Landslide susceptibility assessment using geotechnical characterization of collapsible and dispersive soils at Soche Hill, Blantyre, Malawi

Landslides present an increasing hazard in Sub-Saharan Africa, particularly in rapidly urbanizing highland areas with unstable soils and extreme rainfall. This study investigates the geotechnical behavior of surface soils on Soche Hill, Blantyre, Malawi, to assess landslide susceptibility under hydromechanical stress. Five soil profiles (SP.1–SP.5) were analyzed for collapsibility, dispersivity, Atterberg limits, dry unit weight, porosity, and shear strength parameters. Soils with low clay content (e.g., SP.2 and SP.3) exhibited high collapsibility (K = − 2.0 and − 1.1), while clay-rich samples (e.g., SP.4 and SP.1) displayed greater cohesion but severe dispersivity (scores = 6 and 5). SP.3 showed the highest plasticity index (32.9%) and porosity (41%), suggesting a strong shrink–swell and moisture response. Shear strength parameters, derived empirically from plasticity and clay content, revealed significant spatial variability: cohesion ranged from 13.9 to 25.5 kPa and friction angles from 21.5° to 26.7°. Infinite slope modeling under progressive saturation (ru = 0.0–0.8) showed up to 40% reduction in Factor of Safety (FS), with the most unstable conditions at SP.1 and SP.4. Principal Component Analysis and Spearman’s correlation (ρ ≥ 0.8) revealed strong linkages between plasticity, collapsibility, dispersivity, and slope failure risk. Spatial interpolation and weighted overlay modeling produced a Landslide Susceptibility Index (LSI) map that correctly identified high-risk zones, corroborating damage patterns observed during Cyclone Freddy. A composite Soil Risk Index (SRI) confirmed SP.4 (+ 1.66) and SP.1 (+ 0.04) as high- and medium-risk zones. These results underscore the value of integrating geotechnical and spatial analyses for localized hazard mitigation.