Investigation of Slope Stability Conditions Along a Major Highway in Ondo State

Slope failures and landslides along highway corridors in the hilly terrain of Ondo State, South West Nigeria, create safety hazards and recurring road closure incidents. This study investigates slope stability conditions along a 25-kilometre section of the Ondo-Akure highway, focusing on identified high-risk slope zones. Geotechnical investigation including trial pit excavation, soil sampling, and laboratory testing (shear strength parameters, particle size, and Atterberg limits) was conducted at twelve high-risk slope locations. Slope geometry was surveyed using Total Station, and groundwater levels were assessed through piezometers installed at three critical sites. Stability analyses were performed using the simplified Bishop method for circular failure surfaces, with factor of safety calculations for dry, partially saturated, and fully saturated conditions. Results indicate that three slopes have factors of safety below 1.5 under saturated conditions, indicating potential instability during prolonged heavy rainfall. Factor of safety values range from 0.89 to 2.34 depending on moisture condition. Slopes with existing erosion features show significantly lower factors of safety. The most critical zone involves an actively eroding cut slope above the carriageway with a calculated factor of safety of 0.89 under fully saturated conditions. The study recommends priority slope stabilisation at the three critical locations using a combination of surface drainage, vegetation establishment, and toe buttress installation.

Keywords: slope stability, highway geotechnics, factor of safety, Ondo State, South West Nigeria