Characterization Of Deformation And Strength For Municipal Solid Waste

Landfilling is one of the most economic and feasible means of disposing municipal solid waste (MSW). Settlement and strength characterization of municipal solid waste are the key issues in MSW landfill management. It is important of quantification of the impact of important factors on the settlement and shear strength properties for design and management of landfill, and also for capacity estimation and stabilization analysis of landfill. In addition, researching on the stress-strain relationships of MSW and then perfecting the constitutive model of MSW is of great significance to establishing MSW theory system. Considering the complexity of MSW composition and diversity of landfill conditions and special characteristics of biodegradation of organic matters, degradation discipline of organic fractions and settlement and strength characteristics of MSW were investigated systematically in this thesis by large number of laboratory tests.MSW is a refuse composed of various materials with different properties and characterized by high compaction and biodegradation. Because of complexity of settlement and degradation mechanisms of MSW, it is impossible to study above mentioned properties using traditional geotechnical testing technique merely and test specification has not been unified up to now. A test equipment with settlement measurement unit, landfill gas collection unit, leachate collection and recirculation unit, temperature control unit, loading unit, vertical and leveling device has been developed using theory and techniques of traditional geotechnical testing, mechanical layout and environmental science. In addition, exploratory research on the test method has been conducted and stage loading mode was proposed in simulating the whole settlement and degradation process of MSW. Test results showed that using the method proposed in this thesis could simulate the deformation process better than that of compaction after degradation. In order to investigate the impact of initial compaction, composition and degradation conditions on the long-term settlement of MSW, large numbers of compaction-degradation tests were conducted. The strength and deformation behavior of MSW of different composition were investigated with triaxial CD test under loading-unloading-reloading conditions for the first time. Based on the test results, a bilinear strain hardening model was developed and a complete set of model parameters including unloading-reloading modulus and Poisson ratio were determined. Study shows that MSW composition and its compaction density have remarkable effect on the characteristics of MSW stress-strain relationship. MSW were sorted two types based on R_OI defined as the ratio of organic fraction to inorganic fraction. Different models were used to describe corresponding types of MSW. It is found that MSW stress-strain relationship present marked strain hardening characteristics when the majority of its composition is organic component and a bilinear model was proved to be more reasonable to describe this type of MSW. Whereas, MSW has similar stress-strain behavior with soil when the majority of its composition is inorganic component, and hyperbola model can be used to describe the stress-strain relationship of this type of MSW.Based on the degradation tests, an improved index was proposed here to describe the biodegradation extent of MSW, namely introduce the normalized values of C/L with the initial data as the index of biodegradation extent of MSW. In addition, a sigmoid curve of index of biodegradation extent and time was observed and an expression between them was proprosed. Validity and advantage of the expression are reexamined based on degradation test results and published degradation results from landfill sites. According to the features of old MSW, specimens were prepared in laboratory and the impact of biodegradation extent on the deformation and strength properties was researched. To exclude the influence of void ratio, triaxial tests on fresh and old MSW with the same initial void ratio were performed. Study shows that the landfill stability will be enhanced with the increasing of biodegradation extent of MSW.Old MSW recycle and landfill site redevelopment are key issues of sustainable development of landfilling disposal strategy and so it is significant to research the biodegradation properties and mechanical properties of old MSW. Otherwise, excavating or preparing old MSW specimen is a principle problem for researching its mechanical characteristics. Therefore, based on the results of triaxial tests on old MSW degraded in laboratory, a method of using simulated materials to research mechanical properties of old MSW was proposed and furthermore, sampling principal and guideposts were suggested. Simulated materials were sampled and tested according to the characteristics of real old MSW and the results of triaxial tests. Research showed that it is feasible using simulated materials to investigate the characteristics of old MSW, in addition, it can reduce the time of preparing old MSW specimens and the input of manpower and material resources.