## 3 Results and Discussion  
### Slope Stability Analyses Incorporating Reinforcements  
For the purpose of determining the locations of most probable slope failures on the landfill, there was a network of benchmarks installed at the site. The network helped in defining failure endangered zones, for which the approach of the observational method was applied. It allowed the analysis of the vertical and horizontal displacement measurements of the landfill body during exploitation time [24; 25]. This type of monitoring system allowed the evaluation of the behavior of all slope movement influenced by different groundwater flow conditions, and engineering works execution. The observations revealed a higher increment in horizontal displacement than vertical displacement. This means the increase in slopes steepness, which, in the near future, could induce severe slope failure. Such behavior was confirmed by numerical analyses conducted with the use of limit equilibrium method (LEM) [26]. The result for a selected cross-section is presented in Figure 5.  
The monitoring and modeling of horizontal displacements gave precise information on which slopes should be engineered or reinforced, to assure the overall stability. In 1993, reclamation works on the landfill began. They included safety improvements in terms of engineering the landfill body. To reinforce the slope and to improve the bearing capacity of the access road to the landfill, a retaining wall was also constructed (south-east landfill slope on Figures 6 and 7). By the further carving of the slope, installation of a horizontal uniaxial HDPE geogrid was also proposed. The reason for such heavy modifications was that on the southern slope, the space was very limited due to land ownership issues. The main objective of the reclamation works was to allow as much disposal of municipal waste on the landfill as possible. However, due to the composition of the ballast waste, which did not meet the specific filling material requirements, a new method of compaction and slope filling was adopted. Figure 4: Observational method approach used in the present study. The reason for that was to basically make the material useful for landfill access road construction. The works were also executed by applying the observational method approaches. Based on the waste mechanical characteristics and compaction difficulties, the final decision on mixing the waste with well-graded material was made. Figure 5: Slope stability analyses for a bare slope, using limit equilibrium method (LEM) indicating low value of FOS. Figure 6: Geotechnical safety assessment based on “traffic lights” approach applying observational method framework—no slope reinforcements. As mentioned previously for the slope stability analyses, the observational method approach of a traffic lights system was adopted. Such an approach had not been considered in any other case studies available in the scientific literature at that time. To classify the safety of landfill slopes, three different levels of computed factors of safety (FOSs) were set:  
1. Green color - FOS > 1.3, the slope is safe (risk level 1), does not require further action (continuous monitoring and maintenance is required). 2. Orange color - FOS = 1.3-1.1, alarming condition (risk level 2), stability uncertain, extensive monitoring required, reinforcements application depending on results. 3. Red color - FOS < 1.1, possibility of exceeding ultimate limit state (risk level 3), it is necessary to adopt an alternative plan and immediate changes in the project. Figure 7: Geotechnical safety assessment based on “traffic lights” approach applying observational method framework—with landfill slopes reinforcement. Based on the FOS results presented in Table 3 it is clear that the safety condition on most of the landfill slopes before the reclamation was classified as alarming or exceeded. It was confirmed by the number of local slope failures and cracks observed in the past on the slopes (eastern slope on cross-section III especially--Figure 6).