# Stability of MSW Landfill Slopes Reinforced with Geogrids
## 1 Introduction  
Today, rapid population growth, urbanization, industrialization, and technological developments have revealed the problem of pollution. Solid waste, which constitutes a large part of the pollution problem, is a situation that needs to be managed and improved in all countries of the world. Solid wastes have polluted and continue to pollute ground and underground waters. After the damage caused by ground and groundwater pollution became visible, the importance of this type of pollution was understood, and laws and regulations were prepared to eliminate the pollution that had occurred. If waste is not stored in a way that causes minimum damage to the environment, it is inevitable that negative effects will occur in the future. Since it is not possible to completely destroy waste, the main objective should be to reduce its mass, that is, to reduce its volume and to store it regularly in a way that will allow for reuse with technological developments that may occur in the following years. Slope stability analyses in MSW landfills are very important both to know how high it can be filled without any movement on the slopes, and to determine the angle of the slopes that will be reconstructed if old or abandoned MSW sites are reused as construction sites. In general, stability problems on slopes in landfills are similar to movements seen on normal slopes. Due to the non-homogeneous materials and non-uniform distribution in a landfill, movement can be observed on the slopes ranging from surface ruptures to large landslides. Similar to all natural or artificial slopes, the stability of slopes in MSW landfills against slipping and collapse under various loads is analyzed by the limit equilibrium method (LEM). In this method, the equilibrium between resisting and sliding forces is investigated on a known or accepted critical slip surface. In slope stability analyses, acertain factor of safety (FS) is determined, and necessary analyses are made. The FS calculated as a result of these analyses reveals whether the slope is stable against failure or the degree of stability. With the finite element method (FEM), which is another method for slope stability analysis, slopes can be analyzed under different soil conditions and load combinations. By designing the slopes in two or three dimensions and using the appropriate soil model, realistic stress and displacement values can be obtained. In addition, necessary analyses can be made in cases where the slope is strengthened with different materials. Two approaches are generally used when performing slope stability analyses with FEM. In the first approach, the gravity increase method, the gravitational acceleration is increased until slope failure. In the second approach, the strength reduction method, cohesion and internal friction angles are reduced until slope failure. There are numerous studies in the literature on the stability of slopes in MSW landfills [1; 2; 3; 4; 5; 6; 7; 8]. In these studies, the slope stability analyses were carried out using both conventional LEM and FEM. [PERSON] et al. [1] analyzed the causes of slope failure at a landfill in California. According to the results obtained from the analyses performed in the study, the FS was determined to be between 0.85 and 1.25 at the failure time. [PERSON] et al. [2] investigated the stability of MSW landfill slopes. According to the results obtained from laboratory tests, field experiments, and back-calculations, it has been shown that the shear strength parameters for landfills can be considered as 35\({}^{\circ}\) and 0-50 kpa for the internal friction angle and cohesion, respectively. In the study carried out by [PERSON] and [PERSON] [3], ten landfill failures were discussed. According to the results obtained, numerical analyses for stability calculations depend more on shear strength than other parameters. [PERSON] et al. [4] carried out slope stability analyses at two existing solid waste landfills in Brazil.