| Domestic waste landfills mainly include two types in China: sanitary landfills and waste dumpsites.There are more than 1,800 sanitary landfills and more than 5,000 waste dumpsites in service.During the operation of in-service sanitary landfills,there are many problems,such as instability and slippage of landfill polluted the urban surface environment,penetration of leachate polluted the urban underground environment,and diffusion of landfill gas polluted the urban atmospheric environment.In addition,the capacity of many sanitary landfills built in the 1990 s is close to saturation.Because it is difficult to find suitable new landfill sites,the plan of vertical expansion based on the original design elevation has been adopted or will be adopted to extend its service life.The solution of the above-mentioned problems mainly includes the stability assessment,seepage analysis,settlement calculation,capacity estimation,etc.All of these require an in-depth understanding of the variation of geotechnical characteristics such as moisture retention capacity,seepage,compression,and strength of the landfill with burial depth,and its essence is the variation of these geotechnical characteristics with overlying stress and landfill age.Therefore,it is of great engineering significance for landfill design and operation to deeply study the stress effect and age effect of landfill geotechnical characteristics,and then reveal their variation with the burial depth.This article was funded by the National Natural Science Foundation of China(51708508)and the project entrusted by Zhejiang University architectural design and Research Institute(19050591-J).The municipal solid waste(MSW)samples at different burial depths were obtained by drilling holes from Anshan Yangeryu,Xian Jiangcungou and Taiyuan Houcun landfills.In order to study the influence of stress compression and biochemical degradation on the parameters of moisture retention capacity(MRC),saturated hydraulic conductivity,primary compression ratio(C’c)and shear strength of MSW,the experiments of primary compression ratio,permeability,compression and direct shear were carried out.The main conclusions were as follows:(1)Carried out 8 sets of moisture retention capacity tests with fresh MSW and 12 sets of moisture retention capacity tests with aged MSW under different overburden stresses and degradation conditions.The results showed that MRC of MSW decreased with an increase in stress,which was mainly caused by the decrease of inter-particle water retention capacity and the increase of intra-particle water release.MRC of MSW decreased faster with elapsed time under more favorable degradation conditions,which was related to the increase of release rate of intra-particle water.A mathematical model was then proposed for the prediction of MRC in which the influences of stress and degradation were considered.The pre-dewatering effects of MSW piles were evaluated by using the proposed model,in which six levels of applied stress and six different degradation conditions were studied.The results can provide theoretical basis for the design of pre-dewatering project of MSW piles.An optimal technical program was then recommended: mixing the cellulose enzyme into MSW with a dosage of about 1.5‰ of the dry mass of MSW,and then applying a stress of about 100 k Pa on top of the MSW pile,and then introducing air into the MSW pile at a rate of 300～400L/kg-dry-MSW/day from the side wall and applying negative pressure at the bottom,and meanwhile controlling the temperature at 40±3℃.Under the above conditions,the dewatering rate of MSW piles is promising to reach above 50% after treating for at least3 days.(2)Carried out 12 sets of saturated hydraulic conductivity tests with aged MSW under different overburden stresses.The test results showed that the saturated hydraulic conductivity of MSW decreased with an increasing effective stress and a decrease in void ratio,which was mainly related to the reduction of the porosity and the more tortuous pore distribution caused by the compression of waste skeleton.The saturated hydraulic conductivity of MSW decreased with an increase in degree of degradation,and a decrease in effective particle size,which was mainly related to the reduction of porosity and the more tortuous pore distribution caused by the decomposition of large-size organic matter into fine particle content.Based on the test results,a prediction model for saturated hydraulic conductivity was proposed which incorporated the effects of stress and degradation,and it was developed to accommodate the calculation of saturated hydraulic conductivity with varied burial depth.In the seepage analysis of MSW landfills,the mathematical model can be used to obtain the variation curve of saturated hydraulic conductivity with burial depth.(3)Carried out 12 sets of primary compression tests with aged MSW under different overburden stresses.The test results showed that the primary compression settlement of MSW increased with a decrease in age and an increase in stress.The C’c increased with an increase in dry basis moisture content,a decrease in dry unit weight and an increase in compressible component content.Based on the test results,a prediction model for primary compression ratio C’c was proposed which incorporated the effects of dry basis moisture content,dry unit weight and compressible component content,and it was developed to accommodate the calculation of primary compression ratio with varied burial depth.The analysis result of the calculation example showed that considering and not considering the C’c with the burial depth would cause a great difference in calculation of the primary compression settlement of the MSW landfills.In the landfill process planning and capacity assessment of MSW landfills,the burial depth effect of C’c needs to be considered.(4)Carried out 12 sets of direct shear tests with aged MSW under different stresses.The test results showed that under the same stress condition,the shear stress increased with an increase in shear strain,and the shear curve showed obvious strain hardening.The cohesion and friction angle both showed an increasing trend with the increasing of shear strain.The cohesion decreased with an increasing degree of degradation,and friction angle increased with an increasing degree of degradation.Based on the test results,a prediction model for shear strength parameters of MSW was proposed which incorporated the effects of strain and degree of degradation,and it was developed to accommodate the calculation of shear strength parameters with varied burial depth.In the stability analysis of the MSW landfill,when the experimental data of the shear strength parameter is lacking,the value can be calculated according to the shear strength parameter-burial depth model proposed in this paper. |
