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Nowadays, a large amount of municipal solid waste (MSW) is generated due to the rapid urbanisation in developing countries leads to the demand for larger and higher capacity landfills. Bioreactor landfill technology has been introduced to accelerate the stability of landfill and to solve the issue of limited landfill area. However, the accelerated degradation of the refuse in bioreactor landfills also considerably changes the geotechnical characteristics of the waste in the landfill and thereby increases the concern for waste stability. Hence, this study aims to analyse the stability of both conventional and bioreactor landfill slope with the effects of waste degradation. Finite element method has been used in the slope stability analysis and the stability is presented by the factor of safety. The objectives of this study are i) to determine and assess the main parameter which influences the stability of the waste slope, ii) to determine the effects of waste degradation to the waste properties and iii) to obtain the factor of safety of the landfill slope using numerical analysis by finite element method. From the literature review, it is found that slope stability of a landfill mainly depends on the geotechnical properties of waste, such as moisture content, unit weight, shear strength parameters and hydraulic conductivity of waste. After the degradation process, engineering properties of field refuse are affected which includes the increased pore-water pressure and unit weight, decreased strength and lower hydraulic conductivity. Based on the analysis of conventional landfill slope stability by using Plaxis software, slope ratio of 1:3, 1:4 and 1:5 calculated safe with 1.69, 2.3 and 2.8 whereas the analysis of bioreactor landfill slope stability calculated safe only for slope ratio of 1:4 and 1:5 with 1.60 and 1.97. Moreover, the factor of safety for steeper slopes is lower and vice versa. From the parametric analysis, it is found that the full height of slope and unit weight of waste input affect the result analysis. This study is significant to evaluate the landfill slope stability with the effects of waste degradation and to ensure both conventional and bioreactor landfill slope stability for long periods.

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This study determines the effectiveness of horizontal drains on a slope which refers to a landslide at Putrajaya, Malaysia, in 2007. The incident buried 27 cars, and about 1200 people were transferred to a safe place. The main cause was identified as continuous and heavy rainfall. In Malaysia, abundant rainfall is common during the wet season. Therefore, one of the remedial measures to improve slope stability is applying horizontal drain to flush out rising groundwater and increase soil strength. In this study, the finite element method package, Plaxis, is used to analyze the slope stability changes in terms of the factor of safety (FOS). The simulation of slope behavior includes the horizontal drain conditions in lowering the groundwater level based on three parameters: the heights, appropriate depth, and inclination angle. The horizontal drains are installed at four different heights of 1 m, 2 m, 8 m, and 28 m from the toe of the slope. Based on the results, the horizontal drain is most efficient to be installed with 15 m long and zero degrees angle at the height of 1 m from the toe of the slope. At 2 m height, the drain should be installed with 26 m long and an angle of 1.8 degrees, and at 8 m height, the drain should be installed with 12 m long and an angle of 4.8 degrees. Moreover, at 28 m, the horizontal drain should be installed 20 m long and at an angle of 8.5 degrees. Therefore, this paper highlights the exceedingly appropriate design of horizontal drains at different heights, lengths, and tilt angles along the slope surface.