| China's domestic municipal solid waste landfills mainly include sanitary landfills and simple landfills,of which there are more than 1800 sanitary landfills and 5000 simple landfills in service.The in-service sanitary landfills are mainly dominated by anaerobic landfills.For this landfill,the problems of anti-tilt failure of the middle drainage system,deformation and failure of the capping system,inaccurate landfill's storage assessment and unpredictable settlement after construction are more prominent,which are all related to the settlement of the municipal solid waste landfills.In addition,domestic waste simple landfills have a large number and distribute widely,which would cause many problems such as environmental disasters and serious waste of land resources.Before landfilling,take aerobic degradation into consideration can accelerate the stabilization of MSW,on the basis of which can achieve harmlessness and recycling of simple landfills.Therefore,in-depth study of the compressive deformation of MSW under anaerobic and aerobic degradation conditions is the basis for ensuring the safe operation and management of in-service sanitary landfills,as well as the acceleration of stabilization of simple landfills and the reuse of landfill sites.With the support of the National Natural Science Foundation project "Study on the Compression Deformation Regulation and Theoretical Model of Municipal Solid Waste under the conditions of Aerobic Degradation,Stress,and Temperature"(51708508),in connection with the typical municipal solid waste in China,three experiments were carried out,including a rapid compression test under progressive loading conditions,long-term compression test under anaerobic degradation conditions and long-term compression test under aerobic degradation conditions.On the basis of these experiments,the regulations and the main influencing factors of main compression,creep compression,aerobic degradation compression and anaerobic degradation compression were studied,the method of settlement model based on the value method,and the prediction method models' parameters of the primary compression and secondary compression considering the influence of multiple factors were established,which would provide the scientific design,safe operation and storage capacity management for aerobic and anaerobic municipal solid waste landfills.The main conclusions are as follows:(1)Six groups of rapid compression tests were carried out,using three different types of MSW samples,namely fresh high food waste content,fresh non-food waste content and basic degraded waste content.The model size was ?300 mm × 1000 mm and the stress application ranged from 10 kPa to 400 kPa,each level of stress was maintained for 48 h.The revised main compression indices of fresh high food waste content,fresh non-food waste content and basic degraded waste content were 0.295?0.314,0.227?0.229 and 0.174?0.187,respectively.Based on the test results and literature data in this paper,it is found that the modified main compression index increases with the increase of moisture content,decreases with the increase of dry weight and increases with the increase of compressible component content.On this basis,a modified main compression index prediction model with water content,dry weight and compressible component content as main parameters was established.(2)Six groups of long-term compression test of MSW was carried out.There were 3 groups of anaerobic and aerobic degradation conditions,the overburden stress were 30,50,and 100 kPa.The model size was ?560 mm × 1400 mm and the test duration were 403 days.By conducting three groups of ventilation and oxygen pre-tests,the optimal ventilation rate was determined to be 14.43 L/h/kg dry MSW.Based on the test results of leachate pH,COD and BODs,the aerobic degradation test group was divided into the initial aerobic degradation stage,the hydrolytic acidification stage and the stable aerobic degradation stage,and the anaerobic degradation test group was divided into the initial aerobic degradation stage,hydrolysis Acidification stage and stable methanogenesis stage.The reduction rate of organic matter by anaerobic degradation was less than the aerobic degradation.After the initial aerobic degradation stage and the hydrolysis acidification stage,under the same overstress condition,the total compression strain of the sample under anaerobic degradation was closer to the sample under aerobic degradation s;under stable aerobic degradation stable methanogenesis stage,under the same overburden stress,the compression amount of aerobic degradation was larger than anaerobic degradation,and the gap was more obvious as the overburden stress decreases.Compared with the aerobic and anaerobic degradation conditions,the aerobic degradation conditions had greater compression strain and strain rate in the secondary compression,and were basically provided by the degradation compression.Under different stresses,the difference between the secondary compressive strain and the secondary compressive rate of aerobic and anaerobic conditions was obtained:?Er=-0.084exp(-0.074*?)+0.011,CEr=-1.068exp(-0.036*?)+0.868.(3)Based on four criteria:simplicity of calculation,accuracy of fitting,reliability of prediction and stability of parameters,an evaluation method of settlement model based on entropy method was established;using four MSW large-scale model tests to compress data,nine typical settlement models had been evaluated comprehensively.Based on the evaluation results,the models proposed by Sowers et al.(1973)and Gourc et al.(2010)were finally recommended for predicting the settlement of low food waste content landfills.Hyperbolic models and Chen were recommended(2010)to predict the settlement of high food waste content waste landfills.Based on the test results and literature data in this paper,it is found that the ultimate secondary compressive strain decreases with the increase of stress,decreases with the increase of moisture content on the dry basis,decreases with the increase of dry weight and increases with the increase of organic matter content.The secondary compression rate decreases with the increase of stress,increases with the increase of moisture content of the dry basis,increases with the increase of dry weight,and decreases with the increase of organic matter content.On this basis,a prediction model of ultimate secondary compression strain and secondary compression rate with water content,dry weight,organic matter content and overburden stress as the main parameters was established.(4)Taking Qizishan(Suzhou)MSW Landfill as the engineering background,based on the test results of the basic characteristics of the MSW simples at different depths,the variable of depth was further introduced into the modified primary compression index,the limit secondary compression response and secondary compression rate prediction models.Using the above model,design an example that using different parameter values to analyze the development of the thickness in MSW layer during the landfill process and the change in the slope height of MSW with the horizontal distance after the completion of the landfill.The results show that the revised primary compression index,limit secondary compressive strain and secondary compression rate of fresh municipal solid waste would overestimate the settlement and the capacity of the landfill(about 14.9%in this example).It is recommended that the revised main compression index,limit secondary compressive strain and the change of the secondary compression rate with the burial depth was conducive to more scientific guidance of landfill operations and more accurate capacity.Using the above model,a design example was analyzed to the landfill site under aerobic and anaerobic degradation conditions in settlement and uneven settlement with time;the results show that ventilation and oxygen supply to the landfill after the closure would increase the settlement and capacity(approximately 4.84%in this example),and ventilation and oxygen supply can accelerate landfill stabilization's process.It is recommended to provide ventilation and oxygen for MSW in the landfill,which would be more conducive to the reuse of the site after the MSW is stabilized. |
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