation amount in each city,the landfill height at each landfill increases continuously,so that the problem in stability becomes more and more obvious.The instability of landfills will cause large-scale loss of people and property,and its consequences are often disastrous.At present,the mechanism of landslide caused by high leachate and landfill gas pressure is unclearn The accurate assessment methods and economical and practical control measures is insufficient.In particular,the formation mechanism and distribution of landfill leachate level and landfill gas pressure need to be further studied.The characteristics,degree and regularity of landslides occurring in landfills due to high leachate and gas pressures and the effectiveness of slope stability control measures need to be verified on site.In response to the above problems,it is of great theoretical and practical significance to carry out relevant research on slope stability analysis and slip control of landfills.The research works in this paper are funded by the National Basic Key Research Program(973),via Grant No.2012CB719800 and No.2012CB719806.The research works are also funded by the "2015 Annual Emergency Safety Monitoring Service Project of Xi'an Jiangcungou Landfill" and "Investigation and Evaluation Project of Phase IV(Restructuring and Expansion)of Jiangcungou Landfill in Xi'an".The study of the distribution of leachate pressure and landfill gas pressure in landfills was carried out.Additionally,the stability analysis and control measures based on the distribution of leachate pressure and landfill gas pressure in landfills were carried out.The main research works and conclusions are as follow:(1)The physical and chemical components of Municipal Solid Waste(MSW)and the process of hydromethanation of different components were summarized.The potential for leachate and landfill gas production from MSW was deduced through the principle of conservation of mass.At the same time,the rate of MSW degradation at different particle scales was summarized.According to the different degradation characteristics of rapidly degradable materials and slow degradable materials in MSW,a model for producing gas and leachate from rapidly degradable substances and slow degrading substances was proposed.(2)The paper comes up with an in-situ testing method for multiple landfill leachate levels.On-site tests were carried out at a landfill in northwest China,and leachate levels of each intermediate cover layer were obtained;seepage analysis software was used to simulate and verify reliability of the leachate level recovery test method adopted in the on-site tests,while formation and drainage mechanism of multi-layer leachate levels was simulated.Testing and simulation results show:the perched leachate levels were measured on each intermediate cover layer during the on-site tests,wherein the perched leachate level heights of each layer were not consistent,and confined leachate appeared in some areas;low-permeability intermediate cover layers are the main course for formation of multi-layer leachate levels.(3)Based on the experimental results of on-site and in-house tests on the change of the permeability coefficient of landfill gas along the depth,a model for the variation of the permeability coefficient of landfill gas along the depth was proposed.Based on this model,a one-dimensional steady-state pressure distribution was proposed and compared with the pressure distribution that does not consider the gas permeability coefficient along the depth.At the same time,a one-dimensional transient,a two-dimensional axisymmetric steady state,and a transient pressure distribution,which take into account the variation of the gas permeability coefficient along the depth,are simulated.The value of required suction pressure for different landfill thickness,gas production rate,and gas permeability coefficient was analyzed to provide a reference for landfill gas collection in landfills.(4)Combined the actual production of leachate and gas from the MSW chemical composition,a landfill gas and leachate coupling control equation was established.The control equation was solved by the Comsol numerical software and OpenGeoSys(OGS).Then the simulation of leachate and landfill gas distribution in high-kitchen MSW was simulated.Additionally,the mechanism of interaction between the leachate and landfill gas in landfills was revealed.At the same time,the leachate and landfill gas distribution pattern of MSW in low-kitchen was simulated and compared with the MSW in high-kitchen.On this basis,the influence of the intermediate cover layers on the migration and distribution of the leachate and landfill gas pressure in the landfill was also analyzed.Furthermore,the distribution rule of the two dimensional leachate and landfill gas pressure in high-kitchen was simulated.(5)Based on the distribution of MSW leachate and landfill gas pressure,the slope stability of the landfill was analyzed.Comparing the leachate pressure distribution in the case of only the degradation of leachate and the leachate pressure distribution in the case of the degradation of leachate and landfill gas at the same time,the influence of landfill gas on the liquid pressure was analyzed.The leachate pressure was exported from the COMSOL software and imported into the Slope module via the Pressure Head Spatial Functions in the Geo-Studio software for stability analysis.The effects of landfill gas,kitchen waste content,and landfill time on slope stability were analyzed.In addition,the slope stability of different landfill gas extraction pressures under the same leachate extraction and the slope stability of different leachate extraction under the same landfill gas extraction pressure were analyzed.(6)A field monitoring program was carried out to record the slope failure process of the landfill with multiple intermediate covering layers.The monitoring items include leachate level,surface and deep horizontal displacement within the slope.Based on the monitoring data,analysis was carried out to verify the stability control effects of leachate drainage.The results showed that the maximum slip area was 34760 m2 and the average surface horizontal displacement of the 10th platform was 1.77 m.Dumping near the slope was the main reason for its instability.The closer to the dumping area,the greater the degree of slip and the more significant leachate level rise.Affected by the intermediate covering layers,the failure mode was the local sliding inside the landfill,and the effect of near-slope leachate drainage for the stability control was obvious.(7)The wells that can drainage leachate and landfill gas to control the stability of the landfill at an instability landfill was carried out.From the surface horizontal displacement,deep lateral displacement,leachate level,landfill gas pressure and components to evaluate the method.Using the seepage and stability analysis software to analysis the effects of stability control of pumping time,pumping rate and arrangement.It is shown that after using this method the maximum slip area reduced from the monitoring area of 68%to 17%,deep average slip rate reduced from 2.43mm/d to 0.95 mm/d.The maximum pumping rate of 10 drainage guide gas wells is 279m3/d,and the average pumping rate is 164 m3/d.Simulation analysis shows that the faster of the requirements of landfill stability when the longer pumping time,the greater pumping rate and the more well row number.Under the condition of the same flow rate,three rows is superior to the arrangement of row 1 and 2 in terms of slip control.Under the condition of the same row number,the effect of row spacing of 10 m is superior to the effect of row spacing to 20 m.(8)The method of using compressed air drainage wells to control the instability landfill was carried out in Northwest China.There are 34 drainage wells with an average depth of 15m at the landfill.The surface horizontal displacement,deep lateral displacement and leachate level were analyzed before and after the treatment.Using the software to analysis the effect of this method under different conditions.It is shown that after using the compressed air drainage well,the maximum surface slip rate reduced from 237.3mm/d to 78.7mm/d.The maximum pumping rate of the compressed air drainage well was 255m3/d,and the average pumping rate was 132 m3/d.The analysis showed that the stability of landfill was found to be closely correlated to pumping time and rate.Affected by the depth of the compressed air drainage well,the maximum safety factor increases by about 20%.Compressed air drainage wells are low in cost and time consuming.The effect of compressed air drainage wells in landfill has been proved to be successful in practice,and can provide reference for stability control of landfills.