Research On Time-dependent Constitutive Models For Municipal Solid Waste And Service Performance Of Landfills

With the rapid rural-urban integration and ascending municipal solid waste(MSW)production,as well as the successive closure of some landfills,the problem of MSW dumped in landfills exceeding the design capacity is quite common in China.Confronted with the huge amount of MSW and the difficulty in new landfill sites,on the one hand,existing landfills have to enlarge the storage capacity by vertical and/or vertical expansion,and the storage capacity of the individual landfill is getting bigger and bigger,therefore,large-scale and intensive landfills prevail in China;on the other hand,the sustainable landfilling has been developing in China,and measures are taken to enhance the biodegradation of MSW to promote the settlement,and stabilized landfills are excavated to release the storage capacity.After MSW is buried,great deformation is generated due to the self-weight,overburden pressure,mechanical creep,and biodegradation.Compression characteristics of MSW are fundamental to every aspect of landfills,including design,construction,operation,management,and so on.Under the trend of large-scale intensification,waste settlement will have significant impacts on the service performance of landfills,and analyzing the problems such as slope stability and integrity of liner system influenced by landfill settlement is particularly important;At the same time,sustainable landfilling advocates the promotion of landfill settlement to increase the storage capacity of landfills,and reasonably predicting and calculating the settlement and storage capacity of landfills are quite urgent;In addition,balancing the relationships between settlement and storage capacity of landfills and slope stability and liner system integrity is very critical in the context of large-scale,intensive and sustainable landfills.The research works in this paper were funded by the National Basic Research Program of China(973 Program),via Grant No.2012CB719800 and No.2012CB719806.Following researches were carried out in six aspects under the doctoral topic "Research on time-dependent constitutive models for municipal solid waste and service performance of landfills":(1)engineering properties of MSW in different countries(focusing on the compression characteristics of MSW)were summarized,compared,and analyzed;(2)a one-dimensional equivalent-times model for MSW was established based on its compression characteristics;(3)a constitutive model for MSW considering the time-dependent strain was developed;(4)a practical approach for calculating the settlement and storage capacity of landfills based on the space and time discretization of the landfilling process was proposed;(5)a slope stability analysis method for landfills considering the settlement was introduced;and(6)the mechanical behavior analysis of one-layer HDPE geomembrane liner system based on the MSW constitutive model was carried out.The main conclusions obtained in this paper are as follows:(1)Fresh MSW can be classified into high food waste content(HFWC)and low food waste content(LFWC)MSWs according to the wet-based content of food waste.Differences in physical components not only caused differences in chemical compositions,but also are the internal causes of the differences in the engineering properties of HFWC and LFWC MSWs.Compared with LFWC MSW,HFWC MSW has a similar field capacity but higher initial water content,and the leachate production is significantly higher because large amount of intra-particle water is released rapidly with the biodegradation of food waste;the hydraulic permeability of HFWC MSW is an order of magnitude lower;in addition,HFWC MSW has a faster CH4 generation rate but a lower CH4 generation potential.(2)The compassion process of MSW can be divided into stress-dependent primary compression and time-dependent secondary compression.Biodegradation can reduce the primary compressibility of MSW,and influence the secondary compression of MSW,which can reach or even exceed the main compression under enhanced biodegradation conditions.The stress-strain relationship of MSW shows the feature of strain hardening;with the increase of strain,the cohesion and friction angle of HFWC MSW both increase,but this trend is not obvious for LFWC MSW;furthermore,the friction angle increases but the cohesion decreases with the increase of fill age for HFWC MSW,but LFWC MSW does not conform to this trend.(3)The one-dimensional equivalent-times model for MSW can be divided into two forms:Type I and Type II,and the simulation results can well match with the measured results;Type I is suitable for predicting the settlement in landfills with a distinct biodegradation progress of MSW,a high content of organics in MSW or a short fill age,while Type II is good at predicting the settlement in landfills with a distinct progress of creep compression,a low content of organics in MSW or a long fill age,and Type II is recommended when the biodegradation environment is unclear.Both the two model types converge to a finite value for settlement,and the predicted results of Type I are more conservative than that of Type II,and the composition of MSW should be considered to determine the reliability of the model’s results.Model parameter arameter c rises with the increase of fill age of MSW;and parameter Φ is stress-dependent and there is no significant relationship between Φ and the fill age of MSW.(4)The proposed constitutive model for MSW includes the time-dependent strains due to mechanical creep and biodegradation.Three sources contribute to the hardening of MSW due to volumetric strain:the time-independent plastic volumetric strain,the time-dependent volumetric mechanical creep strain,and the time-dependent volumetric strain due to the biodegradation of MSW.The model parameters can be determined from laboratory or field tests on MSW,MSW composition analysis,or values reported in the literature,and simulation results comparison highlights the sensitivity of the time-dependent settlement predicted by the model to the kinetic rate parameter k.(5)The proposed practical approach for calculating the settlement and storage capacity of landfills converts the problem of landfill settlement and storage capacity into the problem of settlement and storage capacity of column units through the space and time discretization of the landfilling process.The effects of the fluctuation of the main leachate level and the variation in the unit weight of the MSW on the overburden effective stress are considered by introducing the main leachate level’s proportion and the unit weight and buried depth curve.The landfill settlement and storage capacity calculation program,LANDFILL,has a variety of results display functions.The simulation case of the phase IV of Shanghai Laogang Landfill verified the reliability of the proposed approach and program,and it was shown that the discretization of the landfilling process is quite important for obtaining accurate results;settlement and differential settlement calculations are requited for the design of internal structures,and the optimization of the structural design should be based on the settlement of the landfill.(6)The proposed slope stability analysis method for landfills considering settlement includes two steps:the practical approach is first utilized to obtain the landfill settlement and storage capacity,and then the factor of safety of the deformed landfill slope is estimated by the limit-equilibrium method.The main leachate level is of importance to the landfill storage capacity and stability;reducing the filling rate of MSW has multiple advantages:the requirement to control the main leachate level thickness in the MSW body can be diminished,more waste can be filled,and the stability of landfills can be sustained;although the landfill storage capacity can be effectively improved by increasing the height of MSW body,at the same time,it becomes more important to control the main leachate level in the MSW body to sustain the landfill stability;and reducing the slope angle will help weaken the requirement of the main leachate level to sustain the landfill stability,but not necessarily decrease the landfill expansion ratio.In addition,the factor of safety varies with time due to the development of landfill settlement and the biodegradation process.Therefore,the landfill stability assessment should be associated with the construction stage.(7)In the HDPE geomembrane liner system,the axial tensile strain usually occurs at the upper part of the slope.Compared with the Modified Cam-Clay model,more reliable results can be obtained using the MSW constitutive model.The time-dependent strains caused by mechanical creep and biodegradation are significant to the tensile strain development in the liner system;decreasing the filling rate of MSW increases the maximum axial tensile strain in the liner system while increasing the landfill storage capacity;extending the intermediate bench width reduces the maximum axial tensile strain in the liner system but the effect is limited;and increasing the number of intermediate bench is beneficial to reduce the influence of the long-term settlement on the axial strain development in the liner system;in addition,decreasing the side slope inclination is the most effective way to reduce the maximum axial tensile strain in the liner system.