Dynamic Chracteristics And Stability Analysis Of Muncipal Solid Waste In Bioreactor Landfills
Bioreactor landfills are operated to enhance refuse decomposition, gas production, and waste stabilization. The major aspect of bioreactor landfill operation is the recirculation of generated leachate back through the refuse mass. Due to the accelerated decomposition and settlement of solid waste, bioreactor landfills are gaining popularity as an alternative to the conventional landfill designed under Subtitle D regulations. However, recirculation of leachate or addition of water to accelerate the waste decomposition changes the biodegradation and geotechnical characteristics of waste mass. Our understanding of changes in engineering characteristics of solid waste with accelerated decomposition is limited. There is a need to understand and explain the changes in characteristics associated with stability analysis of landfills. The objective of this research is to study the changes in strength characteristics of municipal solid waste (MSW) in bioreactor landfills with time and decomposition. In order to understand the changes in strength properties of MSW in bioreactor landfills with time and decomposition an experimental program was developed. Eight small scale reactors, representative of bioreactor landfill, were built in laboratory, and samples were prepared to represent different phases of decomposition. The state of decomposition was quantified by methane yield, pH, and volatile solids. The geotechnical testing program (CD triaxial test and resonant column tests) was performed to determine shear strength and dynamic characteristics of MSW in bioreactor landfill. The test results indicated that the measured shear strength and dynamic properties of MSW are significantly affected by the degree of decomposition. The friction angle of MSW decreased from 26.7° in Phase I to 19° in Phase IV. The shear modulus increased from 2.11 MPa in Phase I to 12.56 MPa in Phase IV. The change in shear strength and dynamic properties of MSW was attributed to the breakdown in fibrous nature of solid waste particles with degradation. Based on the measured experimental results, stability analyses were conducted. The finite element program PLAXIS and limit equilibrium program STABL was used to analyze the stability of MSW in bioreactor landfills. The stability was evaluated at both under normal and those during earthquake conditions. The factor of safety estimated using PLAXIS and GSTABL under both the conditions decreased as the solid waste degraded with time. The results indicates that the stability of bioreactor landfills should be evaluated using the strength characteristics determined as a function of time and decomposition rather than using average values.