Lion Mountain Landslide in Non-urbanized Terrain: Changing the Myth of Landslide Occurrence in Western Sierra Leone

Abstract

Freetown has documented one of the most devastating landslides in the world in 2017. Many debates in the media, few scientific papers and technical documents, have argued with eloquence ascertaining human factors, particularly deforestation and urbanization, as the dominant causative factor. This notion seems to be widely accepted for all other slides by the communities, government agencies and departments. Therefore, this work attempts to expand on existing public knowledge by demonstrating the less influential or insignificant human factors which can have impacts on certain landslide occurrences in the Freetown Layered-Complex. The representative landslide considered for this study occurred beyond the vicinity of urbanized zone. Therefore, to establish a clear understanding of the actual causative factors, fieldwork and laboratory investigations were undertaken. During the field survey, we assessed the rock type, discontinuities, geomorphology and hydrological influence of the landslide. The specific rock series underlying the landslide was confirmed through thin section analysis at the National Minerals Agency (NMA). DCP tests and laboratory analyses enhanced the derivation of geotechnical properties of the residual soil/regolith.This work systematically presented how natural conditions, such as: geology (rock types and tectonic signatures), geomorphology, hydrology and the geotechnical properties of the slope soil, have interplayed in the occurrence of the landslide event. In addition, the slip surface of the landslide occurred at a depth below the reach of plant activities (2.6 m). This information may help modify public messages by institutions and can be a source of useful information for the country’s Landslide Disaster Management Department (LDMD).