Insights into the structurally controlled Sangwana landslide, KwaZulu-Natal, South Africa

The Sangwana landslide, a structurally controlled rock slope failure, occurred on 18 December 2019 on Sangwana mountain, central KwaZulu-Natal (KZN), South Africa. The landslide occurred in sub-horizontally bedded, fractured arkosic sandstones and quartz-arenites of the Natal Group, which forms the extensive, deeply dissected plateau topography inland of Durban. To assess the influence of geological fractures on the formation of the Sangwana landslide, detailed field mapping, along with analyses of historical and newly acquired airborne imagery, was conducted. Rocscience software (DIPS and RocSlope) was utilised for kinematic analyses to determine kinematic admissibility and preliminary quantitative slope stability analysis. The results demonstrated that intersecting joint sets (JS1 and JS2) and bedding planes (S0) created discrete blocks capable of forming topple and wedge failures. Kinematic analysis indicated that while toppling is structurally prevalent (with a 29.1% probability), quantitative limit-equilibrium analysis identifies wedge sliding as the most critical mechanical instability. This study provides compelling evidence that the slope failure initiated and propagated along pre-existing fractures. The failure was triggered by antecedent and extreme rainfall, which saturated the fractures, significantly reducing their shear strength. The integrated approach, which combines field mapping, kinematic analysis, deterministic stability analysis, and rainfall assessment, has proven highly effective in this study, providing a robust framework for assessing structurally controlled landslides.