Geo-risk analysis of slopes bounding a deep gully erosion site in Uyo, Akwa-Ibom State, Southeastern Nigeria

Abstract

The stability of slopes around a deep gully erosion site is the subject of this article. The site geology is the Coastal Plain Sands. To carry out stability analysis, the slopes were first characterized by deployment of cone and standard penetration tests (CPT and SPT) equipment. The slopes are highly stratified, and consist of numerous layers up to the depth of investigation which is about 37.0 m. The slope is made up of five layers of materials up to the gully valley floor. The soils of the five layers in succession from the top are clayey sand (SC), clayey/silty sand (SC–SM), gravelly clay (GC), poorly graded gravel with well graded sand mixture (GP–SW), and well graded sand (SW). Two locations on the eastern side of the of the gully site which poses serious risk to lives and properties were chosen for the analysis. The first position with the highest slope of 16.0 m gave a critical factor of safety less than 1.5 based on simplified Bishop method of analysis indicating unstable slope, while the second location results in a factor of safety greater than 1.5 suggesting a more stable slope, although there is a potential of sheet erosion on the slope at this location.