The geotechnical characteristics of landslides on the sedimentary and metamorphic terrains of South-East Nigeria, West Africa

Abstract

Landslides in Nigeria occur in various forms and vary in mode, scale and frequency. The variations appear to be significantly controlled by geologic setting, hence the need to study in detail the distinctive features that differentiate landslides in sedimentary environments from those on metamorphic localities. The aim is to understand the actual features impacted on the landslides by geology, from which future predictions of occurrence and identification of instability could be based. The recognition of features that are characteristic of certain geologic setting may be a major step in early warning development in Nigeria. To achieve the objectives detailed mapping of the study area was carried out using topographic maps, aerial photographs and multiple field surveys. The landslides on the sedimentary terrain were mainly shallow, low-volume movements, material slumps and short runout slides some of which activated on slopes that followed the dip of strata, along wavy shear surfaces controlled by impermeable bedding planes. Runoff-triggered movements caused by erosion of channel bed and banks and failures caused by sediment bulking of runoff with material eroded from headwater slopes are also common on the sedimentary terrain. In comparison, the landslides on the metamorphic terrain were complex translational and rotational movements and mudslides on steep slopes sometimes involving a combination of slide and flow with curved headscarps and slickensided shear surfaces. The looseness of slope materials and their relatively low strength parameters account for the dominance of landslides on the sedimentary zone.