Evaluation of landslide areas with limit equilibrium and three dimensional stability analysis

Landslide stability analyses have been a significant research topic for many years, leading to the development of various methods in this field. Among the most commonly used methods, the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM) offer different advantages and limitations. LEM is based on calculating the force or moment equilibrium of the soil mass along the slip surface; however, it cannot adequately represent slope movements. In contrast, FEM supported by the Strength Reduction Method (SRM) provides more realistic results since it does not require prior determination of the slip surface. In this study, safety factors and slip surfaces were determined using 2D LEM, 2D FEM, and 3D FEM methods for three landslide sites: Zığ (Baku, Azerbaijan), Balaban, and Pekmez (Istanbul, Turkey). The stability of these landslide sites was evaluated. Among these three study areas, the Zığ landslide site was examined as a complex geometry, whereas the Balaban and Pekmez sites were considered simple geometries. The landslide sites were analyzed using software developed by RocScience: Phase2 (Version: 8.005) for 2D finite element (FEM) analyses, Slide (Version: 6.005) for 2D limit equilibrium (LEM) analyses, and Plaxis 3D (Version: 2024.1.0.1060) for 3D finite element (FEM) analyses. Based on field measurements and inclinometer data, safety factors and slip surfaces were determined using 2D LEM, 2D FEM, and 3D FEM for all three sites. The slip surface obtained from the 2D LEM method was adjusted based on inclinometer data, ensuring consistency with field observations. Inclinometer data served as the primary source of validation for all three sites, enhancing the accuracy of the analyses and highlighting the advantages and limitations of each method. For the Zığ landslide site, stability analyses were conducted on two cross-sections (Section 1-1 and Section 2-2), considering complex geometric structures and heterogeneous soil conditions. The safety factor obtained using the 2D FEM method was slightly higher than that calculated with the 2D LEM method. Regarding slip surfaces, the 2D FEM method identified a deeper slip surface compared to the 2D LEM method. In 3D FEM analyses, the safety factor was found to be higher than those obtained with both 2D LEM and 2D FEM methods. The 3D FEM method, by considering lateral boundary effects and three-dimensional deformation mechanisms, modeled the slip surface at a deeper level than 2D LEM but at a more moderate level compared to 2D FEM. For the Balaban landslide site, stability analyses were performed on two cross-sections (Section 1-1 and Section 2-2) within a simple geometry consisting of four geological units. In this site, the safety factor calculated using the 2D FEM method was lower than that obtained with the 2D LEM method. Regarding critical slip surfaces, the 2D FEM method provided a broader slip surface model compared to 2D LEM, yielding results consistent with the reference slip surface. In 3D FEM analyses, the safety factor was found to be higher than those obtained with 2D LEM and 2D FEM. The 3D FEM method, by considering lateral force effects and deformation mechanisms, modeled the slip surface at a deeper level. For the Pekmez landslide site, stability analyses were conducted on four cross-sections (Section 1-1, Section 2-2, Section 3-3, and Section 4-4) within a simple geometry consisting of three geological units. In these analyses, the safety factor calculated using the 2D FEM method was lower than that obtained with the 2D LEM method. Regarding slip surfaces, the 2D FEM method provided results consistent with the 2D LEM method and overlapped with the reference slip surface. In 3D FEM analyses, the safety factor was found to be higher than those obtained with 2D LEM and 2D FEM methods. The 3D FEM method, by considering lateral force effects, modeled the slip surface at the deepest level. The safety factor calculated using the 2D FEM method was found to be closer to that obtained with the reference 2D LEM method for both complex (Zığ) and simple geometries (Balaban and Pekmez). This can be attributed to the ability of 2D FEM to model the stress-strain relationship more realistically. Regarding slip surfaces, the 2D FEM method provided deeper slip surfaces in complex geometries (Zığ) and results more consistent with the 2D LEM method in simple geometries (Balaban and Pekmez). The 3D FEM results yielded higher safety factors and deeper slip surfaces in all study areas compared to both 2D LEM and 2D FEM methods. This is attributed to the 3D FEM method's ability to consider lateral force effects and three-dimensional deformation mechanisms. The 3D FEM method provided a more realistic and reliable assessment of landslide stability, especially in complex sites. This study holds significant importance in evaluating the effectiveness of 2D and 3D analysis methods in determining safety factors and slip surfaces for engineering projects. It also aims to present a framework aligned with the existing literature.