Slope Stability Analysis of the Afaha Ekpenedi Integrated Farm Site, Esit Eket, Eastern Niger Delta, Nigeria

Abstract

Natural and or engineered soils slopes failure can be very catastrophic with attendant sliding and or slumping downslope and infrastructural developments on slopes are at risk of instability and failure. Slope instability may be initiated by combination of forces gravitation, seepage water, erosion by run off, sudden lowering of water table adjacent to the slope and earthquakes. The proposed development of an integrated farm to include fisheries and aquaculture, poultry and piggery with associated feed and product processing factories, office complexes and power supply units situated at the slope toe region with slope angle of about 32⁰; and visible and large tension cracks having measured and calculated depths ranging from 0.4m – 0.6m and 4.63 - 5.13m respectively in a perennial coastal microclimatic rainfall zone. Geotechnical investigation and slope stability analysis was considered for a sustainable project development. 7 borings were carried out to a depth of 4m using hand auger and samples collected and tested. The soil profile depicts a two critical layers’ problem, one with a 0 – 2.0m layer of CI-CH clayey-silt with porosity 0.44 – 0.59, void ratio 1.04 – 1.3, hydraulic conductivity 2.04 x 10^-3 – 2.95 x 10^-3, undrained cohesion 34 – 40kN/m², friction angle 6⁰ – 8⁰ and a second layer at 2 – 4.0m of CI – CH clayey-sand of 0.45 – 0.51 porosity, 0.98 - 1.22 void ratio, hydraulic conductivity 1.37 x 10^-3 – 1.17 x 10^-2cm/sec, undrained cohesion 34 – 40KN/m² and friction angle 6⁰ - 9⁰. Three slope design sections were evaluated for instability using the analytical method of Fellenius. Results yielded mobilized cohesion and friction angles of 11.61KN/m² and 7.9⁰, 12.40KN/m² and 5.9⁰, and 12.12KN/m² and 5.9⁰ respectively. Design sector 2 had a failed slope with factor of safety 0.91, while the other 2 slope design sectors were under limit equilibrium conditions. Slope retaining walls were recommended with rectangular foundation bearing capacities of 123.7KN/m², 133.9KN/m² and 116.2KN/m² based on the soil’s shear strength respectively for each of the slope design sectors.

Keywords: Mobilized Cohesion, Friction Angle, Factor of Safety, Limit Equilibrium

DOI: 10.7176/JEES/13-9-05

Publication date: November 30^th 2023