Slope Stability Analysis of Steep Tropical Volcanic Soil Slopes Stabilized by Moisture-Controlled Compaction

Abstract

Landslides on steep slopes composed of tropical volcanic soils constitute a major geotechnical hazard because intensive weathering and variations in water content can substantially affect soil shear strength and slope stability. This study evaluated the effectiveness of moisture-controlled compaction in improving the stability of a steep roadside slope in Aek Godang Arbaan Village, Onanganjang District, Humbang Hasundutan Regency, North Sumatra, Indonesia. The engineering properties of the soil were determined through laboratory Proctor compaction and direct shear tests to obtain the optimum moisture content (OMC), maximum dry density, cohesion, and internal friction angle. Slope stability was assessed using the Fellenius and Bishop limit equilibrium methods and a three-dimensional finite element model developed in PLAXIS 3D using the φ–c reduction approach. Three moisture conditions were considered: 14.70% on the dry side of the OMC, an OMC of 19.25%, and 23.80% on the wet side. The Proctor test yielded an OMC of 19.25% and a maximum dry density of 1.614 g/cm³. At the OMC, the factors of safety calculated using the Fellenius, Bishop, and PLAXIS 3D methods were 1.51, 1.66, and 1.997, respectively. The Bishop method produced a slightly higher factor of safety under the dry-side condition of 14.70% (1.69) than at the OMC (1.66), whereas the Fellenius and PLAXIS 3D analyses exhibited different trends. Nevertheless, when maximum dry density and the combined results of the three stability analyses were considered, the OMC of 19.25% was identified as the most favourable overall compaction condition. PLAXIS 3D provided additional insight into the stress distribution, deformation pattern, and progressive failure mechanism that could not be represented by the limit equilibrium methods. The large deformation values generated during the φ–c reduction analysis indicate numerical progression towards failure and should not be interpreted as predicted service-condition displacements. These findings demonstrate that controlling the water content during compaction at or near the OMC can enhance the stability of steep tropical volcanic soil slopes and provide a practical basis for slope stabilisation and landslide-risk mitigation.