Municipal solid waste (MSW) landfill: A source of microplastics? -Evidence of microplastics in landfill leachate.

Municipal solid waste (MSW) is a refuse composed of various materials with different properties. Some of the components are stable while others degrade as a result of biological and chemical processes. These aspects impart to MSW a complex behavior that has been modeled, with many limitations, within the concepts of soil mechanics. In this paper, a framework to model the MSW mechanical behavior is proposed based on results from laboratory tests, such as triaxial compression and confined compression of large samples. It is suggested that two different effects command MSW mechanical behavior: (a) the reinforcement of MSW by synthetic fibers (composed by many types of polymers) and (b) the behavior of the MSW paste, without fibers. Accordingly, two distinct frameworks were used to represent the main MSW characteristics: (a) a critical state framework for MSW paste and (b) an elastic perfectly plastic framework for waste fibers, with a time lag for fiber loading (function fm). The proposed model is capable of...

Constitutive Model for Municipal Solid Waste

Seismic response of large underground structures in liquefiable soils subjected to horizontal and vertical earthquake excitations

Landfills remain an attractive disposal route for municipal solid waste because in most cases it is more economical than other alternatives such as incineration and composting. The post-closure development of landfilled areas becomes essential as urban growth reaches landfill boundaries. However, physicochemical and biological processes inevitably hinder the beneficial use of such lands because of gas and leachate generation coupled with significant settlement. The rate and magnitude of landfill deformations are often nonuniform resulting in differential settlements that can have devastating effects on the integrity of any structure erected on the landfill. Differential settlements also eventually result in problems such as surface ponding, development of cracks, and failure of the cover system, including tearing of geomembrane, as well as damage of gas collection and drainage pipes. The ability to predict settlement becomes a key issue in the design and construction of landfills. Numerous models have been developed to simulate the movement of solid waste. This article presents a critical review of such models, describes the applicability and usage of these models, and discusses the necessity to develop a multiphase, multicomponent comprehensive model integrating gas generation, leachate movement, and refuse settlement.

Modeling settlement in MSW landfills: a critical review

Some recent studies have indicated that the liquefaction potential of saturated sand can be greatly reduced if the sand can be made slightly unsaturated. One way to reduce the degree of saturation of sand is to inject gas into sand. This approach offers a cost-effective solution for mitigating liquefaction hazard over a large area. However, it is not easy to inject gas into sand in a uniform manner. A biogas method was developed in this study to overcome this difficulty. In this method, denitrifying bacteria are used to generate tiny, inert nitrogen gas bubbles in sand. Shaking table tests using a fully instrumented laminar box are conducted on both saturated sand and sand containing microbially generated nitrogen gas bubbles. Comparisons of the results of these tests indicate that the pore water pressure generated in the partially saturated sand was much smaller than that in saturated sand. Thus the proposed method is effective in reducing the liquefaction potential of sand.

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Mitigation of liquefaction of saturated sand using biogas

The municipal solid waste landfill suffers from large post-closure settlement that occurs over an extended period of time. A large differential settlement may impair foundations, utilities, and other associated facilities constructed on top of a landfill. It may also lead to breakage of the geomembrane and damage of the cover system in a modern municipal solid waste landfill. The waste material exhibits heterogeneous engineering properties that vary over locations and time within a landfill. These factors, combined with the fact that a landfill is not fully saturated, render a traditional soil mechanics approach less attractive for settlement prediction. An empirical approach of expressing settlement rate using logarithmic and power relationships is commonly used in conjunction with an observational procedure. In this paper, validity of these functions is reexamined based on published settlement results from three landfill sites. A hyperbolic function is proposed as an improved tool to simulate the settlement-time relationships, as well as to detect final settlement. The relationships between the parameters of these empirical functions and water content are examined.

Estimation of municipal solid waste landfill settlement

This study focused on the behavior of a large-diameter burial pipe with special reference to its stability against flotation subject to soil liquefaction. Centrifugal modeling technique was used where the results are presented for a total of eight shaking table tests conducted on the burial pipe in a laminar box under 30g gravitational field. The ground was prepared with Nevada sand at a relative density of 38% and shaken with a sinusoidal wave at an amplitude of 0.5g. The use of a viscous fluid in a saturated soil deposit satisfied the time scaling relationships of both dynamic and dissipation phenomena. The centrifugal modeling technique simulated flotation of pipeline as the soil liquefied. A technique that used gravels and geosynthetic material was used to mitigate flotation. The response of the soil deposit, in terms of acceleration and excess pore pressure, was investigated. The uplifting of the pipe, earth pressure response and ground surface deformation were also presented. Based on the test results, a design procedure was proposed for the burial pipe in resisting flotation due to soil liquefaction. The deadweight and stiffness of the gravel unit, which was confined by geosynthetic, were important items in design.

Centrifugal modeling of seismic behavior of large-diameter pipe in liquefiable soil

The tensile behavior of three commonly used polymeric geogrids (polypropylene, polyester, and high-density polyethylene) under cyclic loading was investigated. The tests were strain controlled and were conducted for 100 cycles at different load ratios. The stiffness and damping ratio of geogrids at all load cycles were compared with the primary loading curve. The stiffness increased while the damping ratio decreased with more loading cycles at any load ratio. A higher load ratio decreased the stiffness ratio and increased the damping ratio. The strengths of prestressed and virgin specimens were also compared. Negligible strength reduction resulted from short-term cyclic loading. The strength obtained from static tests appears reasonable when used for design considering short-term cyclic loading. An empirical relationship is proposed to determine the strain accumulated from cyclic loading under different load ratios.

Tensile Properties of Geogrids under Cyclic Loadings

Seismic analysis of sliding wedge: extended Francais–Culmann's analysis

A coupled stress-flow finite element procedure, based on dynamic Biot equations, was used to analyze the behavior of pipe buried in liquefiable soil. The governing equations, soil constitutive model, finite element discretization and solutions were described. The results of analysis were compared with two cases of dynamic centrifuge test of soil deposit and pipe conducted at 30 g acceleration field. The horizontal soil deposit was analyzed followed by the deposit having a buried pipe of diameter 10 cm (3 m in prototype). The deposit was composed of loose Nevada sand that was saturated with a viscous solution in satisfying the similitude rules of time for the dynamic event and diffusion phenomena. The response of the ground, such as acceleration and excess pore water pressure, and the earth pressure and uplifting of the pipe, were presented and compared. The results of analysis indicated that a coupled stress-flow finite element procedure where the soil was expressed by Pastor–Zienkiewicz Mark-III model was able to simulate the dynamic response of the soil and pipe up to the stage of liquefaction. Several other issues related to the analysis were discussed.

Toshinori Kawabata

Papers

Estimation of municipal solid waste landfill settlement

Centrifugal modeling of seismic behavior of large-diameter pipe in liquefiable soil

Tensile Properties of Geogrids under Cyclic Loadings

Seismic analysis of sliding wedge: extended Francais–Culmann's analysis

Finite element analysis of pipe buried in saturated soil deposit subject to earthquake loading