The effects of daily cover soils on shear strength of municipal 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 collected 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. The addition or recirculation of leachate to accelerate the waste decomposition changes the geotechnical characteristics of waste mass. The daily cover soils, usually up to 20-30% of total MSW volumes in the landfill, may also influence the decomposition and shear strength behavior of MSW. The objective of this paper is to study the effects of daily covers soils on the shear strength properties of municipal solid waste (MSW) in bioreactor landfills with time and decomposition. Two sets of laboratory-scale bioreactor landfills were simulated in a laboratory, and samples were prepared to represent different phases of decomposition. The state of decomposition was quantified by methane yield, pH, and volatile organic content (VOC). Due to decomposition, the matrix structure of the degradable solid waste component was broken down and contributed to a significant decrease in the reinforcing effect of MSW. However, the daily cover soil, a non-degradable constituent of MSW, remains constant. Therefore, the interaction between daily cover soil particles and MSW particles will affect shear strength behavior. A number of triaxial tests were performed to evaluate the shear strength of MSW. The test results indicated that the shear strength of MSW was affected by the presence of cover soils. The friction angle of MSW with the presence of cover soil is higher than the friction angle of MSW without any cover soils. The friction angle of MSW increased from 27 degrees to 30 degrees due to the presence of cover soils for Phase 1 samples. The increased strength was attributed to the friction nature of sandy soil that was used as daily covers soils. Therefore, the effects of cover soils on the shear strength properties of MSW should be evaluated and taken into consideration during stability analyses and design.