# Stability of MSW Landfill Slopes Reinforced with Geogrids
## 3 Results and Discussion
### Reinforced Analyses
#### 3.2.1 Effect of Geogrid Number  
In the analyses, the effect of placing geogrid reinforcements at various spacings on the slope stability was investigated by using the PLAXIS computer program. For this purpose, as a result of the analyses carried out previously without reinforcements, a solid waste landfill with a slope angle of \(\beta\) = 26.56\({}^{\circ}\) whose safety number is FS = 0.99, that is, 1V/2H slope, H = 30 m high, was considered, and analyses were carried out by placing geogrid reinforcements at different spacings. In the analyses, other the parameters were the internal friction angle, \(\phi\) = 15\({}^{\circ}\); cohesion, c = 10 kN/m\({}^{2}\); dilatation angle, \(\psi\) = 0\({}^{\circ}\); modulus of elasticity, E = 6500 kN/m\({}^{2}\); Poisson's ratio, \(\gamma\) = 0.45; and the unit volume weight, \(\gamma\) = 9 kN/m\({}^{3}\). Geogrid spacings (h) were taken as certain ratios of the slope height (H) and made dimensionless in terms of H/h. In the analyses, an EA value of 1100 kN/m was entered into the program as the material property of the geogrid reinforcement layers. In the FEM analysis, while determining the initial stresses of the landfill slope, first, the gravitational force was applied since the MSW landfill slope is not horizontal. In the second stage, geogrids were activated, and in the third stage, stability analyses were performed with the phi-c reduction method, and FS values were obtained. A numerical analysis set was carried out to determine the effects of the geogrid reinforcement on the slope stability. The Analysis was carried out in seven sets. Table 6 summarizes the analysis sets with the constant and variable parameters used. When the results obtained from the reinforced analyses are evaluated, it is seen that the stability of the slope can be increased significantly depending on the geogrid reinforcement spacing and the geogrid number values in the case of using geogrid reinforcements. According to Table 7 and Figure 10, as the reinforcement spacing decreases, in other words, as the number of reinforcements increases, higher FS values are obtained. However, since the FS value is generally desired to be higher than 1.50 (FS \(>\) 1.50) in slope stability analyses, it has been observed that this criterion cannot be provided if the geogrid reinforcement spacing is 10 m (N = 3) or more. According to the results, the safety factor value (FS = 0.99), which is less than 1.50 in the unreinforced condition, becomes greater than 1.50 (FS = 1.84) when N = 4 geogrid reinforcements are placed in the MSW landfill slope. Although the FS \(>\) 1.50 criterion is provided for other reinforcement spacings (H/h = 30, H/h = 15, H/h = 10, H/h = 6 and H/h = 5), it is seen that the H/h = 4 combination is more suitable for an economical solution in the model considered. Analyses were performed to investigate the effect of the weight of the geogrids on the FS of the MSW landfill slope. In the analyses, factory values were used for the geogrid weight, and the weight was applied to the slope as a surcharge load. As a result of the analyses, no change was observed in the FS of the slope in the case of the N = 4 geogrid, while a decrease of approximately 2% in the FS of the slope was observed in the analyses performed for the N = 30 geogrid. The failure plane occurring in the MSW landfill slope can be obtained by plotting the shear strain increment. In the unreinforced condition, the failure plane is quite distinct and extends to the toe region of the slope (Figure 11a). The failure plane moved away from the toe region due to the increase in the number of geogrids placed in the slope (Figure 11b-e). This is due to the high tensile strength of the geogrid reinforcements [45]. As the number of geogrids placed in the slope increases, in other words, as the vertical distance between the geogrids decreases, the slip circle is formed in a thinner region. When the studies on the reinforcement of the MSW landfill slope using geogrids were examined, it was seen that a similar behavior was obtained.