Geotechnical and GIS-based environmental factors and vulnerability studies of the Okemesi landslide, Nigeria

Landslide is a geological hazard typically associated with extreme events such as earthquakes, heavy rainfall, volcanic eruptions, changes in groundwater level, etc. This study was carried out in Okemesi-Ekiti (also known as Okemesi), Southwest Nigeria, with the purpose of using remote sensing and GIS technologies to analyze the environmental factors (grain size, direct shear strength resistance, rainfall data, wet density, surface, and slope) resulting in the occurrence of the Okemesi landslide. The study also aimed to conduct a vulnerability analysis in the study area to identify regions with a probability of landslide occurrence. The grain size analysis of the soil in the Okemesi landslide area showed that slope materials comprised 17.14% gravel, 59.31% sand, and 19.48% fines, thus the soil type could be classified as poorly graded gravely sand with a high possibility of landslide occurrence. The geomorphic characteristics of the study area was characterized by slopes ranging from 0.00 & DEG; to 49.00 & DEG;, while most slopes in the area were less than 8.00 & DEG;. The slope aspect direction was mainly in south (157.51 & DEG;-202.50 & DEG;), southwest (202.51 & DEG;-247.50 & DEG;), west (247.51 & DEG;-292.50 & DEG;), and north (0.00 & DEG;-22.50 & DEG; and 337.51 & DEG;-360.00 & DEG;). The highlands were primarily bounded by the slope directions of north (0.00 & DEG;-22.50 & DEG; and 337.51 & DEG;-360.00 & DEG;), northeast (22.51 & DEG;-67.50 & DEG;), east (67.51 & DEG;-112.51 & DEG;), and southeast (112.51 & DEG;-157.50 & DEG;), which indicated the potential direction of mass movement. The study area can be divided into three vulnerability zones: high, medium, and low, with the area percentages of 9.00%, 61.80%, and 29.20%, respectively. The analysis suggested that the Okemesi landslide was likely triggered by rainfall, which might have weakened the physical structure of slope materials. Understanding the causes and impacts of landslides is crucial for policy-makers to implement measures to mitigate landslide hazards, protect infrastructure, and prevent the loss of life in the landslide-prone regions.