Numerical Analysis Of Seismic Responses Of Landfills With High Leachate Levels And Verification By Centrifugal Model Tests

Although incineration has been widely used for disposing with decontamination in recent years,landfill is still one of the most important ways in MSW(municipal solid waste)disposal of China.Most landfills in China are very large and usually have steep slopes and huge heights due to the shortage of land capable of being used for human activities.The stability of a landfill is very important for that there will be a disastrous impact on economy of the society and lives of the public together with the environment once a landslide occurs in the landfill.The leachate levels in landfills of China are relatively high as a result of high kitchen waste content,which increases the likelihood of the occurrence of failure in landfills.Moreover,large quantities of MSW landfills are located in areas that are more vulnerable to earthquakes because of China's special location in a seismic belt.Earthquake ground motion rises the uncertainty of failure of landfills.Hence there is a necessity of research on the seismic stability of landfills with high leachate levels.Nonlinear analysis has been proved to be a reasonable and effective method in studying the seismic responses of a structure.Centrifugal model test,taken by some scholars in recent years,has become a significant way to study the seismic responses or stability of MSW landfills.Combination of nonlinear analysis and centrifuge tests will be an ideal way to study the responses and failure mechanism of landfills under earthquakes.In this paper,numerical landfill models including prototypes,hypergravity models with pore water and hypergravity models with a viscous pore fluid have been built.Then the nonlinear analysis grounded on finite difference method(FDM)has been carried out to study the seismic responses of landfills.As for the seismic responses of a hypergravity landfill model with water,results of FDM analysis have been compared with those that were observed in centrifuge tests;hence the responses of pore pressure,horizontal acceleration and displacement and the potential failure modes of landfills under impacts of leachate levels and magnitudes of ground motions have been clarified.At last,time-history of safety factor has been taken to analyze the seismic stability of landfills and the impacts of strength parameters on the stability of landfills.The main work and conclusions of this paper are as follows.1.Governing equations for analysis of seismic responses of prototypes,hypergravity models with pore water and hypergravity models with a viscous fluid have been given based on some rational assumptions.Similarity relations between a hypergravity model with a viscous pore fluid and a prototype and between a hypergravity model with pore water and a prototype have been explained.Different numerical models have also been built.Then relevant parameters for numerical analysis were noted or explained.Disparities of seepage between a hypergravity model with pore water and a prototype bring about a conflict of similarity relations in fluid constitutive equation;thus similarity relations of acceleration,displacement,pore pressure,etc.between them cannot be met perfectly.Adoption of hysteresis damping that reflects the nonlinearity of MSW and maximum dynamic shear modulus related to mean principal stress in the numerical analysis is reasonable.The ideal elastic-plastic model and Mohr-Coulomb criteria taken in this paper neglect the strain hardening and dilatancy characteristics of MSW and hence make some difference exist between the results of numerical study and the reality.2.FDM nonlinear analysis has been conducted to study the seismic responses of pore pressure,acceleration and displacement of a prototype landfill or a hypergravity model with a viscous pore fluid at a high leachate level.Responses of the two numerical models have been compared to illustrate the similarity obtained from governing equations.The results have shown that there is a consistency in responses between the prototype and the model when scaling the results of the model according to the similarity relations.As far as a prototype landfill with a high leachate level is concerned,a negative excess pore pressure occurs in the process of a ground motion,which dissipates slowly after the motion.Moreover,the horizontal peak ground acceleration(HPGA)at a point increases when the elevation of the point ascends.However,increment of the HPGA at a point decreases when the ground motion is relatively strong.3.Analyses of the seismic responses of pore pressure,acceleration and displacement and the development of volumetric strain,plastic zone,shear strain and deformation of a hypergravity landfill model with pore water under impacts of different leachate levels and magnitudes of ground motions have also been carried out.Impacts of an ascent in leachate level or an intensification of a ground motion on the seismic responses and stability of a landfill model have been noted and the potential failure mode of a landfill under a strong ground motion then has been explained.Comparison between a hypergravity model with pore water and a prototype has been taken to study the influence of the disparities in seepage between the two landfills.The results have shown that both of them exhibited similar seismic responses,despiting some small difference in the values.It is important to notice that there is also a negative excess pore pressure,as a result of cyclic shear and tension force,in the hypergravity model during a ground motion.But it should be noted that the dissipation of the accumulative excess pore pressure after the motion in a hypergravity model with pore water is obviously fast while the dissipation of the excess pore pressure in a prototype is much slower,causing the absolute maximum excess pore pressure in the prototype is larger in most scenarios.The development of volumetric strain,plastic zone,shear strain and deformation of the models under a strong ground motion has indicated that the potential sliding surface in a MSW landfill with a double-sided slope is noncircular,which begins at zones below the right top of slope,extends towards lower-left direction until reaching the bottom surface of MS W,then extends alongside the bottom and finally slides out at the toe of the left side of the slope of the landfill.4.As for the seismic responses of hypergravity landfill models with pore water,results of numerical study have been compared with those observed in centrifuge tests.Similarities and disparities of the responses of pore pressure,acceleration and displacement between the two studies have been noted.Reasons why the disparities occurred have also been explained:assumptions,parameters,disadvantages of the constitutive model,factors regarding test equipment and procedure,etc.caused the difference between them.Whether or not the centrifugal model tests use water can project the seismic responses of prototype landfills has been discussed.It has been concluded that results observed in centrifuge tests,though the excess pore pressure and settlement estimated via tests will be smaller,can approximately project the seismic responses of prototype landfills by taking the similarity relations.5.Based on information saved at different time in FDM analysis,time-history curves of safety factors of hypergravity models with pore water under different leachate levels or ground motions of different intensities have been obtained.Impacts of strength parameters on seismic stability of landfills have also been studied.When using parameters corresponded to a recommended strain level,the results have shown that landfills with low leachate levels can remain stable during the ground motions and that landfills with high leachate levels also have ability to resist failure when the input ground motions are not strong while landfills with high leachate levels become unstable during strong ground motions.When taking bigger strength parameters in the study,the stability of landfills improves.However,failure also occurs in landfills in the process of the strongest ground motion.Consequently,further study regarding the dynamic strength parameters of MSW is necessary.