Electrical tomographic and geotechnical investigation of landslide occurrence at Agbona Hill in Okemesi, Southwestern Nigeria

An integrated investigation of possible factors responsible for reoccurring landslide at Agbona Hill of Okemesi, in a typical Basement Complex terrain of southwestern Nigeria is presented in this paper. The landslide which rises to about 18.62 m at the point of failure, strikes N-S direction, and measured about 8.54 m in width and extends down the slope in length of about 265 m. The landslide which occurred immediately to the north of residential area resulted into damages of buildings and the road along the path of the mass movement. The investigation which involved resistivity imaging along four traverses with length varying between 180 and 210 m in the N-S direction was complemented with eight vertical electrical soundings and geotechnical analysis of soils samples at ten selected points. The inferred lithological depth sections from electrical properties revealed the nature of both the upper and lower sections of the landslide with thin topsoil, having resistivity and thickness values ranging from 104 to 871 Ω-m and 0.8–1.5 m, respectively, observed to be characterized by loose sand to clayey sand and quartzite rubbles. The internal structures also revealed underlying layer classified into fairly to poorly weathered bedrock or indurated quartzite rocks. The resistivity ranges from 297 to 4381 Ω-m and the thicknesses varying from 1.3 to 9.9 m. Both sections exhibited intermediate to steep internal slope with a series of fracturing observed over the outcropped quartzite ridge that extends deeply into these layers. The fractures serve as conduit for the groundwater to flow to the surface, thereby accelerating the weathering of the rock. The results of geotechnical study showed that the upper segment of the landslide comprise of highly weathered materials characterised by sandy clay or silt materials with plasticity index of the fines indicating slight medium plasticity, thereby serving as a trigger to the landslide under prolonged and torrential rainfall arising from the movement in the soils by the flow of groundwater which seeps to the surface through the fractures in the rock mass.