# Stability of MSW Landfill Slopes Reinforced with Geogrids
## 3 Results and Discussion
### Reinforced Analyses
#### 3.2.5 Effect of Material Model on the Reinforced Slope  
In this study, the Mohr-Coulomb material model (MCM) was used to analyze the slope stability of the MSW landfill. MCM has been used by many researchers in the analysis of MSW landfill slopes because it gives acceptable results and the parameters required for modeling can be obtained more easily than other advanced material models [7; 46; 47; 48]. The creep deformation and humidity distribution of MSW plays a key role in the stability of MSW landfill slopes [49; 50]. Given this, the Mohr-Coulomb model cannot accurately represent the macroscopic response without considering the mechanical creep. Finite element analyses were performed using the soft soil creep model (SSCM) in the PLAXIS program for investigating the mechanical creep effect on the stability behavior of the MSW landfill slope.  
The SSCM is used to model the time-dependent behavior of soft soils. Cohesion, c) internal friction angle, \(\phi\); dilatation angle, \(\psi\); modified compression index, \(\lambda^{*}\); modified swelling index, \(\kappa^{*}\); and the modified creep index, \(\mu^{*}\), are entered as input parameters in the model. The modified compression index, \(\lambda^{*}\); the modified swelling index, \(\kappa^{*}\); and the  
Figure 18: Variation in FS with \(\gamma\) for the reinforced slope.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Unit Weight (\(\gamma\)) (kN/m\({}^{3}\))** & **Factor of Safety (FS)** \\ \hline
9 & 1.84 \\
10 & 1.74 \\
11 & 1.65 \\
12 & 1.58 \\
13 & 1.52 \\ \hline \hline \end{tabular}
\end{table}
Table 12: Analysis results for different unit weight values (N = 4).  
modified creep index, \(\mu^{\star}\), can be obtained both from an isotropic compression test and an oedometer test [17]. These parameters can be calculated from Equations (1)-(3).  
\[\lambda^{*}=\frac{C_{c}}{2.3(1+e)} \tag{1}\]  
\[\kappa^{*}\approx\ \frac{2}{2.3}\frac{C_{r}}{(1+e)} \tag{2}\]  
\[\mu^{*}\approx\ \frac{C_{a}}{2.3(1+e)} \tag{3}\]  
where \(C_{c}\) is the compression index, \(C_{r}\) is the recompression index, \(C_{a}\) is the secondary compression index, and \(e\) is the initial void ratio. The parameters used in the analyses were obtained from the studies available in the literature. [PERSON] and [PERSON] [20] conducted compressibility tests and reported that the \(C_{c}\) value is between 0.4-0.9, \(C_{a}\) ranges from 0.03 to 0.009, and the initial void ratio, \(e\), ranges between 1.0 and 3.0. [PERSON] et al. [51] presented the results of one-dimensional compression tests on MSWs with varying degrees of degradation and found that values of the recompression index, \(C_{r}\), varied from 0.003 to 0.017. In the analyses performed using the SSCM, mean values were taken and the input parameters, the modified compression index, \(\lambda^{*}\); the modified swelling index, \(\kappa^{*}\); and the modified creep index, \(\mu^{\star}\), were obtained using \(C_{c}\) = 0.65, \(C_{a}\) = 0.0195, \(C_{r}\) = 0.01, and \(e\) = 2.  
When Table 13 and Figure 19 are examined, it is seen that the FS values obtained using the SSCM in both the unreinforced and reinforced conditions are lower than the values obtained using MCM. While the difference between the FS values is 2% in the unreinforced case, this difference is around 3.3% in the reinforced case.