Stability assessment of the Pekmez landslide area: a comparison of the 2D Limit Equilibrium Method (2D LEM), 2D Finite Element Method (2D FEM), and 3D Finite Element Method (3D FEM)

This study evaluates the slope stability conditions of the Pekmez landslide area, located on the western side of Istanbul, using three different engineering analysis methods: the 2D Limit Equilibrium Method (LEM), the 2D Finite Element Method (FEM), and the 3D Finite Element Method (FEM). The primary aim of the study is to compare the performance of these methods under site-specific conditions and determine which approach offers more realistic results for engineering applications Various engineering geological data-including field boreholes, laboratory tests, and topographic maps-were collected for analysis. The study area was evaluated through four cross-sections that represent three distinct geological units and exhibit relatively uniform topography with limited lithological variability. Each method offers a different level of analysis depth and accuracy: the 2D LEM offers fast and practical solutions, the 2D FEM better models the stress-strain relationship, while the 3D FEM enables the most comprehensive assessment by incorporating three-dimensional ground behavior. Across all sections, 2D FEM analyses yielded 0.67% to 1.27% lower safety factors compared to 2D LEM. Meanwhile, 3D FEM results were 1.7% to 13.3% higher than those from 2D LEM, and 2.6% to 14.4% higher than those from 2D FEM. These discrepancies were found to be consistent with previously published literature. In terms of slip surface geometry, 2D analyses tend to reveal shallower and more circular failure patterns, whereas 3D analyses identify deeper and more complex deformation mechanisms. Notably, deep slip surfaces detected in Section 4-4 through 3D FEM analysis were not observed in the 2D models, underscoring the need for deep inclinometer data for more accurate interpretations. In conclusion, while 2D analysis methods may suffice for preliminary evaluations, 3D analyses are essential for developing more reliable engineering solutions in complex geotechnical and topographic settings.